Amplified role of potential HONO sources in O3 formation in North China Plain during autumn haze aggravating processes.

Co-occurrences of high concentrations of PM 2.5 and ozone (O 3) have been frequently observed in haze-aggravating processes in the North China Plain (NCP) over the past few years. Higher O 3 concentrations on hazy days were hypothesized to be related to nitrous acid (HONO), but the key sources of HONO enhancing O 3 during haze-aggravating processes remain unclear. We added six potential HONO sources, i.e., four ground-based (traffic, soil, and indoor emissions, and the NO 2 heterogeneous reaction on ground surface (Het ground)) sources, and two aerosol-related (the NO 2 heterogeneous reaction on aerosol surfaces (Het aerosol) and nitrate photolysis (Phot nitrate)) sources into the WRF-Chem model and designed 23 simulation scenarios to explore the unclear key sources. The results indicate that ground-based HONO sources producing HONO enhancements showed a rapid decrease with height, while the NO + OH reaction and aerosol-related HONO sources decreased slowly with height. Phot nitrate contributions to HONO concentrations were enhanced with aggravated pollution levels. The enhancement of HONO due to Phot nitrate on hazy days was about 10 times greater than on clean days and Phot nitrate dominated daytime HONO sources (∼ 30 %–70 % when the ratio of the photolysis frequency of nitrate (Jnitrate) to gas nitric acid (JHNO3) equals 30) at higher layers (>800 m). Compared with that on clean days, the Phot nitrate contribution to the enhanced daily maximum 8 h averaged (DMA8) O 3 was increased by over 1 magnitude during the haze-aggravating process. Phot nitrate contributed only ∼ 5 % of the surface HONO in the daytime with a Jnitrate/JHNO3 ratio of 30 but contributed ∼ 30 %–50 % of the enhanced O 3 near the surface in NCP on hazy days. Surface O 3 was dominated by volatile organic compound-sensitive chemistry, while O 3 at higher altitudes (>800 m) was dominated by NO x -sensitive chemistry. Phot nitrate had a limited impact on nitrate concentrations (<15 %) even with a Jnitrate/JHNO3 ratio of 120. These results suggest the potential but significant impact of Phot nitrate on O 3 formation, and that more comprehensive studies on Phot nitrate in the atmosphere are still needed. [ABSTRACT FROM AUTHOR]