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
(HOx) which attributes HOx 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 HOx 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 HOx 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 HOx reacts with ozone (O3), carbon
monoxide (CO), reactive nitrogen compounds (NOy), non-methane
hydrocarbons (NMHCs) and peroxyacyl nitrates (PAN), the contributions to these
species are also modified by the advanced HOx tagging method V1.1.
The contributions to NOy, 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.
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 HOx. The ratio of OH to HO2 in an air parcel
strongly depends on the chemical background, in particular on the composition
of nitrogen oxides NOx (= NO +NO2) and non-methane hydrocarbons
(NMHC) .
HOx 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. ). Hence, HOx 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. ). 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
HOx concentrations are measured on a local scale, which is generally
difficult to compare with global models e.g..
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 . On
regional and global scales, no direct HOx 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) . As emissions of
CH3CCl3 steadily decline, 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,
which compares well with the observation-based results from
and .
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 . To
do so, we use a “tagging” method: the theoretical framework of this tagging
method is given in and , and the
implementation is described in . 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
(NOy, 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 NOy concentration, while emissions of e.g. C2H4,
C3H6 and HCHO contribute to the NMHC concentration. This tagging method considers the
competition of NOy, CO and NMHC in producing and destroying O3.
The tagging method of the long-lived species O3, CO, PAN,
NOy
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,
NOy 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
.
However, the short-lived species HOx 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.
and introduced a modified approach for
tagging HOx: 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 HOx chemistry, the
contributions of each source category to OH and HO2 are determined.
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, NOy and NMHC.
The contributions to long-lived and short-lived species are closely linked
(see Fig. ). For example, the reaction
OH+O3⟶HO2+O2
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
HOx, presented by , is referred to as the HOx tagging method V1.0. It did not consider all relevant reactions for the
production and loss of HOx. In particular, the reactions which are
important in the stratosphere were not taken into account. Moreover, the
steady-state assumption between HOx production and loss was not
fulfilled. In this study, we present a revised version V1.1 of the
HOx tagging method, largely improving these shortcomings. It
includes the main chemical reactions of HOx 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 HOx tagging method V1.1 in
Sect. 3. In Sect. 4, the results are compared with the tagging method
V1.0 by . Finally, Sect. 5 concludes the methods and the
results of this study.
Model description of EMAC and MECO(n)
To evaluate the further developed HOx tagging method we use the
same model set-up as . 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
. 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; ). 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
and . 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 NOx are scaled to approximately
4 Tg(N) a-1 (parameterized according to ). The submodel
ONEMIS calculates NOx emissions from soil
(parameterized according to ) and biogenic C5H8
emissions (parameterized according to ). Direct CH4
emissions are not considered, and instead pseudo-emissions are calculated using
the submodel TNUDGE . This submodel relaxes the mixing
ratios in the lowest model layer towards observations by Newtonian relaxation
(more details are given by ).
To show the effect of the HOx 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 . To test the
HOx 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 . A detailed chemical
evaluation of the set-up is given in .
Both model simulations are based on the quasi chemistry-transport model
(QCTM) mode in which the chemistry is decoupled from the dynamics
. The anthropogenic emissions are taken from the MACCity
emission inventory . The TAGGING submodel (as described by
) 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, NOy, 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.
Tagging method of short-lived speciesTagging method V1.0
The tagging method V1.0 described by determines the
contribution of source categories to O3, NOy, 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 (O3ant), road
traffic emissions (O3tra), ship emissions (O3shp), air traffic
emissions (O3air), biogenic emissions (O3bio), biomass burning
(O3bb), lightning (O3lig), methane decomposition (O3CH4),
nitrous oxide decomposition (O3N2O) and stratospheric ozone production
(O3str). 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
and , respectively.
The reduced HOx reaction system V1.1 describes the main reactions of
HOx 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 HOx chemistry (reactions marked with “o” in last
column of Table ). It included the main production and loss
reactions of HOx with O3, NOy, 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 ) was not regarded. It
was combined with Reaction (11), CO+ OH ⟶H+CO2 (see
Table ), to
CO+OH⟶CO2+H⟶O2CO2+HO2⟹CO+OH⟶O2CO2+HO2.
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 ). 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. .)
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 .
