An advanced method of contributing emissions to short-lived chemical species (OH and HO₂): the TAGGING 1.1 submodel based on the Modular Earth Submodel System (MESSy 2.53)

To mitigate the human impact on climate change, it is essential to determine the contribution of emissions to the concentration of trace gases. In particular, the source attribution of short-lived species such as OH and HO₂ is important as they play a crucial role for atmospheric chemistry. This study presents an advanced version of a tagging method for OH and HO₂ (HO_x) which attributes HO_x concentrations to emissions. While the former version (V1.0) only considered 12 reactions in the troposphere, the new version (V1.1), presented here, takes 19 reactions in the troposphere into account. For the first time, the main chemical reactions for the HO_x chemistry in the stratosphere are also regarded (in total 27 reactions). To fully take into account the main HO₂ source by the reaction of H and O₂, the tagging of the H radical is introduced. In order to ensure the steady-state assumption, we introduce rest terms which balance the deviation of HO_x production and loss. This closes the budget between the sum of all contributions and the total concentration. The contributions to OH and HO₂ obtained by the advanced tagging method V1.1 deviate from V1.0 in certain source categories. For OH, major changes are found in the categories biomass burning, biogenic emissions and methane decomposition. For HO₂, the contributions differ strongly in the categories biogenic emissions and methane decomposition. As HO_x reacts with ozone (O₃), carbon monoxide (CO), reactive nitrogen compounds (NO_y), non-methane hydrocarbons (NMHCs) and peroxyacyl nitrates (PAN), the contributions to these species are also modified by the advanced HO_x tagging method V1.1. The contributions to NO_y, NMHC and PAN show only little change, whereas O₃ from biogenic emissions and methane decomposition increases in the tropical troposphere. Variations for CO from biogenic emissions and biomass burning are only found in the Southern Hemisphere.

1 Introduction

The radicals hydroxyl (OH) and hydroperoxyl (HO₂) are crucial for atmospheric chemistry. Both radicals are very reactive and have a lifetime of only a few seconds. OH is frequently converted to HO₂ and vice versa. Thus, OH and HO₂ radicals are closely linked and often referenced together as the chemical family HO_x. The ratio of OH to HO₂ in an air parcel strongly depends on the chemical background, in particular on the composition of nitrogen oxides NO_x (= NO + NO₂) and non-methane hydrocarbons (NMHC) (Heard and Pilling, 2003).

HO_x impacts global warming and local air quality in various ways: by reacting with greenhouse gases such as methane (CH₄) and ozone (O₃), OH reduces their atmospheric residence time (e.g. Stevenson et al., 2006; Voulgarakis et al., 2013; Righi et al., 2015). Hence, HO_x controls the impact of CH₄ and O₃ on global warming. Moreover, being the main oxidizer in the troposphere, OH is involved in the decomposition of pollutants and in the production of ground-level ozone, photochemical smog and secondary organic aerosols (e.g. Lawrence et al., 2001; Heard and Pilling, 2003). Consequently, to quantify the human impact on climate and air quality, it is essential to understand the distribution and variability of OH and HO₂ in the atmosphere.

However, the determination of OH and HO₂ concentrations in the atmosphere is still challenging due to their short lifetimes. In field campaigns HO_x concentrations are measured on a local scale, which is generally difficult to compare with global models (e.g. Ren et al., 2003; Olson et al., 2006). For certain environments, such as the marine boundary layer, model studies compare well with measurements. Other regions, such as unpolluted forest areas, show large discrepancies (Heard and Pilling, 2003; Stone et al., 2012). On regional and global scales, no direct HO_x measurements are available. So far, OH concentration and its inter-annual variability can only be estimated indirectly by measurements and emission rates of methyl chloroform (CH₃CCl₃) (Prinn et al., 2005; Montzka et al., 2011). As emissions of CH₃CCl₃ steadily decline, Liang et al. (2017) suggest an alternative method: they combine several trace gases such as CH₂F₂, CH₂FCF₃, CH₃CHF₂ and CHClF₂ in a gradient-trend-based two-box model approach to derive a global OH concentration of 11.2 × 10⁵ molec cm⁻³. Overall, global chemistry climate models estimate a tropospheric OH concentration of around 11 × 10⁵ molec cm⁻³ (Naik et al., 2013), which compares well with the observation-based results from Prinn et al. (2005) and Liang et al. (2017).

To mitigate the human impact on climate change or pollution in general, it is crucial to determine the contribution of an emission sector to the concentration of certain chemical species (Grewe et al., 2012; Clappier et al., 2017). To do so, we use a “tagging” method: the theoretical framework of this tagging method is given in Grewe et al. (2010) and Grewe (2013), and the implementation is described in Grewe et al. (2017). This method splits up all chemical species which are important for O₃ production and destruction into 10 source categories: emissions from anthropogenic non-traffic (e.g. industry and households), road traffic, shipping, aviation, biogenic sources, biomass burning, lightning, methane (CH₄) and nitrous oxide (N₂O) decompositions and stratospheric ozone production. Subsequently, the contributions of these sources to the concentrations of O₃, CO, OH, HO₂, peroxyacyl nitrates (PANs), reactive nitrogen compounds (NO_y, e.g. NO, NO₂, HNO₄) and non-methane hydrocarbons (NMHC) are diagnosed. The contribution calculations are based on chemical reaction rates, online emissions (e.g. lightning), offline emissions (e.g. road traffic) and deposition rates. Emissions of NO and NO₂ contribute to the NO_y concentration, while emissions of e.g. C₂H₄, C₃H₆ and HCHO contribute to the NMHC concentration. This tagging method considers the competition of NO_y, CO and NMHC in producing and destroying O₃.

The tagging method of the long-lived species O₃, CO, PAN, NO_y and NMHC and of the short-lived species OH and HO₂ is based on the same principles of apportioning the contributions. (In this study, O₃, CO, PAN, NO_y and NMHC are denoted as long-lived species because their atmospheric lifetime is significantly longer then the lifetime of OH and HO₂.) However, the implementation for long-lived and short-lived species differs. For the long-lived species, each source tracer is transported, receives the corresponding online or offline emissions, is deposited and reacts with other species. Based on these processes, the tagging method determines the concentration of the source tracers. A detailed description of the implementation of the tagging method for long-lived species is given in Grewe et al. (2017).

However, the short-lived species HO_x are not transported and experience neither emission nor deposition. Thus, the same implementation of the tagging method as for long-lived species is not possible. Tsati (2014) and Grewe et al. (2017) introduced a modified approach for tagging HO_x: since the lifetime of OH and HO₂ is very short, a steady state between the production and destruction of OH and HO₂ is assumed. Using the main chemical reactions of HO_x chemistry, the contributions of each source category to OH and HO₂ are determined.

Figure 1 Sketch of the chemistry used in advanced tagging mechanism V1.1. Blue boxes indicate tagged long-lived species, and orange boxes display tagged short-lived species. Green boxes represent the emissions of CO, NO_y and NMHC.

The contributions to long-lived and short-lived species are closely linked (see Fig. 1). For example, the reaction

$$\mathrm{OH}+{\mathrm{O}}_{\mathrm{3}}⟶{\mathrm{HO}}_{\mathrm{2}}+{\mathrm{O}}_{\mathrm{2}}$$

involves the long-lived species O₃ and the short-lived species OH and HO₂. Hence, this reaction is considered in the implementation of the tagging method for long-lived and short-lived species. The contribution of, for example, shipping emissions to O₃ influences the contribution of shipping emissions to HO₂: the higher the contribution to O₃, the more HO₂ is attributed to shipping emissions. Furthermore, OH from shipping emissions destroys O₃ and thus reduces the contribution of shipping emissions to O₃.

The implementation of the tagging method for the short-lived species HO_x, presented by Grewe et al. (2017), is referred to as the HO_x tagging method V1.0. It did not consider all relevant reactions for the production and loss of HO_x. In particular, the reactions which are important in the stratosphere were not taken into account. Moreover, the steady-state assumption between HO_x production and loss was not fulfilled. In this study, we present a revised version V1.1 of the HO_x tagging method, largely improving these shortcomings. It includes the main chemical reactions of HO_x chemistry in the troposphere and stratosphere. This is enabled by introducing the tagging of the hydrogen radical (H). Special care is taken for the steady-state assumption.

