Atmospheric measurements at Mt. Tai – Part II: HONO budget and radical (ROx + NO3) chemistry in the lower boundary layer.

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 13 % of the observed HONO, leading to a strong unknown source strength of up to 3 ppbv h -1. Among the additional sources, the NO 2 uptake on the ground surface dominated (∼ 70 %) nighttime 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 Δ HONO /Δ NO x ratio of 0.7 % for direct emissions from vehicle exhaust was inferred, and a new method to quantify its contribution to the observations was proposed and discussed. Aerosol-derived sources, including the NO 2 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 the foot station. It was also a substantial radical source throughout the daytime, with contributions higher than O 3 photolysis to OH initiation. Moreover, we found that OH dominated the atmospheric oxidizing capacity in the daytime, while modeled NO 3 appeared to be significant at night. Peaks of modeled NO 3 time series and average diurnal variation reached 22 and 9 pptv, respectively. NO 3 -induced reactions contribute 18 % of nitrate formation potential (P (HNO 3)) and 11 % of the isoprene (C 5 H 8) oxidation throughout the whole day. At night, NO 3 chemistry led to 51 % and 44 % of P (HNO 3) or the C 5 H 8 oxidation, respectively, implying that NO 3 chemistry could significantly affect nighttime secondary organic and inorganic aerosol formation in this high-O 3 region. Considering the severe O 3 pollution in the NCP and the very limited NO 3 measurements, we suggest that besides direct measurements of HO x and primary HO x precursors (O 3 , HONO, alkenes, etc.), NO 3 measurements should be conducted to understand the atmospheric oxidizing capacity and air pollution formation in this and similar regions. [ABSTRACT FROM AUTHOR]