Reconciling modelled and observed Δ17O(NO3-) in Beijing winter haze with
heterogeneous chlorine chemistry
Abstract
The air quality in Chinese megacities has been improved as indicated by
large decreases in fine particulate matter (PM2.5) due to remarkable
decreases in key precursors (e.g., SO2, NOx) after the implementation of
strict mitigation strategies. However, nitrate concentrations in PM2.5
(p-NO3-) have not decreased and mass fractions of p-NO3- in PM2.5 have
increased, especially during wintertime haze events. Discerning chemical
mechanisms leading to nitrate growth during haze events is critical to
implement effective mitigation policies. Chemical transport models
incorporating oxygen isotope anomaly of nitrate (Δ17O(NO3-)) have been
widely used to investigate nitrate formation mechanisms, showing general
consensus on the modelled and observed Δ17O(NO3-). However, under
Beijing haze days, the same model tends to underestimate observed
Δ17O(NO3-). Here we compiled reported Δ17O(NO3-) data in Beijing haze
along with relevant observational parameters (e.g., OH total reactivity,
peroxyl radical concentrations), tested assumptions on Δ17O of key
precursors (e.g., OH and NO2), re-calculated Δ17O(NO3-) and compared
with observations. Our results indicate that considering heterogeneous
N2O5 reactions on Cl–containing aerosols with a ClNO2 yield of
~ 0.75 can explain the observed high Δ17O(NO3-).
According to the Δ17O(NO3-) data, this heterogeneous N2O5 + Cl-
chemistry can explain ~ 60% of nighttime nitrate
production and makes daytime and nocturnal pathways equally important in
winter Beijing haze. Meanwhile, the high yield of ClNO2 means that on
the following day the subsequent photolysis of ClNO2 would enhance
atmospheric oxidation capacity and promote haze pollution, highlighting
the critical role of reactive chlorine chemistry in air
pollution/chemistry in inland cities