The paper is structured as follows: after introducing the model set-up in Sect. 2, we present the advanced HO_x tagging method V1.1 in Sect. 3. In Sect. 4, the results are compared with the tagging method V1.0 by Grewe et al. (2017). Finally, Sect. 5 concludes the methods and the results of this study.

2 Model description of EMAC and MECO(n)

To evaluate the further developed HO_x tagging method we use the same model set-up as Grewe et al. (2017). A global climate simulation is performed with the ECHAM/MESSy Atmospheric Chemistry (EMAC) chemistry climate model. EMAC is a numerical chemistry and climate simulation system that includes submodels describing tropospheric and middle atmosphere processes and their interaction with oceans, land and human influences (Jöckel et al., 2010). It uses the second version of the Modular Earth Submodel System (MESSy2.53) to link multi-institutional computer codes. The core atmospheric model is the 5th generation European Centre Hamburg general circulation model (ECHAM5; Roeckner et al., 2006). For the present study we apply EMAC in the T42L90MA resolution, i.e. with a spherical truncation of T42 (corresponding to a quadratic Gaussian grid of approx. 2.8 by 2.8^∘ in latitude and longitude) with 90 vertical hybrid pressure levels up to 0.01 hPa. For the simulation presented in this study, the time span of July 2007 to December 2008 is considered: half a year as a spin-up and 1 year for the analysis.

For the chemical scheme, we use the submodel MECCA (Module Efficiently Calculating the Chemistry of the Atmosphere), which is based on Sander et al. (2011) and Jöckel et al. (2010). The chemical mechanism includes 218 gas-phase, 12 heterogeneous and 68 photolysis reactions. In total 188 species are considered. It regards the basic chemistry of OH, HO₂, O₃, CH₄, nitrogen oxides, alkanes, alkenes, chlorine and bromine. Alkynes, aromatics and mercury are not considered.

Total global emissions of lightning NO_x are scaled to approximately 4 Tg(N) a⁻¹ (parameterized according to Grewe et al., 2001). The submodel ONEMIS (Kerkweg et al., 2006) calculates NO_x emissions from soil (parameterized according to Yienger and Levy, 1995) and biogenic C₅H₈ emissions (parameterized according to Guenther et al., 1995). Direct CH₄ emissions are not considered, and instead pseudo-emissions are calculated using the submodel TNUDGE (Kerkweg et al., 2006). This submodel relaxes the mixing ratios in the lowest model layer towards observations by Newtonian relaxation (more details are given by Jöckel et al., 2016).

To show the effect of the HO_x tagging method on a regional scale, a further simulation with the coupled model system MESSyfied ECHAM and COSMO models nested n times (MECO(n)) is performed. The nested system couples the global chemistry climate model EMAC online with the regional chemistry climate model COSMO/MESSy (Kerkweg and Jöckel, 2012a, b). To test the HO_x tagging in MECO(n), we conduct a simulation using one COSMO/MESSy nest over Europe with a resolution of 0.44^∘. EMAC is applied in a horizontal resolution of T42 with 31 vertical levels. The period from July 2007 to December 2008 is simulated. The set-up of the simulation is identical to the one described in Grewe et al. (2017). A detailed chemical evaluation of the set-up is given in Mertens et al. (2016).

Both model simulations are based on the quasi chemistry-transport model (QCTM) mode in which the chemistry is decoupled from the dynamics (Deckert et al., 2011). The anthropogenic emissions are taken from the MACCity emission inventory (Granier et al., 2011). The TAGGING submodel (as described by Grewe et al., 2017) is coupled to the detailed chemical solver MECCA from which it obtains information about tracer concentrations and reaction rates. Based on this information, it calculates the contributions of source categories to O₃, CO, NO_y, PAN and NMHC concentrations. The contributions of OH and HO₂ are calculated with the advanced method V1.1 presented in the next section. The implementation is based on MESSy2.53 and will be available in MESSy2.54.

3 Tagging method of short-lived species

3.1 Tagging method V1.0

The tagging method V1.0 described by Grewe et al. (2017) determines the contribution of source categories to O₃, NO_y, CO, NMHC, PAN, OH and HO₂ concentrations. A total of 10 source categories are considered, and every species included in the tagging method is decomposed into these categories: for example, the concentration of O₃ is split up into O₃ produced by anthropogenic non-traffic (e.g. industry) emissions (${\mathrm{O}}_{\mathrm{3}}^{\mathrm{ant}}$), road traffic emissions (${\mathrm{O}}_{\mathrm{3}}^{\mathrm{tra}}$), ship emissions (${\mathrm{O}}_{\mathrm{3}}^{\mathrm{shp}}$), air traffic emissions (${\mathrm{O}}_{\mathrm{3}}^{\mathrm{air}}$), biogenic emissions (${\mathrm{O}}_{\mathrm{3}}^{\mathrm{bio}}$), biomass burning (${\mathrm{O}}_{\mathrm{3}}^{\mathrm{bb}}$), lightning (${\mathrm{O}}_{\mathrm{3}}^{\mathrm{lig}}$), methane decomposition (${\mathrm{O}}_{\mathrm{3}}^{{\mathrm{CH}}_{\mathrm{4}}}$), nitrous oxide decomposition (${\mathrm{O}}_{\mathrm{3}}^{{\mathrm{N}}_{\mathrm{2}}\mathrm{O}}$) and stratospheric ozone production (${\mathrm{O}}_{\mathrm{3}}^{\mathrm{str}}$). These tagged species go through the same chemical reactions and the same deposition loss processes as O₃. The tagging method uses a combinatoric approach to determine the contributions: it redistributes the production and loss rates of each species to the 10 source categories according to the concentrations of the tagged species. Details on the tagging theory and implementation in EMAC and MECO(n) are found in Grewe (2013) and Grewe et al. (2017), respectively.

Table 1 The reduced HO_x reaction system V1.1 describes the main reactions of HO_x chemistry in the troposphere and stratosphere. These 27 reactions are used for the tagging method V1.1. In the column “tropos.” (“stratos.”), reactions which are important in the troposphere (stratosphere) are marked. In the column “V1.1”, reactions marked with “o” were already included in V1.0. Reactions marked with “x” are added in V1.1. Reactions marked with “(x)” were only partly taken into account in V1.0. The numbers of reactions are referenced in the text.

For the first time, V1.0 determined the contribution of source categories to OH and HO₂ concentrations. The tagging method V1.0 was based on 12 reactions for the HO_x chemistry (reactions marked with “o” in last column of Table 1). It included the main production and loss reactions of HO_x with O₃, NO_y, NMHC, CO and CH₄. V1.0 only regarded reactions which are important in the troposphere. Reactions which mainly occur in the stratosphere were not taken into account. However, the main HO₂ production by the Reaction (1) H + O₂ ⟶ HO₂ (see Table 1) was not regarded. It was combined with Reaction (11), CO + OH ⟶ H + CO₂ (see Table 1), to

$$\begin{array}{ll}{\displaystyle }\mathrm{CO}+\mathrm{OH}& {\displaystyle }⟶{\mathrm{CO}}_{\mathrm{2}}+\mathrm{H}\\ {\displaystyle }& {\displaystyle }\stackrel{{\mathrm{O}}_{\mathrm{2}}}{\mathit{⟶}}{\mathrm{CO}}_{\mathrm{2}}+{\mathrm{HO}}_{\mathrm{2}}\\ {\displaystyle }⟹\mathrm{CO}+\mathrm{OH}& {\displaystyle }\stackrel{{\mathrm{O}}_{\mathrm{2}}}{\mathit{⟶}}{\mathrm{CO}}_{\mathrm{2}}+{\mathrm{HO}}_{\mathrm{2}}.\end{array}$$

But not all H radicals in the troposphere are produced by the reaction of CO + OH. Reactions (7) OH + O(³P), (10) H₂ + OH and (28) HCHO + hv also produce H (Table 2). These reactions were neglected in V1.0. Thus, only 80 % of the H production and therefore only 80 % of the HO₂ production by Reaction (1) was considered in the troposphere. In the stratosphere, the reaction of CO + OH becomes less important and most H is produced by Reactions (7) and (28). Consequently, only 6 % of the H and thus of the HO₂ production by Reaction (1) was regarded in this approach. (Numbers are derived from an EMAC simulation as described in Sect. 2.)

