Sulfate formation during heavy winter haze events and the potential contribution from heterogeneous SO<sub>2</sub> + NO<sub>2</sub> reactions in the Yangtze River Delta region, China
Rapid sulfate formation is recognized as a key
characteristic of severe winter haze in China. However, air quality models
tend to underestimate sulfate formation during heavy haze periods, and
heterogeneous formation pathways have been proposed as promising mechanisms
to reduce gaps between observation and model simulation. In this study, we
implemented a reactive SO2 uptake mechanism through the SO2+NO2 heterogeneous reactions in the Comprehensive Air Quality Model with
Extensions (CAMx) to improve simulation of sulfate formation in the Yangtze
River Delta (YRD) region. Parameterization of the SO2+NO2
heterogeneous reactions is based on observations in Beijing and considered
both the impact of relative humidity and aerosol pH on sulfate formation.
Ammonia is reported to be critical for the formation of secondary inorganic
aerosols. Estimation of ammonia emissions is usually associated with large
uncertainties and models tend to underestimate ammonia concentrations
substantially. Sensitivity tests were conducted to evaluate the influence of
the SO2+NO2 heterogeneous reactions as well as ammonia
emissions on modeled sulfate concentrations during a period with several
heavy haze episodes in the YRD region. Base case model results show large
underestimation of sulfate concentrations by 36 % under polluted
conditions in the YRD region. Adding the SO2+NO2 heterogeneous
reactions or doubling ammonia emissions alone leads to slight model
improvement (∼6 %) on simulated sulfate concentrations in
the YRD region. However, model performance significantly improved when both
the SO2+NO2 heterogeneous reactions and doubled ammonia
emissions were included in the simulation: predicted sulfate concentrations
during polluted periods increased from 23.1 µg m−3 in the base
scenario to 29.1 µg m−3 (representing an increase of 26 %).
Aerosol pH is crucial for the SO2+NO2 heterogeneous reactions,
and our calculated aerosol pH is always acidic and increased by 0.7 with
doubled ammonia emissions. Modeling results also show that this reactive
SO2 uptake mechanism enhanced sulfate simulations by 1 to 5 µg m−3 for the majority of the eastern and central parts of China, with more
than 20 µg m−3 increase in sulfate concentrations over the
northeastern plain. These findings suggest that the SO2+NO2
heterogeneous reactions could be potentially important for sulfate formation
in the YRD region as well as other parts of China. Further studies are
needed to constrain the uncertainties associated with the parameterization
of the SO2+NO2 heterogeneous reactions based on local data
as well as to evaluate this mechanism in other regions. In addition, ammonia
emissions were found to be a key driving variable of the spatial patterns of
sulfate enhancement due to the new pathway. Substantial efforts are needed
to improve the accuracy of the ammonia emission inventory.
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