Atmospheric Measurements at the Foot and the Summit of Mt. Tai – Part II: HONO Budget and Radical (RO<sub>x</sub> + NO<sub>3</sub>) Chemistry in the Lower Boundary Layer

Abstract. In the summer of 2018, a comprehensive field campaign, with measurements on HONO and related parameters, was conducted at the foot (150 m a.s.l.) and the summit of Mt. Tai (1534 m a.s.l.) in the central North China Plain (NCP). With the implementation of a 0-D box model, the HONO budget with six additional sources and its role in radical chemistry at the foot station were explored. We found that the model default source, NO + OH, could only reproduce the observed HONO by 13 %, leading to a strong unknown source strength up to 3 ppbv h−1. Among the additional sources, the NO2 uptake on the ground surface dominated (~70 %) night-time HONO formation, and its photo-enhanced reaction dominated (~80 %) daytime HONO formation. Their contributions were sensitive to the mixing layer height (MLH) used for the parameterizations, highlighting the importance of a reasonable MLH for exploring ground-level HONO formation in 0-D models and the necessity of gradient measurements. A HONO / NOx ratio of 0.7 % for the direct emission was inferred and a new method to quantify its contribution to the observations was proposed and discussed. Aerosol-derived sources, including the NO2 uptake on the aerosol surface and the particulate nitrate photolysis, did not lead to significant HONO formation, with their contributions lower than NO + OH. HONO photolysis in the early morning initialized the daytime photochemistry at both the foot and the summit stations and also was a substantial radical source throughout the daytime, with contributions higher than or about one-quarter of O3 photolysis to OH initiation at the foot and the summit stations, respectively. Moreover, we found that OH dominated the atmospheric oxidizing capacity in the daytime, while NO3 appeared to be significant at night. Peaks of NO3 time series and diurnal variation reached 22 and 9 pptv, respectively. NO3 induced reactions contribute 18 % of nitrate formation potential (P(HNO3)) and 11 % of the isoprene (C5H8) oxidation throughout the whole day. At night, NO3 chemistry led to 51 % or 44 % of P(HNO3) or the C5H8 oxidation, respectively. NO3 chemistry may significantly affect night-time secondary organic and inorganic aerosol formation in this high-O3 region, implying that NO3 chemistry could significantly affect night-time secondary organic and inorganic aerosol formation in this high-O3 region. Considering the severe O3 pollution in the NCP and the very limited NO3 measurements, we suggest that besides direct measurements of HOx and primary HOx precursors (O3, HONO, alkenes, etc.), NO3 measurements should be conducted to understand the atmospheric oxidizing capacity and air pollution formation in this and similar regions.