Table 2 The reduced H reaction system describes the main reactions of H. In the column “tropos.” (“stratos.”), reactions which are important in the troposphere (stratosphere) are marked. The numbers of the reactions correspond to the numbers in Table 1.

In the troposphere, the most important reactions not covered in V1.0 are Reaction (1) H + O₂, Reaction (15) NO₂ + HO₂ and Reaction (18) for the decomposition of HNO₄. In the stratosphere, Reactions (1) H + O₂, (5) HO₂ + O(³P) and (7) OH + O(³P) play a leading role and were not included in V1.0.

Most reaction rates used in the tagging method correspond to the production and loss rates directly provided by the chemical scheme MECCA of EMAC. However, for reactions with NMHC, the reaction rates were obtained indirectly. The reaction rate of OH with NMHC (Reaction 21, Table 1) was determined via the production rates of CO by assuming that each reaction of OH with NMHC produces one CO molecule. This method neglects all intermediate oxidation reactions of NMHC and considers only these reactions when NMHC is finally oxidized to CO. For the reaction rates of NO_y and HO₂ with NMHC (Reactions 22 and 23), only the reaction of HO₂ with the methylperoxy radical (CH₃O₂) was considered.

To derive the contributions to OH and HO₂, a steady state between HO_x production and loss was assumed. However, the steady-state assumption was not completely fulfilled for V1.0 (see Sect. 3.4). Moreover, the sum of the contributions of the 10 source categories to the OH and HO₂ concentrations did not equal the total OH and HO₂ concentrations. It deviated by about 70 %.

3.2 Reduced HO_x reaction system V1.1

OH and HO₂ react with many chemical species. To reduce the calculation time of a simulation, we reduce the HO_x chemistry used in the chemical scheme MECCA to the most important reactions which occur in the troposphere and stratosphere. We consider only reactions with a tropospheric or stratospheric annual mean reaction rate larger than 10⁻¹⁵ $\mathrm{mol}{\mathrm{mol}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$ (see Table 1). Hence, we increase the number of reactions from 12 (V1.0) to 27 (V1.1), which still constitutes a reduced set of reactions compared to the full chemical scheme MECCA used in EMAC. In the following, we call this set reduced HO_x reaction system V1.1.

The reactions which are important in the troposphere are indicated in Table 1. As stated above, Reaction (1) of H and O₂ dominates the HO₂ production in the troposphere. It produces 49 % of tropospheric HO₂. In V1.0, only part of this HO₂ source was regarded (see Sect. 3.1). The most important HO₂ loss is the reaction with NO (Reaction 14), followed by the reaction with itself producing H₂O₂ (Reaction 3), which accounts for 32 and 12 % of tropospheric HO₂ loss. The production via H₂O and O(¹D) produces about 21 % of tropospheric OH (Reaction 2). The excited oxygen radical (O(¹D)) originates from the photolysis of O₃. Reaction (14) of NO and HO₂ also produces 32 % of tropospheric OH. OH is mostly destroyed by CO (Reaction 11, 38 %), followed by NMHC (Reaction 21, 25 %).

In the stratosphere different chemical reactions become important. Here, OH is mainly destroyed by O₃, producing 40 % of stratospheric HO₂. The reaction is partly counteracted by the Reaction (14), which produces 21 % of OH and destroys 24 % of HO₂. Since large quantities of O₃ are found in the stratosphere, O₃ or the excited oxygen radical (O(³P)) destroys about 62 % of HO₂. Reactions with NMHC, CO and CH₄ play only a minor role in the stratosphere.

Reactions of OH and HO₂ with chlorine and bromide were not considered in V1.0. We add these reactions, which take place only in the stratosphere, to the tagging method V1.1. Reactions (21) to (25) involve the chemical family NMHC, which contains several species such as formaldehyde (HCHO), ethylene (C₂H₄) and propane (C₃H₈). The rate for Reaction (21) is determined by adding up the rates of all reactions of OH with each single species of the family NMHC. The reaction rate (23) contains all rates of the reactions between the species of the chemical families NO_y and NMHC. All reaction rates are directly derived by the MECCA mechanism of EMAC.

Table 1 does not consider all reactions with annual reaction rates larger than 10⁻¹⁵ $\mathrm{mol}{\mathrm{mol}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$. The photolysis of hydrogen peroxide (H₂O₂), hypochlorous acid (HOCl) and hypobromous acid (HOBr) is excluded from the reduced HO_x reaction system V1.1 as the tagging method cannot be applied. The specific reasons are explained in Appendix Appendix A.

3.3 Deductions of tagged species

To derive how much OH and HO₂ is produced and destroyed by a source category i, the tagging approach described in Grewe et al. (2010, 2017) is used. In general, bimolecular reactions with two chemical species A + B ⟶ C are tagged as follows: each tagged species is split up into its contribution from n source categories $A={\sum }_{i=\mathrm{1}}^n{A}^i$, $B={\sum }_{i=\mathrm{1}}^n{B}^i$ and $C={\sum }_{i=\mathrm{1}}^n{C}^i$. These contributions ($A^i,B^i,C^i$) go through the same reactions as their main species (A, B, C). If A from category i reacts with B from category j, then the resulting species C belongs half to the category i and half to the category j:

$$\begin{array}{}\text{(1)}& A^i+B^j⟶{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{C}^i+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{C}^j.\end{array}$$

Table 3 Annual mean of OH, HO₂ and H production and loss rates (air mass weighted) in 10⁻¹³ mol mol⁻¹ s⁻¹ for the total rates (derived from the complete chemical scheme MECCA in EMAC) and for the rates of the reduced reaction system of the tagging method V1.0 and V1.1. The first row gives the rates for the troposphere, and the second row for the stratosphere (written in italic).

Consequently, the production P and loss L of a species from the category i (here LossAⁱ, LossBⁱ and ProdCⁱ) are determined by regarding all possible combinations of the reaction between Aⁱ and B^j:

$$\begin{array}{ll}{\displaystyle }{\text{LossA}}^i& {\displaystyle }={\text{LossB}}^i={\text{ProdC}}^i\\ {\displaystyle }& {\displaystyle }=k\left(A^i{B}^i+\sum _{j=\mathrm{1},j\ne i}^n{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{A}^i{B}^j+\sum _{j=\mathrm{1},j\ne i}^n{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{A}^j{B}^i\right)\\ \text{(2)}& {\displaystyle }& {\displaystyle }={\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}R\left({\displaystyle \frac{A^i}{A}}+{\displaystyle \frac{B^i}{B}}\right),\end{array}$$

with k being the reaction rate coefficient and R=k A B being the respective reaction rate. For unimolecular reactions A ⟶ B + C, the distribution of categories from the educts is completely passed to the products:

$$\begin{array}{}\text{(3)}& {\text{LossA}}^i={\text{ProdB}}^i={\text{ProdC}}^i=R{\displaystyle \frac{A^i}{A}},\end{array}$$

with the reaction rate R=kA.

As described above, the long-lived species O₃, CO, NO_y and NMHC are tagged according to the tagging method described in Grewe et al. (2017). To limit memory demand, other species such as H₂, H₂O₂, CH₄, ClO and BrO are not tagged (as in V1.0). Here, different approaches are derived to retain the ratio of the contribution to total concentration $\frac{A^i}{A}$.

1. If a tagged species reacts with a non-tagged species, the non-tagged species does not contribute and the tagging method for a unimolecular reaction is applied (see Eq. 2). Examples are Reactions (9), (10) and (13).

