Abstract
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 13 % of the observed HONO, leading to a strong unknown source strength of up to 3 ppbv h−1. Among the additional sources, the NO2 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 ΔHONO/ΔNOx 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 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 the foot station. It was also a substantial radical source throughout the daytime, with contributions higher than O3 photolysis to OH initiation. Moreover, we found that OH dominated the atmospheric oxidizing capacity in the daytime, while modeled NO3 appeared to be significant at night. Peaks of modeled NO3 time series and average 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 % and 44 % of P(HNO3) or the C5H8 oxidation, respectively, implying that NO3 chemistry could significantly affect nighttime 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.
Highlights
IntroductionNumerous field campaigns coupled with model simulations have been conducted worldwide to understand summertime atmospheric chemistry as it is linked to the regional air quality and global climate (Alicke et al, 2003; Elshorbany et al, 2012; Heard et al, 2004; Kanaya et al, 2009, 2013; Michoud et al, 2012; Ren et al, 2003; Rohrer and Berresheim, 2006; Tan et al, 2017; Travis et al, 2020)
The fast conversion of HO2 to OH (Reaction R1) as part of the radical propagation cycle, primary OH mainly originates from photolysis reactions, including O3 (Reactions R2 to R4), HONO (Reaction R5), HCHO (Reactions R6 to R9, and R1), and H2O2 (Reaction R10), and the ozonolysis of alkenes
Very few NO3 measurements are available in China (Lu et al, 2019; Suhail et al, 2019), while its high concentration and important role in chemical oxidation presented in this study indicate the necessity of direct NO3 measurements in the North China Plain (NCP), where summertime O3 levels are substantially increasing (Han et al, 2020; Li et al, 2019; Sun et al, 2016, 2019)
Summary
Numerous field campaigns coupled with model simulations have been conducted worldwide to understand summertime atmospheric chemistry as it is linked to the regional air quality and global climate (Alicke et al, 2003; Elshorbany et al, 2012; Heard et al, 2004; Kanaya et al, 2009, 2013; Michoud et al, 2012; Ren et al, 2003; Rohrer and Berresheim, 2006; Tan et al, 2017; Travis et al, 2020). HONO photolysis is reported to be an important or even the major OH source in the lower atmosphere of polluted regions, with a contribution of 20 %–90 % (Alicke et al, 2003; Elshorbany et al, 2009; Kleffmann et al, 2005; Platt et al, 1980; Slater et al, 2020; Whalley et al, 2021; Xue et al, 2020). This process still needs more global quantification due to the incomplete understanding of HONO formation and its vertical distribution in the atmosphere (Kleffmann, 2007). A state-ofart summary of the reported HONO sources can be found in our recent study (Xue et al, 2020)
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