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Abstract:
The heterogeneous reaction of N2O5 on Cl-containing aerosols (heterogeneous N2O5
+ Cl chem-
istry) plays a key role in chlorine activation, NOx recycling, and consequently O3 and PM2.5 formation. In this
study, we use the GEOS-Chem model with additional anthropogenic and biomass burning chlorine emissions
combined with updated parameterizations for the heterogeneous N2O5
+ Cl chemistry (i.e., the uptake coef-
ficient of N2O5 (γN2O5
) and the ClNO2 yield (ϕClNO2
)) to investigate the impacts of chlorine chemistry on air
quality in China, the role of the heterogeneous N2O5
+ Cl chemistry, and the sensitivity of air pollution formation
to chlorine emissions and parameterizations for γN2O5
and ϕClNO2
. The model simulations are evaluated against
multiple observational datasets across China and show significant improvement in reproducing observations of
particulate chloride, N2O5, and ClNO2 when including anthropogenic chlorine emissions and updates to the
parameterization of the heterogeneous N2O5
+ Cl chemistry relative to the default model. The simulations show
that total tropospheric chlorine chemistry could increase annual mean maximum daily 8 h average (MDA8) O3
by up to 4.5 ppbv but decrease PM2.5 by up to 7.9 µgm
−3
in China, 83 % and 90 % of which could be attributed to
the effect of the heterogeneous N2O5
+ Cl chemistry. The heterogeneous uptake of N2O5 on chloride-containing
aerosol surfaces is an important loss pathway of N2O5 as well as an important source of O3 and hence is partic-
ularly useful in elucidating the commonly seen ozone underestimations relative to observations. The importance
of chlorine chemistry largely depends on both chlorine emissions and the parameterizations for the heteroge-
neous N2O5
+ Cl chemistry. With the additional chlorine emissions, the simulations show that annual MDA8 O3
in China could be increased by up to 3.5 ppbv. The corresponding effect on PM2.5 concentrations varies largely
with regions, with an increase of up to 4.5 µgm
−3
in the North China Plain but a decrease of up to 3.7 µgm
−3
in the Sichuan Basin. On the other hand, even with the same chlorine emissions, the effects on MDA8 O3 and
PM2.5 in China could differ by 48 % and 27 %, respectively, between different parameterizations.