2. Using the family concept as described in Grewe et al. (2017) allows for the assumption that all tags are distributed equally among the species within the same chemical family.

   $$\begin{array}{}\text{(4)}& {\displaystyle \frac{{\mathrm{NO}}^i}{\mathrm{NO}}}={\displaystyle \frac{{{\mathrm{NO}}_{\mathrm{2}}}^i}{{\mathrm{NO}}_{\mathrm{2}}}}={\displaystyle \frac{{{\mathrm{HNO}}_{\mathrm{4}}}^i}{{\mathrm{HNO}}_{\mathrm{4}}}}={\displaystyle \frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}}\end{array}$$

   As mentioned in Grewe et al. (2017), all species which are frequently converted back and forth to ozone are considered as an “ozone storage” (Crutzen and Schmailzl, 1983). These species together with O₃ are lumped into one chemical family: ozone. Both O(¹D) and O(³P) belong to this chemical family. Hence, as in Grewe et al. (2017), we apply the family concept and set

   $$\begin{array}{}\text{(5)}& {\displaystyle \frac{\mathrm{O}({}^{\mathrm{1}}\mathrm{D}{)}^i}{\mathrm{O}\left({}^{\mathrm{1}}\mathrm{D}\right)}}={\displaystyle \frac{\mathrm{O}({}^{\mathrm{3}}\mathrm{P}{)}^i}{\mathrm{O}\left({}^{\mathrm{3}}\mathrm{P}\right)}}={\displaystyle \frac{{{\mathrm{O}}_{\mathrm{3}}}^i}{{\mathrm{O}}_{\mathrm{3}}}}.\end{array}$$

3. In Reaction (1), neither H nor O₂ is tagged. To obtain the ratio $\frac{{{\mathrm{HO}}_{\mathrm{2}}}^i}{{\mathrm{HO}}_{\mathrm{2}}}$, we set up an extra tagging of H itself. As the H radical is very reactive, we assume that H production balances H loss (see Sect. 3.4). Table 2 presents the main reactions for H, which still constitute a subset of full H chemistry implemented in MECCA. Based on Table 2, we set up the H production ProdHⁱ and H loss LossHⁱ for the contribution of a source category i.

   $$\begin{array}{ll}{\displaystyle }{\text{ProdH}}^i& {\displaystyle }={\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{7}}\left({\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}+{\displaystyle \frac{{{\mathrm{O}}_{\mathrm{3}}}^i}{{\mathrm{O}}_{\mathrm{3}}}}\right)+R_{\mathrm{10}}{\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}\\ \text{(6)}& {\displaystyle }& {\displaystyle }+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{11}}\left({\displaystyle \frac{{\mathrm{CO}}^i}{\mathrm{CO}}}+{\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}\right)+R_{\mathrm{28}}{\displaystyle \frac{{\mathrm{NMHC}}^i}{\mathrm{NMHC}}}\end{array}$$
   $$\begin{array}{}\text{(7)}& {\displaystyle }{\text{LossH}}^i=R_{\mathrm{1}}{\displaystyle \frac{{\mathrm{H}}^{\mathrm{i}}}{\mathrm{H}}}\end{array}$$

   As mentioned above, the family concept also sets $\frac{{\mathrm{HCHO}}^i}{\mathrm{HCHO}}=\frac{{\mathrm{NMHC}}^i}{\mathrm{NMHC}}$. Since the steady-state assumption applies for H (see Sect. 3.4), the H production per source category i ProdHⁱ equals the loss LossHⁱ. After setting Eqs. (5) and (5) equal to each other, we obtain

   $$\begin{array}{ll}{\displaystyle \frac{{\mathrm{H}}^i}{\mathrm{H}}}& {\displaystyle }={\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{\displaystyle \frac{R_{\mathrm{7}}}{R_{\mathrm{1}}}}\left({\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}+{\displaystyle \frac{{{\mathrm{O}}_{\mathrm{3}}}^i}{{\mathrm{O}}_{\mathrm{3}}}}\right)+{\displaystyle \frac{R_{\mathrm{10}}}{R_{\mathrm{1}}}}{\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}\\ \text{(8)}& {\displaystyle }& {\displaystyle }+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{\displaystyle \frac{R_{\mathrm{11}}}{R_{\mathrm{1}}}}\left({\displaystyle \frac{{\mathrm{CO}}^i}{\mathrm{CO}}}+{\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}\right)+{\displaystyle \frac{R_{\mathrm{28}}}{R_{\mathrm{1}}}}{\displaystyle \frac{{\mathrm{NMHC}}^i}{\mathrm{NMHC}}}.\end{array}$$

Figure 2 Contribution of 10 source categories to OH in 10⁻¹⁵ mol mol⁻¹. Zonal means of the year 2008 are shown. First and third columns show the tagging method V1.1. Second and forth columns show the tagging method V1.0. Simulation is performed with EMAC.

Figure 3 Contribution of 10 source categories to HO₂ in 10⁻¹⁴ mol mol⁻¹. Zonal means of the year 2008 are shown. First and third columns show the tagging method V1.1. Second and forth columns show the tagging method V1.0. Simulation is performed with EMAC.

These different approaches are applied to the reduced HO_x reaction system V1.1 (Table 1) to derive the contributions of source categories to OH and HO₂ in Sect. 3.5.

3.4 Steady-state assumption

The steady-state assumption of the HO_x chemistry is the basic principle of the tagging method for short-lived species (Tsati, 2014; Grewe et al., 2017). In steady state, the production and loss of OH and HO₂ balance each other. Table 3 shows the annual means of the HO_x and H production and loss rates of the reduced reaction system for the tagging methods V1.0 and V1.1 as well as the total production and loss rates derived from the complete chemical scheme MECCA in EMAC. The production and loss rates are obtained from an EMAC simulation following the set-up described in Sect. 2. Note that for V1.0 no values for the H production and loss are available since the tagging of H was not considered in V1.0.

In general, total OH production (derived by MECCA) equals total OH loss in the troposphere and stratosphere. The same holds for HO₂ and H. In the troposphere, the OH loss of V1.1 and V1.0 represents the total OH loss in the troposphere well. However, the OH production for V1.1 and V1.0 differs by 12 % from the total OH production. Considering HO₂ in the troposphere, the total production and loss rates are well reflected by V1.1. In contrast, the HO₂ production and loss of V1.0 differs by 14 and 41 % from the total rates.

In the stratosphere, V1.1 represents the total rates very well. However, the OH production of V1.1 misses 10 % of the total OH production. Since V1.0 was only developed for the troposphere, not all reactions which are important in the stratosphere were considered. Thus, the OH and HO₂ production and loss rates of V1.0 considerably underestimated the total production and loss rates.

The reduced H reaction system in V1.1 (Table 2) represents the total H production and loss in the troposphere very well. However, in the stratosphere H loss in V1.1 deviates by 17 % from the total H loss.

Summing up, the reduced HO_x reaction system V1.1 represents the total HO_x production and loss in the troposphere and stratosphere well. V1.1 reproduces the HO_x chemistry better than V1.0. However, OH production in the troposphere and stratosphere as well as H loss in the stratosphere of V1.1 deviate from the total rates derived by MECCA. Thus, the steady state for the reduced HO_x and H reaction system (Tables 1 and 2) is not completely fulfilled.

But steady state between production and loss is crucial for the tagging method for short-lived species. To re-establish steady state, it would be necessary to include the complete HO_x and H chemistry in the tagging method. However, this is not possible as the tagging method of short-lived species does not apply to all reactions of the HO_x and H chemistry (for examples see Appendix Appendix A). Moreover, tagging all chemical species of the HO_x and H chemistry with the implementation of long-lived species would significantly increase the memory demand of a climate simulation (for a detailed discussion see Sect. 6 in Grewe et al., 2017). Consequently, we introduce the rest terms resOH, resHO₂ and resH for OH, HO₂ and H to compensate for the deviations from steady state. Each rest term is calculated by subtracting the production rate of the reduced reaction system from the loss rate (Tables 1 and 2). The resulting rest terms are shown in the Supplement (Fig. S1).

Considering the rest terms resOH, resHO₂ and resH leads to the closure of the budget. In V1.0, the sum of the contributions from all source categories did not balance the total concentration. The averaged deviations for OH and HO₂ in the troposphere were about 70 % of the total concentrations. Since the stratosphere was not considered in V1.0, the deviations were even larger (104 % for OH and 89 % for HO₂). In V1.1, the sum of OH and HO₂ now balances the total OH and HO₂ concentrations. The deviations are negligible (below 10⁻³ %). Consequently, including the rest terms in the tagging method is mandatory for the steady-state assumption and also closes the budget.

