Impacts of additional HONO sources on O3 and PM2.5 chemical coupling and control strategies in the Beijing-Tianjin-Hebei region of China

The objective of this work is to examine the impacts of additional HONO sources on the chemical interaction between ozone (O-3) and particulate matter with a diameter <= 2.5 mu m (PM2.5). Three additional HONO sources, i.e. HONO emissions, the reaction of photo-excited nitrogen dioxide (NO2*) with water vapour (H2O), and NO2 heterogeneous reaction on aerosol surfaces, were inserted into the fully coupled Weather Research and Forecasting-Chemistry model to evaluate O-3 and PM2.5 concentration enhancements in the Beijing-Tianjin-Hebei (BTH) region during August 2007. Results show that the additional HONO sources significantly increase O-3 and PM2.5 concentrations during daytime. Up to 9 ppb enhancements of O-3 and 32 mu g m(-3) increases in PM2.5 are found at seven urban sites over the BTH region. O-3 increases are closely connected to PM2.5 increases over urban areas during daytime when the additional HONO sources are taken into account. PM2.5 inorganic components of SO42-, NO3-, and NH4+ are increased by 5-18, 10-58 and 10-40%, respectively, over urban areas during daytime. The simultaneous increment of O-3 and PM2.5 during daytime due to the additional HONO sources is related to the increasing oxidants (OH, H2O2 and O-3) that enhance the atmospheric oxidising capacity. The concentration variations of O-3 and PM2.5 under a variety of NOx, volatile organic compound and ammonia (NH3) emission control scenarios show that the additional HONO sources increase the sensitivity of O-3 and PM2.5 concentrations to the changes of NOx emissions. An increase of the PM2.5 sensitivity to changes in NH3 emissions is also found. This indicates that without considering the additional HONO sources, the effectiveness of emission control strategies in reducing O-3 and PM2.5 concentrations would be significantly underestimated.