Exploring HONO formation and its role in driving secondary pollutants formation during winter in the North China Plain

Daytime HONO photolysis is an important source of atmospheric hydroxyl radicals (OH). Knowledge of HONO formation chemistry under typical haze conditions, however, is still limited. In the Multiphase chemistry experiment in Fogs and Aerosols in the North China Plain in 2018, we investigated the wintertime HONO formation and its atmospheric implica-tions at a rural site Gucheng. Three different episodes based on atmospheric aerosol loading levels were classified: clean periods (CPs), moderately polluted periods (MPPs) and severely polluted periods (SPPs). Correlation analysis revealed that HONO formation via heteroge- neous conversion of NO2 was more efficient on aerosol surfaces than on ground, highlight-ing the important role of aerosols in promoting HONO formation. Daytime HONO budget analysis indicated a large missing source (with an average production rate of 0.66 +/- 0.26, 0.97 +/- 0.47 and 1.45 +/- 0.55 ppbV/hr for CPs, MPPs and SPPs, respectively), which strongly correlated with photo-enhanced reactions (NO2 heterogeneous reaction and particulate ni-trate photolysis). Average OH formation derived from HONO photolysis reached up to (0.92 +/- 0.71), (1.75 +/- 1.26) and (1.82 +/- 1.47) ppbV/hr in CPs, MPPs and SPPs respectively, much higher than that from O 3 photolysis (i.e., (0.004 +/- 0.004), (0.006 +/- 0.007) and (0.0035 +/- 0.0034) ppbV/hr). Such high OH production rates could markedly regulate the atmospheric oxida-tion capacity and hence promote the formation of secondary aerosols and pollutants.(c) 2022 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.