3.5 Determination of HO_x contributions

Taking the above considerations into account, we finally derive the OH and HO₂ production and loss terms per source category i. In the reduced HO_x reaction system V1.1 (Table 1), OH is produced by the Reactions (2) H₂O + O(¹D), (4) HO₂ + O₃, (5) HO₂ + O(³P), (14) NO + HO₂, (19) HONO + hv and (20) HNO₃ + hv. Applying the partitioning described in Sect. 3.3, the OH production for a source category i ProdOHⁱ is determined as follows.

$$\begin{array}{ll}{\displaystyle }{\text{ProdOH}}^i& {\displaystyle }=\mathrm{2}\cdot R_{\mathrm{2}}{\displaystyle \frac{{{\mathrm{O}}_{\mathrm{3}}}^i}{{\mathrm{O}}_{\mathrm{3}}}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{4}}\left({\displaystyle \frac{{{\mathrm{HO}}_{\mathrm{2}}}^i}{{\mathrm{HO}}_{\mathrm{2}}}}+{\displaystyle \frac{{{\mathrm{O}}_{\mathrm{3}}}^i}{{\mathrm{O}}_{\mathrm{3}}}}\right)\\ {\displaystyle }& {\displaystyle }+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{5}}\left({\displaystyle \frac{{{\mathrm{HO}}_{\mathrm{2}}}^i}{{\mathrm{HO}}_{\mathrm{2}}}}+{\displaystyle \frac{{{\mathrm{O}}_{\mathrm{3}}}^i}{{\mathrm{O}}_{\mathrm{3}}}}\right)\\ {\displaystyle }& {\displaystyle }+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{14}}\left({\displaystyle \frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}}+{\displaystyle \frac{{{\mathrm{HO}}_{\mathrm{2}}}^i}{{\mathrm{HO}}_{\mathrm{2}}}}\right)\\ \text{(9)}& {\displaystyle }& {\displaystyle }+R_{\mathrm{19}}{\displaystyle \frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}}+R_{\mathrm{20}}{\displaystyle \frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}}\end{array}$$

OH is destroyed by the Reactions (6) OH + O₃, (7) OH + O(³P), (8) HO₂ + OH, (9) H₂O₂ + H, (10) H₂ + OH, (11) CO + OH, (12) CH₄ + OH, (13) ClO + OH, (16) NO + OH, (17) NO₂ + OH, (21) NMHC + OH and (24) NMHC + OH. The OH loss per source category i LossOHⁱ is

$$\begin{array}{ll}{\displaystyle }{\text{LossOH}}^i& {\displaystyle }={\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{6}}\left({\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}+{\displaystyle \frac{{{\mathrm{O}}_{\mathrm{3}}}^i}{{\mathrm{O}}_{\mathrm{3}}}}\right)+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{7}}\left({\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}+{\displaystyle \frac{{{\mathrm{O}}_{\mathrm{3}}}^i}{{\mathrm{O}}_{\mathrm{3}}}}\right)\\ {\displaystyle }& {\displaystyle }+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{8}}\left({\displaystyle \frac{{{\mathrm{HO}}_{\mathrm{2}}}^i}{{\mathrm{HO}}_{\mathrm{2}}}}+{\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}\right)+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{9}}\left({\displaystyle \frac{{{\mathrm{HO}}_{\mathrm{2}}}^i}{{\mathrm{HO}}_{\mathrm{2}}}}+{\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}\right)\\ {\displaystyle }& {\displaystyle }+R_{\mathrm{10}}{\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{11}}\left({\displaystyle \frac{{\mathrm{CO}}^i}{\mathrm{CO}}}+{\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}\right)+R_{\mathrm{12}}{\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}\\ {\displaystyle }& {\displaystyle }+R_{\mathrm{13}}{\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{16}}\left({\displaystyle \frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}}+{\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}\right)\\ {\displaystyle }& {\displaystyle }+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{17}}\left({\displaystyle \frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}}+{\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}\right)\\ {\displaystyle }& {\displaystyle }+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{21}}\left({\displaystyle \frac{{\mathrm{NMHC}}^i}{\mathrm{NMHC}}}+{\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}\right)\\ \text{(10)}& {\displaystyle }& {\displaystyle }+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{24}}\left({\displaystyle \frac{{\mathrm{NMHC}}^i}{\mathrm{NMHC}}}+{\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}\right).\end{array}$$

HO₂ is produced by Reactions (1) H + O₂, (6) OH +, O₃, (9) H₂O₂ + OH, (13) ClO + OH, (18) HNO₄, (23) NMHC + NO_y, (24) NMHC + OH and (25) NMHC + hv. However, H in Reaction (1) is not tagged. To be able to determine the HO₂ production by Reaction (1) $R_{\mathrm{1}}\frac{{\mathrm{H}}^i}{\mathrm{H}}$, we apply the introduced H tagging (see Sect. 3.3) and replace $\frac{{\mathrm{H}}^i}{\mathrm{H}}$ with Eq. (6). In addition, Reaction (13) constitutes a simplified reaction producing 0.94 ⋅ HO₂. Consequently, the HO₂ production per source category i ProdHO2ⁱ is

$$\begin{array}{ll}{\displaystyle }{\text{ProdHO}}_{\mathrm{2}}^i& {\displaystyle }={\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{6}}\left({\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}+{\displaystyle \frac{{{\mathrm{O}}_{\mathrm{3}}}^i}{{\mathrm{O}}_{\mathrm{3}}}}\right)+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{7}}\left({\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}+{\displaystyle \frac{{{\mathrm{O}}_{\mathrm{3}}}^i}{{\mathrm{O}}_{\mathrm{3}}}}\right)\\ {\displaystyle }& {\displaystyle }+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{9}}\left({\displaystyle \frac{{{\mathrm{HO}}_{\mathrm{2}}}^i}{{\mathrm{HO}}_{\mathrm{2}}}}+{\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}\right)+R_{\mathrm{10}}{\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}\\ {\displaystyle }& {\displaystyle }+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{11}}\left({\displaystyle \frac{{\mathrm{CO}}^i}{\mathrm{CO}}}+{\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}\right)+\mathrm{0.94}\cdot R_{\mathrm{13}}{\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}\\ {\displaystyle }& {\displaystyle }+R_{\mathrm{18}}{\displaystyle \frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{23}}\left({\displaystyle \frac{{\mathrm{NMHC}}^i}{\mathrm{NMHC}}}+{\displaystyle \frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}}\right)\\ {\displaystyle }& {\displaystyle }+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{24}}\left({\displaystyle \frac{{\mathrm{NMHC}}^i}{\mathrm{NMHC}}}+{\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}\right)+R_{\mathrm{25}}{\displaystyle \frac{{\mathrm{NMHC}}^i}{\mathrm{NMHC}}}\\ \text{(11)}& {\displaystyle }& {\displaystyle }+R_{\mathrm{28}}{\displaystyle \frac{{\mathrm{NMHC}}^i}{\mathrm{NMHC}}}.\end{array}$$

The HO₂ loss is determined by Reactions (3) HO₂ + HO₂, (4) HO₂ + O₃, (5) HO₂ + O(³P), (8) HO₂ + OH, (14) NO + HO₂, (15) NO₂ + HO₂, (22) NMHC + HO₂, (26) ClO + HO₂ and (27) BrO + HO₂. Hence, the HO₂ loss per source category i LossHO2ⁱ is