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 ) 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 NOy 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
HOx production and loss was assumed. However, the steady-state
assumption was not completely fulfilled for V1.0 (see
Sect. ). 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 %.
Reduced HOx reaction system V1.1
OH and HO2 react with many chemical species. To reduce the calculation
time of a simulation, we reduce the HOx 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 10-15molmol-1s-1
(see Table ). 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 reducedHOxreaction system V1.1.
The reactions which are important in the troposphere are indicated in
Table . 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. ). 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 NOy and
NMHC. All reaction rates are directly derived by the MECCA mechanism of EMAC.
Table does not consider all reactions with annual reaction
rates larger than 10-15molmol-1s-1. The photolysis of
hydrogen peroxide (H2O2), hypochlorous acid (HOCl) and hypobromous acid
(HOBr) is excluded from the reduced HOx reaction system V1.1 as
the tagging method cannot be applied. The specific reasons are explained in
Appendix .
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
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=∑i=1nAi, B=∑i=1nBi and C=∑i=1nCi. These contributions (Ai,Bi,Ci) 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:
Ai+Bj⟶12Ci+12Cj.
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 Bj:
LossAi=LossBi=ProdCi=kAiBi+∑j=1,j≠in12AiBj+∑j=1,j≠in12AjBi2=12RAiA+BiB,
with k being the reaction rate coefficient and R=kAB being
the respective reaction rate. For unimolecular reactions A⟶B+C,
the distribution of categories from the educts is completely passed
to the products:
LossAi=ProdBi=ProdCi=RAiA,
with the reaction rate R=kA.
As described above, the long-lived species O3, CO, NOy and NMHC
are tagged according to the tagging method described in . 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
AiA.
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. ). Examples are Reactions (9), (10) and (13).
Using the family concept as described in allows for the
assumption that all tags are distributed equally among the species within the
same chemical family.
NOiNO=NO2iNO2=HNO4iHNO4=NOyiNOy
As mentioned in , all species which are frequently
converted back and forth to ozone are considered as an “ozone storage”
. 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 , we apply the family concept
and set
O(1D)iO(1D)=O(3P)iO(3P)=O3iO3.
In Reaction (1), neither H nor O2 is tagged. To obtain the ratio
HO2iHO2, 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. ).
Table presents the main reactions for H, which still constitute a
subset of full H chemistry implemented in MECCA.
Based on Table , we set up the H production ProdHi and H
loss LossHi for the contribution of a source category i.
ProdHi=12R7OHiOH+O3iO3+R10OHiOH6+12R11COiCO+OHiOH+R28NMHCiNMHCLossHi=R1HiH
As mentioned above, the family concept also sets
HCHOiHCHO=NMHCiNMHC. Since the steady-state
assumption applies for H (see Sect. ), the H
production per source category i ProdHi equals the loss LossHi. After
setting Eqs. () and () equal to each other, we
obtain
HiH=12R7R1OHiOH+O3iO3+R10R1OHiOH8+12R11R1COiCO+OHiOH+R28R1NMHCiNMHC.
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.
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 HOx reaction
system V1.1 (Table ) to derive the contributions of source
categories to OH and HO2 in Sect. .
Steady-state assumption
The steady-state assumption of the HOx chemistry is the basic
principle of the tagging method for short-lived species . In steady state, the production and loss of OH and HO2 balance
each other. Table shows the annual means of the HOx 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. . 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 ) 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 HOx reaction system V1.1 represents
the total HOx production and loss in the troposphere and
stratosphere well. V1.1 reproduces the HOx 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 HOx and H reaction system
(Tables and ) 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 HOx 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 HOx and
H chemistry (for examples see Appendix ). Moreover,
tagging all chemical species of the HOx 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
). 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 and ). 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.
Determination of HOx 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
HOx reaction system V1.1 (Table ), 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. , the OH production for a source category
i ProdOHi is determined as follows.
ProdOHi=2⋅R2O3iO3+12R4HO2iHO2+O3iO3+12R5HO2iHO2+O3iO3+12R14NOyiNOy+HO2iHO29+R19NOyiNOy+R20NOyiNOy
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
LossOHi=12R6OHiOH+O3iO3+12R7OHiOH+O3iO3+12R8HO2iHO2+OHiOH+12R9HO2iHO2+OHiOH+R10OHiOH+12R11COiCO+OHiOH+R12OHiOH+R13OHiOH+12R16NOyiNOy+OHiOH+12R17NOyiNOy+OHiOH+12R21NMHCiNMHC+OHiOH10+12R24NMHCiNMHC+OHiOH.
