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 (O₃) and particulate matter with a diameter ≤ 2.5 µm (PM_2.5). 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 O₃ and PM_2.5 concentration enhancements in the Beijing–Tianjin–Hebei (BTH) region during August 2007. Results show that the additional HONO sources significantly increase O₃ and PM_2.5 concentrations during daytime. Up to 9 ppb enhancements of O₃ and 32 µg m⁻³ increases in PM_2.5 are found at seven urban sites over the BTH region. O₃ increases are closely connected to PM_2.5 increases over urban areas during daytime when the additional HONO sources are taken into account. PM_2.5 inorganic components of SO₄^2–, NO₃^– and NH₄⁺ are increased by 5–18, 10–58 and 10–40%, respectively, over urban areas during daytime. The simultaneous increment of O₃ and PM_2.5 during daytime due to the additional HONO sources is related to the increasing oxidants (OH, H₂O₂ and O₃) that enhance the atmospheric oxidising capacity. The concentration variations of O₃ and PM_2.5 under a variety of NO_x, volatile organic compound and ammonia (NH₃) emission control scenarios show that the additional HONO sources increase the sensitivity of O₃ and PM_2.5 concentrations to the changes of NO_x emissions. An increase of the PM_2.5 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 O₃ and PM_2.5 concentrations would be significantly underestimated.