$$\begin{array}{ll}{\displaystyle }{\text{LossHO}}_{\mathrm{2}}^i& {\displaystyle }=R_{\mathrm{3}}{\displaystyle \frac{{{\mathrm{HO}}_{\mathrm{2}}}^i}{{\mathrm{HO}}_{\mathrm{2}}}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{4}}\left({\displaystyle \frac{{{\mathrm{HO}}_{\mathrm{2}}}^i}{{\mathrm{HO}}_{\mathrm{2}}}}+{\displaystyle \frac{{{\mathrm{O}}_{\mathrm{3}}}^i}{{\mathrm{O}}_{\mathrm{3}}}}\right)\\ {\displaystyle }& {\displaystyle }+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{5}}\left({\displaystyle \frac{{{\mathrm{HO}}_{\mathrm{2}}}^i}{{\mathrm{HO}}_{\mathrm{2}}}}+{\displaystyle \frac{{{\mathrm{O}}_{\mathrm{3}}}^i}{{\mathrm{O}}_{\mathrm{3}}}}\right)+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{8}}\left({\displaystyle \frac{{{\mathrm{HO}}_{\mathrm{2}}}^i}{{\mathrm{HO}}_{\mathrm{2}}}}+{\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}\right)\\ {\displaystyle }& {\displaystyle }+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{14}}\left({\displaystyle \frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}}+{\displaystyle \frac{{{\mathrm{HO}}_{\mathrm{2}}}^i}{{\mathrm{HO}}_{\mathrm{2}}}}\right)\\ {\displaystyle }& {\displaystyle }+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{15}}\left({\displaystyle \frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}}+{\displaystyle \frac{{{\mathrm{HO}}_{\mathrm{2}}}^i}{{\mathrm{HO}}_{\mathrm{2}}}}\right)\\ {\displaystyle }& {\displaystyle }+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{22}}\left({\displaystyle \frac{{\mathrm{NMHC}}^i}{\mathrm{NMHC}}}+{\displaystyle \frac{{{\mathrm{HO}}_{\mathrm{2}}}^i}{{\mathrm{HO}}_{\mathrm{2}}}}\right)+R_{\mathrm{26}}{\displaystyle \frac{{{\mathrm{HO}}_{\mathrm{2}}}^i}{{\mathrm{HO}}_{\mathrm{2}}}}\\ \text{(12)}& {\displaystyle }& {\displaystyle }+R_{\mathrm{27}}{\displaystyle \frac{{{\mathrm{HO}}_{\mathrm{2}}}^i}{{\mathrm{HO}}_{\mathrm{2}}}}.\end{array}$$

Section 3.4 shows that the steady-state assumption for OH and HO₂ is justified when the rest terms resOH, resHO₂ and resH are regarded. Therefore, the rest terms are divided by the number of source categories n to add them to the contributions of a category i. In steady state, production of OHⁱ and HO₂ⁱ equals the loss.

$$\begin{array}{}\text{(13)}& {\displaystyle }& {\displaystyle }{\text{ProdOH}}^i-{\text{LossOH}}^i+\text{resOH}/n=\mathrm{0}\text{(14)}& {\displaystyle }& {\displaystyle }{\text{ProdHO}}_{\mathrm{2}}^i-{\text{LossHO}}_{\mathrm{2}}^i+{\text{resHO}}_{\mathrm{2}}/n+\text{resH/}n=\mathrm{0}\end{array}$$

Equations (6) and (6) are rewritten as follows:

$$\begin{array}{}\text{(15)}& {\displaystyle }& {\displaystyle }\mathrm{0}=A^i-L^{\mathrm{OH}}{\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}+P^{\mathrm{OH}}{\displaystyle \frac{{\mathrm{HO}}_{\mathrm{2}}^i}{{\mathrm{HO}}_{\mathrm{2}}}}+{\displaystyle \frac{\mathrm{resOH}}{n}},\text{(16)}& {\displaystyle }& {\displaystyle }\mathrm{0}=B^i+P^{{\mathrm{HO}}_{\mathrm{2}}}{\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}-L^{{\mathrm{HO}}_{\mathrm{2}}}{\displaystyle \frac{{{\mathrm{HO}}_{\mathrm{2}}}^i}{{\mathrm{HO}}_{\mathrm{2}}}}+{\displaystyle \frac{{\text{resHO}}_{\mathrm{2}}}{n}}+{\displaystyle \frac{\text{resH}}{n}},\end{array}$$

with the variables P^OH, L^OH, $P^{{\mathrm{HO}}_{\mathrm{2}}}$, $L^{{\mathrm{HO}}_{\mathrm{2}}}$, Aⁱ and Bⁱ as follows (compare to Grewe et al., 2017 Eqs. 25 to 28).

$$\begin{array}{}\text{(17)}& {\displaystyle }{\displaystyle }{P}^{\mathrm{OH}}={\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{4}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{5}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{14}}-{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{8}}\end{array}$$

$$\begin{array}{ll}{\displaystyle }{L}^{\mathrm{OH}}& {\displaystyle }={\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{6}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{7}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{8}}+R_{\mathrm{9}}+R_{\mathrm{10}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{11}}\\ \text{(18)}& {\displaystyle }& {\displaystyle }+R_{\mathrm{12}}+R_{\mathrm{13}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{16}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{17}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{21}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{24}}\end{array}$$

$$\begin{array}{ll}{\displaystyle }{P}^{{\mathrm{HO}}_{\mathrm{2}}}& {\displaystyle }={\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{6}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{7}}+R_{\mathrm{9}}+R_{\mathrm{10}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{11}}\\ \text{(19)}& {\displaystyle }& {\displaystyle }+\mathrm{0.94}\cdot R_{\mathrm{13}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{24}}-{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{8}}\end{array}$$

$$\begin{array}{ll}{\displaystyle }{L}^{{\mathrm{HO}}_{\mathrm{2}}}& {\displaystyle }=\mathrm{2}\cdot R_{\mathrm{3}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{4}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{5}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{8}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{14}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{15}}\\ \text{(20)}& {\displaystyle }& {\displaystyle }+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{22}}+R_{\mathrm{26}}+R_{\mathrm{27}}\end{array}$$

$$\begin{array}{}\text{(21)}& \begin{array}{rl}{A}^i& =\mathrm{2}\cdot R_{\mathrm{2}}{\displaystyle \frac{{{\mathrm{O}}_{\mathrm{3}}}^i}{{\mathrm{O}}_{\mathrm{3}}}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{4}}{\displaystyle \frac{{{\mathrm{O}}_{\mathrm{3}}}^i}{{\mathrm{O}}_{\mathrm{3}}}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{5}}{\displaystyle \frac{{{\mathrm{O}}_{\mathrm{3}}}^i}{{\mathrm{O}}_{\mathrm{3}}}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{14}}{\displaystyle \frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}}\\ & +R_{\mathrm{19}}{\displaystyle \frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}}+R_{\mathrm{20}}{\displaystyle \frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}}-{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{6}}{\displaystyle \frac{{{\mathrm{O}}_{\mathrm{3}}}^i}{{\mathrm{O}}_{\mathrm{3}}}}\\ & -{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{7}}{\displaystyle \frac{{{\mathrm{O}}_{\mathrm{3}}}^i}{{\mathrm{O}}_{\mathrm{3}}}}-{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{11}}{\displaystyle \frac{{\mathrm{CO}}^i}{\mathrm{CO}}}-{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{16}}{\displaystyle \frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}}\\ & -{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{17}}{\displaystyle \frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}}-{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{21}}{\displaystyle \frac{{\mathrm{NMHC}}^i}{\mathrm{NMHC}}}-{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{24}}{\displaystyle \frac{{\mathrm{NMHC}}^i}{\mathrm{NMHC}}}\end{array}\end{array}$$

$$\begin{array}{}\text{(22)}& \begin{array}{rl}{B}^i& ={\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{6}}{\displaystyle \frac{{{\mathrm{O}}_{\mathrm{3}}}^i}{{\mathrm{O}}_{\mathrm{3}}}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{7}}{\displaystyle \frac{{{\mathrm{O}}_{\mathrm{3}}}^i}{{\mathrm{O}}_{\mathrm{3}}}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{11}}{\displaystyle \frac{{\mathrm{CO}}^i}{\mathrm{CO}}}+R_{\mathrm{18}}{\displaystyle \frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}}\\ & +{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{23}}\left({\displaystyle \frac{{\mathrm{NMHC}}^i}{\mathrm{NMHC}}}+{\displaystyle \frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}}\right)+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{24}}{\displaystyle \frac{{\mathrm{NMHC}}^i}{\mathrm{NMHC}}}\\ & +R_{\mathrm{25}}{\displaystyle \frac{{\mathrm{NMHC}}^i}{\mathrm{NMHC}}}+R_{\mathrm{28}}{\displaystyle \frac{{\mathrm{NMHC}}^i}{\mathrm{NMHC}}}-{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{4}}{\displaystyle \frac{{{\mathrm{O}}_{\mathrm{3}}}^i}{{\mathrm{O}}_{\mathrm{3}}}}-{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{5}}{\displaystyle \frac{{{\mathrm{O}}_{\mathrm{3}}}^i}{{\mathrm{O}}_{\mathrm{3}}}}\\ & -{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{14}}{\displaystyle \frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}}-{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{15}}{\displaystyle \frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}}-{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{R}_{\mathrm{22}}{\displaystyle \frac{{\mathrm{NMHC}}^i}{\mathrm{NMHC}}}\end{array}\end{array}$$

Figure 4 Contribution of shipping emissions to OH and HO₂ in 10⁻¹⁵ mol mol⁻¹. Monthly means of ground-level values in August 2007 are shown. Simulation is performed with MECO(n).