HO2 is produced by Reactions (1) H +O2, (6) OH +, O3, (9)
H2O2+ OH, (13) ClO + OH, (18) HNO4, (23) NMHC +NOy, (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) R1HiH, we apply the introduced H tagging (see
Sect. ) and replace HiH with
Eq. (). In addition, Reaction (13) constitutes a simplified
reaction producing 0.94 ⋅HO2. Consequently, the HO2 production
per source category i ProdHO2i is
ProdHO2i=12R6OHiOH+O3iO3+12R7OHiOH+O3iO3+12R9HO2iHO2+OHiOH+R10OHiOH+12R11COiCO+OHiOH+0.94⋅R13OHiOH+R18NOyiNOy+12R23NMHCiNMHC+NOyiNOy+12R24NMHCiNMHC+OHiOH+R25NMHCiNMHC11+R28NMHCiNMHC.
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
LossHO2i=R3HO2iHO2+12R4HO2iHO2+O3iO3+12R5HO2iHO2+O3iO3+12R8HO2iHO2+OHiOH+12R14NOyiNOy+HO2iHO2+12R15NOyiNOy+HO2iHO2+12R22NMHCiNMHC+HO2iHO2+R26HO2iHO212+R27HO2iHO2.
Section 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.
13ProdOHi-LossOHi+resOH/n=014ProdHO2i-LossHO2i+resHO2/n+resH/n=0
Equations () and
() are rewritten as follows:
150=Ai-LOHOHiOH+POHHO2iHO2+resOHn,160=Bi+PHO2OHiOH-LHO2HO2iHO2+resHO2n+resHn,
with the variables POH, LOH, PHO2,
LHO2,
Ai and Bi as follows (compare to Eqs. 25 to
28).
POH=12R4+12R5+12R14-12R8LOH=12R6+12R7+12R8+R9+R10+12R1118+R12+R13+12R16+12R17+12R21+12R24PHO2=12R6+12R7+R9+R10+12R1119+0.94⋅R13+12R24-12R8LHO2=2⋅R3+12R4+12R5+12R8+12R14+12R1520+12R22+R26+R27Ai=2⋅R2O3iO3+12R4O3iO3+12R5O3iO3+12R14NOyiNOy+R19NOyiNOy+R20NOyiNOy-12R6O3iO3-12R7O3iO3-12R11COiCO-12R16NOyiNOy-12R17NOyiNOy-12R21NMHCiNMHC-12R24NMHCiNMHCBi=12R6O3iO3+12R7O3iO3+12R11COiCO+R18NOyiNOy+12R23NMHCiNMHC+NOyiNOy+12R24NMHCiNMHC+R25NMHCiNMHC+R28NMHCiNMHC-12R4O3iO3-12R5O3iO3-12R14NOyiNOy-12R15NOyiNOy-12R22NMHCiNMHC
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. () and (),
we finally obtain the contributions of a source category i to the OH and
HO2 concentration (same equations as Eqs. 29 and 30 in
, but with differently defined coefficients).
23OHiOH=AiLHO2+BiPOHLOHLHO2-POHPHO224HO2iHO2=AiPHO2+BiLOHLOHLHO2-POHPHO2
These equations are implemented in the TAGGING submodel, and EMAC
and MECO(n) simulations according to Sect. are
performed. The results for the OH and HO2 contributions are analysed and
compared with V1.0 in the following section.
Annual mean contributions of 10 source categories to O3 concentration in percent.
Results of model simulationsContribution of short-lived species (HOx)
Figures and 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
(Figs. , ).
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. and
). The HOx 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 12R15NOyiNOy to the HO2 loss (Eq. ) and
Reaction (18) adds the term R18NOyiNOy to the HO2
production (Eq. ). As the reaction rate R15 equals the rate
R18, this leads to a larger HO2 production than HO2 loss R18NOyiNOy>12R15NOyiNOy.
Consequently, the addition of Reactions (15) and (18) to the reduced
HOx 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
HOx reaction system V1.1 presents an HO2 source increasing OH and
HO2 contributions. Furthermore, reactions of NMHC with OH, HO2 and
NOy (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 NOy (see
Sect. ),
significantly changing the pattern of biogenic emissions, CH4
decomposition, biomass burning and anthropogenic non-traffic.
