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. Nitrous acid (HONO) in the core city of the Central Plains Economic Region in China was measured using an ambient ion monitor from 9 to 31 January 2019.Measurement time intervals were classified into the following periods inaccordance with the daily mean values of PM2.5: clean days (CDs),polluted days (PDs), and severely polluted days (SPDs). The HONOconcentrations during CD, PD, and SPD periods were 1.2, 2.3, and 3.7 ppbv,respectively. The contributions of the homogeneous reaction, heterogeneousconversion, and direct emissions to HONO sources varied under differentpollution levels. The mean values of the net HONO production of thehomogeneous reaction (POH+NOnet) in CD,PD, and SPD periods were 0.13, 0.26, and 0.56 ppbv h−1, respectively.The average conversions of NO2 (CHONO) in CD, PD, and SPD periodswere 0.72×10-2, 0.64×10-2, and 1.54×10-2 h−1, respectively, indicating that the heterogeneousconversion of NO2 was less important than the homogeneous reaction.Furthermore, the net production of the homogeneous reaction may have beenthe main factor in the increase of HONO under high-NOx conditions(i.e., when the concentration of NO was higher than that of NO2) atnighttime. Daytime HONO budget analysis showed that the mean values of theunknown source (Punknown) during CD, PD, and SPD periods were 0.26,0.40, and 1.83 ppbv h−1, respectively. The values ofPOH+NOnet, CHONO, and Punknownin the SPDs period were comparatively larger than those in other periods,indicating that HONO participated in many reactions. The proportions ofnighttime HONO sources also changed during the entire sampling period.Direct emissions and a heterogeneous reaction controlled HONO production inthe first half of the night and provided a contribution that is larger than that ofthe homogeneous reaction. The proportion of homogenization graduallyincreased in the second half of the night due to the steady increase in NOconcentrations. The hourly level of HONO abatement pathways, except for OH + HONO, was at least 0.22 ppbv h−1 in the SPDs period. The cumulativefrequency distribution of the HONOemission∕HONO ratio (less than20 %) was approximately 77 %, which suggested that direct emission wasnot important. The heterogeneous HONO production increased when the relativehumidity (RH) increased, but it decreased when RH increased further. Theaverage HONO∕NOx ratio (4.9 %) was more than twice the assumedglobally averaged value (2.0 %).

Characteristics, sources, and reactions of nitrous acid during winter at an urban site in the Central Plains Economic Region in China