Effect of Different Combustion Processes on Atmospheric Nitrous Acid Formation Mechanisms: A Winter Comparative Observation in Urban, Suburban and Rural Areas of the North China Plain.

Atmospheric nitrous acid (HONO) is a dominant precursor of hydroxyl (OH) radicals, and its formation mechanisms are still controversial. Few studies have simultaneously explored effects of different combustion processes on HONO sources. Hereby, synchronous HONO measurement in urban (BJ), suburban (XH) and rural (DBT) areas with different combustion processes is performed in the North China Plain in winter. A box model is utilized to analyze HONO formation mechanisms. HONO concentration is the highest at the DBT site (2.51 ± 1.90 ppb), followed by the XH (2.18 ± 1.95 ppb) and BJ (1.17 ± 1.20 ppb) sites. Vehicle exhaust and coal combustion significantly contribute to nocturnal HONO at urban and rural sites, respectively. During a stagnant pollution period, the NO+OH reaction and combustion emissions are more crucial to HONO in urban and rural areas; meanwhile, the heterogeneous reaction of NO ₂ is more significant in suburban areas. Moreover, the production rate of OH from HONO photolysis is about 2 orders of magnitude higher than that from ozone photolysis. Consequently, vehicle exhaust and coal combustion can effectively emit HONO, further causing environmental pollution and health risks. It is necessary to expand the implementation of the clean energy transition policy in China, especially in areas with substantial coal combustion.