To demonstrate the impact of the advanced HOx tagging method on
a regional scale, Fig. shows the contributions of ship
emissions to OH and HO2 in the boundary layer simulated with the high-resolution model MECO(n) (see Sect. ). 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 NOy 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 (, 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
HOx 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 HOx tagging method.
Effects on long-lived species
The tagging of short-lived and long-lived species closely intertwines (see
Fig. ). Changes in the contributions to OH and HO2
influence the contributions to the long-lived tracers O3, NOy,
CO, NMHC and PAN. For example, Fig. shows the zonal mean
of the contributions of the 10 source categories to O3.
present the same figure for the HOx tagging method V1.0 (their
Fig. 4). For consistency, we compare our results with the results of
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. NOy, 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.
Discussion and conclusion
We present an extension of the HOx tagging method described by
. A total of 15 new reactions producing and destroying HOx are
added to the tagging mechanism. In , the HOx tagging
method V1.0 was restricted to the troposphere only. We further include the
reactions which are essential for HOx 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 HOx contributions and the
total HOx concentration deviated by about 70 %. To re-establish
steady state, we add more reactions to the reduced HOx reaction
system and introduce rest terms to balance the deviation of HOx production and loss. This leads to the closure of the budget. Thus, the
tagging mechanism introduced by operates not only for
long-lived but also for short-lived species.
The advanced HOx 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 , and
) 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., to attribute
NOx concentrations to emission sources or to
trace stable isotopic compositions . Also for the source
attribution of tropospheric O3, there are several tagging approaches
attributing tropospheric O3 only to NOx sources
, only to NMHC sources
or to NOy, CO and NMHC emissions
simultaneously .
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.. However, if the underlying chemical
processes are non-linear (as is the case for OH), the perturbation method
largely underestimates the contribution . 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 HOx 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.
Code availability
The Modular Earth Submodel System (MESSy) is continuously
further developed and applied by a consortium of institutions. The usage of
MESSy and access to the source code is licensed to all affiliates of
institutions which are members of the MESSy Consortium. Institutions can
become a member of the MESSy Consortium by signing the MESSy Memorandum of
Understanding. More information can be found on the MESSy Consortium website
(http://www.messy-interface.org, last access: 22 May 2018).
The submodel TAGGING 1.1 will be included
in MESSy version 2.54. The code being used to obtain the presented results is
available upon personal request.
Exclusion of reactions from reduced HOx reaction system V1.1
The annual mean reaction rates of the following three reactions are also
greater than 10-15molmol-1s-1 and thus would usually
be regarded in the reduced
HOx reaction system V1.1.
A1H2O2+hv⟶2OHA2HOCl+hv⟶OH+ClA3HOBr+hv⟶OH+Br
However, the tagging method cannot be applied for these three reactions.
To include the OH production by the photolysis of H2O2 (Reaction ),
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 HOx and H is not valid for
H2O2. Consequently, we exclude the Reaction () from the
HOx 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 and ),
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 () and ()
from the reduced HOx reaction system V1.1.
Considering Reactions (), () and
() in the reduced HOx 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 HOx reaction system V1.1
worsens the steady-state assumption between OH production and loss. The rest
term resOH introduced in Sect. compensates
for this deviation from the production and loss rate.
HOx contributions in the stratosphere
Figures and
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 ).
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).
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.
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.
The supplement related to this article is available online at: https://doi.org/10.5194/gmd-11-2049-2018-supplement.
Competing interests
There are no competing interests.
Acknowledgements
This study has been carried out in the framework of the project VEU2 funded
by DLR. We used the NCAR Command Language (NCL) for data analysis and to
create the figures of this study. NCL is developed by UCAR/NCAR/CISL/TDD and
available online (DOI: 10.5065/D6WD3XH5). We gratefully acknowledge the
computer systems provided by the Deutsches Klimarechenzentrum (DKRZ), which we
used for our simulations. We thank Mattia Righi from DLR for helpful comments.
The article processing charges for this open-access publication
were covered by a Research Centre of the Helmholtz Association.
Edited by: Olaf Morgenstern
Reviewed by: two anonymous referees
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