LAUSR

Abstract. Nitrous acid (HONO) in the core city of the Central Plains Economic Region was measured using an ambient ion monitor from January 9 to 31, 2019. Measurement time intervals were classified into the following periods in accordance with the daily mean values of PM2.5: clean days (CD), pollution days (PD), and severe pollution days (SPD). The HONO concentrations during CD, PD, and SPD were 1.2, 2.3, and 3.7 ppbv, respectively. The contribution of the three sources varied under different pollution levels. The mean values of the net HONO production of the homogeneous reaction (P(OH+NO)net) in CD, PD, and SPD periods were 0.51, 1.03, and 2.18 ppbv h−1, respectively. The average conversions of NO2 (CHONO) in CD, PD, and SPD periods were 0.72 × 10−2, 0.64 × 10−2, and 1.54 × 10−2 ppbv h−1, respectively, indicating that the heterogeneous conversion of NO2 was unimportant. Furthermore, the net production of the homogeneous reaction may have been the main factor for the increase in HONO under high-NOX conditions (i.e., the concentration of NO was higher than that of NO2) at nighttime. Daytime HONO budget analysis showed that the mean values of the unknown source (Punknown) during CD, PD, and SPD periods were 0.26, 0.40, and 1.83 ppbv h−1, respectively. The values of P(OH+NO)net, CHONO, and Punknown in the SPD period were comparatively larger than those in other periods, indicating that HONO participated in many reactions. The proportions of nighttime HONO sources also changed during the entire sampling period. Direct emission and a heterogeneous reaction controlled HONO production in the first half of the night and provided a contribution larger than that of the homogeneous reaction. The proportion of homogenization gradually increased in the second half of the night due to the steady increase in NO concentration. The hourly abatement level of HONO abatement pathways, except for OH + HONO, should be at least 1.47 ppbv h−1 in the SPD period. The cumulative frequency distribution of the HONOemission / HONO ratio (less than 20 %) was approximately 76.7 %, which suggests that direct emission was important. The heterogeneous HONO production increased when the relative humidity (RH) increased from 52 % to 77 %, but it decreased when RH increased further. The average HONO / NOX ratio (4.9 %) was more than twice the assumed globally averaged value (2.0 %).