Effects of Additional HONO Sources on Visibility over the North China Plain

The objective of the present study was to better understand the impacts of the additional sources of nitrous acid (HONO) on visibility, which is an aspect not considered in current air quality models. Simulations of HONO contributions to visibility over the North China Plain (NCP) during August 2007 using the fully coupled Weather Research and Forecasting/Chemistry (WRF/Chem) model were performed, including three additional HONO sources: (2) the reaction of photo-excited nitrogen dioxide (NO2*) with water vapor; (3) the NO2 heterogeneous reaction on aerosol surfaces; and (4) HONO emissions. The model generally reproduced the spatial patterns and diurnal variations of visibility over the NCP well. When the additional HONO sources were included in the simulations, the visibility was occasionally decreased by 20%30% (34 km) in local urban areas of the NCP. Monthly-mean concentrations of NO3-, NH4+, SO42- and PM2.5 were increased by 20%52% (311 g m-3), 10%38%, 6%10%, and 6%11% (917 g m-3), respectively; and in urban areas, monthly-mean accumulation-mode number concentrations (AMNC) and surface concentrations of aerosols were enhanced by 15%20% and 10%20%, respectively. Overall, the results suggest that increases in concentrations of PM2.5, its hydrophilic components, and AMNC, are key factors for visibility degradation. A proposed conceptual model for the impacts of additional HONO sources on visibility also suggests that visibility estimation should consider the heterogeneous reaction on aerosol surfaces and the enhanced atmospheric oxidation capacity due to additional HONO sources, especially in areas with high mass concentrations of NOx and aerosols.