By solving Eqs. (6) and (6), we finally obtain the contributions of a source category i to the OH and HO₂ concentration (same equations as Eqs. 29 and 30 in Grewe et al., 2017, but with differently defined coefficients).

$$\begin{array}{}\text{(23)}& {\displaystyle \frac{{\mathrm{OH}}^i}{\mathrm{OH}}}={\displaystyle \frac{A^i{L}^{{\mathrm{HO}}_{\mathrm{2}}}+B^i{P}^{\mathrm{OH}}}{L^{\mathrm{OH}}{L}^{{\mathrm{HO}}_{\mathrm{2}}}-P^{\mathrm{OH}}{P}^{{\mathrm{HO}}_{\mathrm{2}}}}}\text{(24)}& {\displaystyle \frac{{{\mathrm{HO}}_{\mathrm{2}}}^i}{{\mathrm{HO}}_{\mathrm{2}}}}={\displaystyle \frac{A^i{P}^{{\mathrm{HO}}_{\mathrm{2}}}+B^i{L}^{\mathrm{OH}}}{L^{\mathrm{OH}}{L}^{{\mathrm{HO}}_{\mathrm{2}}}-P^{\mathrm{OH}}{P}^{{\mathrm{HO}}_{\mathrm{2}}}}}\end{array}$$

These equations are implemented in the TAGGING submodel, and EMAC and MECO(n) simulations according to Sect. 2 are performed. The results for the OH and HO₂ contributions are analysed and compared with V1.0 in the following section.

Figure 5 Annual mean contributions of 10 source categories to O₃ concentration in percent.

4 Results of model simulations

4.1 Contribution of short-lived species (HO_x)

Figures 2 and 3 show the zonal mean of OH and HO₂ contributions up to 200 hPa for the 10 source categories derived by V1.1 (first and third columns) and V1.0 (second and forth columns). The zonal mean of OH and HO₂ contributions from 1 to 200 hPa are shown in Appendix Appendix B (Figs. B1, B2). First, the OH and HO₂ contributions of V1.1 are described in the following. For the categories which are determined by anthropogenic emissions, such as shipping, road traffic and anthropogenic non-traffic, the maximum values of OH and HO₂ contributions occur in the lower troposphere in the Northern Hemisphere. This clearly shows that for anthropogenic-dominated categories the OH and HO₂ contributions are caused by anthropogenic emissions. The contributions vary among these categories of surface emissions as not only the amount but also the composition of the emissions differs. For the category aviation, maximum OH contributions are found in the Northern Hemisphere between 200 and 250 hPa. However, the HO₂ contribution has a minimum in this region and a maximum in the lower troposphere. The OH values for the categories CH₄ decomposition, N₂O decomposition, lightning and biogenic emissions are largest in the upper troposphere. Most OH contributions of biomass burning are found in the lower tropical troposphere. In contrast, negative values occur in the upper tropical troposphere. Concerning the HO₂ contribution, the residual categories show a maximum in the tropical lower troposphere. In addition, the category lightning shows a strong HO₂ loss in the upper tropical troposphere, which is caused by Reaction (14).

The results obtained by V1.1 are compared to the OH and HO₂ zonal profiles of V1.0 only in the troposphere (Figs. 2 and 3). The HO_x tagging method V1.0 was only developed for the troposphere. Hence, a comparison in the stratosphere is not reasonable. In general, contributions to OH and HO₂ concentrations of V1.1 are larger in the troposphere compared to V1.0. This overall shift towards larger values is explained by the re-establishment of the steady state and thus the closure of the budget in V1.1. In V1.0 the budget was not closed and thus the contributions were underestimated.

For OH, the categories lightning and aviation show no large changes in the general pattern of the zonal means between V1.0 and V1.1. Considering the HO₂ contributions, no large changes are found for the categories biomass burning, anthropogenic non-traffic, road traffic and shipping.

The contribution of the category aviation to HO₂ in V1.1 shows roughly the same pattern compared to V1.0. However, the HO₂ destruction along the flight path is no longer as pronounced, which is caused by the inclusion of Reactions (15) and (18) in V1.1. Reaction (15) adds the term $\frac{\mathrm{1}}{\mathrm{2}}{R}_{\mathrm{15}}\frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}$ to the HO₂ loss (Eq. 6) and Reaction (18) adds the term $R_{\mathrm{18}}\frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}$ to the HO₂ production (Eq. 6). As the reaction rate R₁₅ equals the rate R₁₈, this leads to a larger HO₂ production than HO₂ loss $\left(R_{\mathrm{18}}\frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}>\frac{\mathrm{1}}{\mathrm{2}}{R}_{\mathrm{15}}\frac{{\mathrm{NO}}_y^i}{{\mathrm{NO}}_y}\right)$. Consequently, the addition of Reactions (15) and (18) to the reduced HO_x reaction system V1.1 constitutes an extra HO₂ source.

Larger values of the categories N₂O decomposition and lightning to HO₂ in the upper troposphere are explained by a larger HO₂ production in V1.1 compared to V1.0. The H tagging in V1.1 considers all relevant HO₂ sources (Reactions 7, 10, 11 and 28) leading to a larger HO₂ production. Also the addition of Reactions (15) and (18) (for an explanation see above) as well as the addition of Reaction (23), which considers more reactions than in V1.0, increases the HO₂ contribution of the categories N₂O decomposition and lightning.

Large changes in pattern are observed for the contributions of biogenic emissions and CH₄ decomposition to OH and HO₂ as well as for the contributions of biomass burning and anthropogenic non-traffic to OH. In V1.1, these categories mainly constitute a source of OH and HO₂ in the troposphere. The addition of Reactions (24) and (25) to the reduced HO_x reaction system V1.1 presents an HO₂ source increasing OH and HO₂ contributions. Furthermore, reactions of NMHC with OH, HO₂ and NO_y (Reactions 21, 22 and 23) are important throughout the whole troposphere. In contrast to V1.0, V1.1 considers all reactions of NMHC with OH, HO₂ and NO_y (see Sect. 3.2), significantly changing the pattern of biogenic emissions, CH₄ decomposition, biomass burning and anthropogenic non-traffic.

To demonstrate the impact of the advanced HO_x tagging method on a regional scale, Fig. 4 shows the contributions of ship emissions to OH and HO₂ in the boundary layer simulated with the high-resolution model MECO(n) (see Sect. 2). The ship paths in the Atlantic, Mediterranean and Red Sea are clearly visible and lead to OH and HO₂ production along these paths. In the polluted area at the coast of Marseille the OH and HO₂ contributions are reduced. In this region NO_y from shipping emissions is larger than in the Mediterranean Sea, causing a reduction of OH and HO₂ by Reactions (14) to (17).

The tagging method V1.0 (Grewe et al., 2017, their Fig. 6) showed negative HO₂ shipping contributions along the ship paths. This was explained by Reaction (14): NO destroys HO₂ and leads to negative contributions. However, in V1.1 HO₂ shipping contributions are positive. The change in sign is caused by the addition of Reactions (15) and (18) to the reduced HO_x reaction system V1.1, which constitutes a net HO₂ production, leading to positive HO₂ contributions (for an explanation see above). The comparison shows that HO₂ contributions in V1.0 were systematically and erroneously underestimated.

To summarize, the contributions to OH and HO₂ concentrations show larger values in V1.1 compared to V1.0. This is explained by the re-establishment of the steady state. For OH, no large changes are found in the categories lightning and aviation. However, large changes are found for biomass burning, CH₄ decomposition and biogenic emissions. For HO₂, no large differences occur in the categories biomass burning, anthropogenic non-traffic, road traffic and shipping. In comparison, the categories biogenic emissions and CH₄ decomposition differ strongly. The differences between the contributions of V1.1 and V1.0 are traced back to the addition of certain reactions to the reduced reaction system considered in the HO_x tagging method.

