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. In the summer of 2018, a comprehensive field campaign, with measurements onHONO and related parameters, was conducted at the foot (150 m a.s.l.) andthe 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 sixadditional sources and its role in radical chemistry at the foot stationwere explored. We found that the model default source, NO + OH, could onlyreproduce 13 % of the observed HONO, leading to a strong unknown sourcestrength of up to 3 ppbv h−1. Among the additional sources, theNO2 uptake on the ground surface dominated (∼ 70 %)nighttime HONO formation, and its photo-enhanced reaction dominated(∼ 80 %) daytime HONO formation. Their contributions weresensitive to the mixing layer height (MLH) used for the parameterizations,highlighting the importance of a reasonable MLH for exploring ground-levelHONO formation in 0-D models and the necessity of gradient measurements. AΔHONO/ΔNOx ratio of 0.7 % for direct emissions fromvehicle exhaust was inferred, and a new method to quantify its contributionto the observations was proposed and discussed. Aerosol-derived sources,including the NO2 uptake on the aerosol surface and the particulatenitrate photolysis, did not lead to significant HONO formation, with theircontributions lower than NO + OH. HONO photolysis in the early morning initialized the daytime photochemistryat the foot station. It was also a substantial radical source throughout thedaytime, with contributions higher than O3 photolysis to OH initiation.Moreover, we found that OH dominated the atmospheric oxidizing capacity inthe daytime, while modeled NO3 appeared to be significant at night.Peaks of modeled NO3 time series and average diurnal variation reached22 and 9 pptv, respectively. NO3-induced reactions contribute 18 % ofnitrate formation potential (P(HNO3)) and 11 % of the isoprene(C5H8) oxidation throughout the whole day. At night, NO3chemistry led to 51 % and 44 % of P(HNO3) or the C5H8oxidation, respectively, implying that NO3 chemistry couldsignificantly affect nighttime secondary organic and inorganic aerosolformation in this high-O3 region. Considering the severe O3pollution in the NCP and the very limited NO3 measurements, we suggestthat besides direct measurements of HOx and primary HOx precursors(O3, HONO, alkenes, etc.), NO3 measurements should be conducted tounderstand the atmospheric oxidizing capacity and air pollution formation inthis and similar regions.

atmospheric chemistry and physics

Atmospheric measurements at Mt. Tai – Part II: HONO budget and radical (RO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; + NO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;) chemistry in the lower boundary layer