Abstract. We compare Community Multiscale Air Quality (CMAQ) model predictions with measured nitrous acid (HONO) concentrations in Beijing,
China, for December 2015. The model with the existing HONO chemistry in CMAQ
severely underestimates the observed HONO concentrations with a normalized
mean bias of −97 %. We revise the HONO chemistry in the model by
implementing six additional heterogeneous reactions in the model: the reaction
of nitrogen dioxide (NO2) on ground surfaces, the reaction of NO2 on
aerosol surfaces, the reaction of NO2 on soot surfaces, the photolysis of
aerosol nitrate, the nitric acid displacement reaction, and the hydrochloric acid
displacement reaction. The model with the revised chemistry substantially
increases HONO predictions and improves the comparison with observed data
with a normalized mean bias of −5 %. The photolysis of HONO enhances
daytime hydroxyl radical by almost a factor of 2. The enhanced hydroxyl
radical concentrations compare favorably with observed data and produce
additional sulfate via the reaction with sulfur dioxide, aerosol nitrate via
the reaction with nitrogen dioxide, and secondary organic aerosols via the
reactions with volatile organic compounds. The additional sulfate stemming
from revised HONO chemistry improves the comparison with observed
concentration; however, it does not close the gap between model prediction
and the observation during polluted days.
Improving the representation of HONO chemistry in CMAQ and examining its impact on haze over China