4.2 Effects on long-lived species

The tagging of short-lived and long-lived species closely intertwines (see Fig. 1). Changes in the contributions to OH and HO₂ influence the contributions to the long-lived tracers O₃, NO_y, CO, NMHC and PAN. For example, Fig. 5 shows the zonal mean of the contributions of the 10 source categories to O₃. Grewe et al. (2017) present the same figure for the HO_x tagging method V1.0 (their Fig. 4). For consistency, we compare our results with the results of Grewe et al. (2017) only for the year 2008.

In general, no large differences between V1.1 and V1.0 for long-lived species are found. The categories biogenic emissions and CH₄ decomposition show an O₃ increase in the tropical troposphere. Stratospheric O₃ production slightly increases in the Southern Hemisphere. Small O₃ changes are found for the categories lightning and N₂O decomposition. Regarding the remaining long-lived species (see Figs. S3–S6), the contribution of biomass burning to CO decreases, while the contributions of biogenic emissions to CO increase in the Southern Hemisphere. The remaining sectors stay rather unchanged. NO_y, NMHC and PAN show only minor changes. Even though major differences in OH and HO₂ occur between V1.0 and V1.1, these do not have a large effect on the long-lived species.

5 Discussion and conclusion

We present an extension of the HO_x tagging method described by Grewe et al. (2017). A total of 15 new reactions producing and destroying HO_x are added to the tagging mechanism. In Grewe et al. (2017), the HO_x tagging method V1.0 was restricted to the troposphere only. We further include the reactions which are essential for HO_x production and loss in the stratosphere. Moreover, we introduce an equivalent tagging method to obtain the contributions to the H radical. This step is mandatory to fully account for the main HO₂ source: the reaction of H with O₂.

In V1.0, the steady-state assumption was not completely fulfilled, resulting in an unclosed budget: the sum of the HO_x contributions and the total HO_x concentration deviated by about 70 %. To re-establish steady state, we add more reactions to the reduced HO_x reaction system and introduce rest terms to balance the deviation of HO_x production and loss. This leads to the closure of the budget. Thus, the tagging mechanism introduced by Grewe et al. (2010) operates not only for long-lived but also for short-lived species.

The advanced HO_x tagging method V1.1 was implemented in the global chemistry climate model EMAC and in the regional model MECO(n). A 1-year simulation was performed in both model systems and compared to V1.0. For most categories, the general zonal pattern of the contributions to OH and HO₂ show minor differences. In contrast, large changes are observed in the category biogenic emissions and CH₄ decomposition, which are traced back to the addition of certain reactions to V1.1. Although the contributions of long-lived and short-lived species influence each other, no large changes are found for long-lived species.

The mechanism presented in this study (and introduced by Tsati, 2014, and Grewe et al., 2017) is the first method for tagging short-lived species. Other studies quantify the source attributions of chemical species with a significantly longer lifetime. The idea of source attribution is applied to attribute CO to different emission types and regions (e.g. Granier et al., 1999; Pfister et al., 2004, 2011), to attribute NO_x concentrations to emission sources (Horowitz and Jacob, 1999) or to trace stable isotopic compositions (Gromov et al., 2010). Also for the source attribution of tropospheric O₃, there are several tagging approaches attributing tropospheric O₃ only to NO_x sources (Lelieveld and Dentener, 2000; Grewe, 2004; Grewe et al., 2012; Emmons et al., 2012), only to NMHC sources (Butler et al., 2011; Coates and Butler, 2015) or to NO_y, CO and NMHC emissions simultaneously (Grewe et al., 2017).

A common technique to quantify the impact of emissions on OH is the so-called perturbation method, which compares two simulations: one simulation with all emissions and one simulation with reduced emissions (e.g. Niemeier et al., 2006; Hoor et al., 2009). However, if the underlying chemical processes are non-linear (as is the case for OH), the perturbation method largely underestimates the contribution (Grewe et al., 2012; Emmons et al., 2012; Mertens et al., 2018). Consequently, the tagging approach presented in this study delivers the actual contribution of the emission source, while the perturbation method displays the impact of the emission reduction.

To conclude, the further developed HO_x tagging method can be used to identify the contribution of anthropogenic emissions to the atmospheric composition. In particular, the contribution of emission sectors to the concentrations of OH and HO₂ in the troposphere and stratosphere can be measured. This method will be applied for re-evaluating the impact of the traffic sector on climate.

Appendix A: Exclusion of reactions from reduced HO_x reaction system V1.1

The annual mean reaction rates of the following three reactions are also greater than 10⁻¹⁵ $\mathrm{mol}{\mathrm{mol}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$ and thus would usually be regarded in the reduced HO_x reaction system V1.1.

$$\begin{array}{}\text{(A1)}& {\displaystyle }{\mathrm{H}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}+hv⟶\mathrm{2}\mathrm{OH}\text{(A2)}& {\displaystyle }\mathrm{HOCl}+hv⟶\mathrm{OH}+\mathrm{Cl}\text{(A3)}& {\displaystyle }\mathrm{HOBr}+hv⟶\mathrm{OH}+\mathrm{Br}\end{array}$$

However, the tagging method cannot be applied for these three reactions.

To include the OH production by the photolysis of H₂O₂ (Reaction A1), we would need to tag H₂O₂. Since the production and the loss of H₂O₂ are not balanced, we cannot assume a steady state. Thus, a similar tagging approach as for HO_x and H is not valid for H₂O₂. Consequently, we exclude the Reaction (A1) from the HO_x tagging method. This reaction contributes about 8 % to the total OH production in the troposphere.

Hypochlorous acid (HOCl) and hypobromous acid (HOBr) are photolysed in the stratosphere and produce OH (Reactions A1 and A1), but HOCl and HOBr are not tagged. Although the steady-state assumption is globally valid, locally the production and loss of HOCl and HOBr are not balanced everywhere. In the stratosphere, for about 65 % of the model grid boxes the production deviates by more than 10 % from the loss of HOCl and HOBr. In particular, in the transition area between day and night in the polar region, the production deviates strongly from the loss. Also at night when the reactions mostly occur, the steady state is not fulfilled everywhere. Moreover, since both species are not radicals, their lifetimes cannot be assumed to be short. Hence, we cannot apply the tagging method, so we have to omit the Reactions (A1) and (A1) from the reduced HOx reaction system V1.1.

Considering Reactions (A1), (A1) and (A1) in the reduced HO_x reaction system V1.1 would lead to a significantly larger OH production in the troposphere representing about 98 % of the total OH production rate derived by MECCA. In the stratosphere, 91 % of the total OH production would be regarded. Hence, excluding these reactions from the reduced HO_x reaction system V1.1 worsens the steady-state assumption between OH production and loss. The rest term resOH introduced in Sect. 3.4 compensates for this deviation from the production and loss rate.

Appendix B: HO_x contributions in the stratosphere

Figures B1 and B2 show the zonal mean of OH and HO₂ from 1 to 200 hPa. As OH concentration strongly rises with increasing height, so do the contributions to OH. The category biomass burning shows negative OH values in the tropopause region. In this region, large CO values from biomass burning also occur. CO effectively destroys OH by Reaction (11), which causes this OH loss. The large OH loss in the lower stratosphere of the category stratospheric O₃ production is mainly caused by the destruction of OH by O₃ (Reaction 6).

The contributions to HO₂ in the stratosphere increases with height as well. The categories biogenic emissions, lightning, biomass burning, anthropogenic non-traffic, road traffic, shipping and aviation show a local maximum at around 5 hPa, which is caused by omitting the photolysis of HOCl (see Appendix Appendix A).

For the category lightning, HO₂ is destroyed by Reaction (14) in the tropopause region. The category N₂O decomposition shows negative values in the lower stratosphere and a strong negative minimum at around 10 hPa, which is also caused by Reaction (14). The local maximum with positive HO₂ contributions indicates that in this region the HO₂ production via Reactions (1) and (6) dominates the HO₂ loss via Reaction (14).

Figure B1 Contributions of 10 source categories to OH in the stratosphere. Zonal means of the year 2010 are shown. Black line indicates the tropopause. Simulation is performed with EMAC. Note the logarithmic scale of the contour levels.

Figure B2 Contributions of 10 source categories to HO₂ in the stratosphere. Zonal means of the year 2010 are shown. Black line indicates the tropopause. Simulation is performed with EMAC. Note the logarithmic scale of the contour levels.