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

Abstract

<jats:p>Abstract. 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 in accordance with the daily mean values of PM2.5: clean days (CDs), polluted days (PDs), and severely polluted days (SPDs). The HONO concentrations during CD, PD, and SPD periods were 1.2, 2.3, and 3.7 ppbv, respectively. The contributions of the homogeneous reaction, heterogeneous conversion, and direct emissions to HONO sources varied under different pollution levels. The mean values of the net HONO production of the homogeneous 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 periods were 0.72×10-2, 0.64×10-2, and 1.54×10-2 h−1, respectively, indicating that the heterogeneous conversion of NO2 was less important than the homogeneous reaction. Furthermore, the net production of the homogeneous reaction may have been the main factor in the increase of HONO under high-NOx conditions (i.e., when 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 POH+NOnet, CHONO, and Punknown in the SPDs 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 emissions and a heterogeneous reaction controlled HONO production in the first half of the night and provided a contribution that is 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 concentrations. The hourly level of HONO abatement pathways, except for OH + HONO, was at least 0.22 ppbv h−1 in the SPDs period. The cumulative frequency distribution of the HONOemission∕HONO ratio (less than 20 %) was approximately 77 %, which suggested that direct emission was not important. The heterogeneous HONO production increased when the relative humidity (RH) increased, 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 %). </jats:p>

The Levels and Sources of Nitrous Acid (HONO) in Winter of Beijing and Sanmenxia

“…The ratio of HONO corr /NO 2 can represent the heterogeneous conversion fraction of NO 2 to HONO due to its first‐order dependence on NO 2 (Jenkin et al., 1988), and is based on the hypothesis that all HONO corr comes from NO 2 heterogeneous reaction. In this study, the average HONO corr /NO 2 values during the whole day were 0.029 ± 0.021 in BJ and 0.037 ± 0.033 in SMX, which was in accordance with previous studies (Hao et al., 2020; Huang et al., 2017). As shown in Figure S10 in Supporting Information , the diurnal profiles of HONO corr /NO 2 showed a tendency with high nighttime values and low daytime values at the two sites.…”

“…The higher k emis in SMX indicated the stronger HONO emission from the sources relating to NO X . These two k emis were comparable to the generally estimated levels of 0.003–0.012 (Hao et al., 2020; Sarwar et al., 2008; Zheng et al., 2020).…”

“…The homogeneous reaction production rate from OH and NO was comparable at the two sites and played a more important role at noontime. The high HONO production rate from OH and NO reaction was also found in other sites (Hao et al., 2020; Xue et al., 2020). The HONO production rate of NO 2 heterogeneous reaction on the ground surface was 0.13 ppbv/hr in BJ and 0.23 ppbv/hr in SMX.…”

“…Based on these studies, the observed HONO can be partly reproduced in the model studies (Y. H. Liu, Nie, et al., 2019; Xue et al., 2020). In urban Zhengzhou and Xi'an of the Fenwei Plain, the homogeneous reaction was more important than the NO 2 heterogeneous reactions and direct emission (Hao et al., 2020), and the known HONO sources only explained 40% of the observed daytime HONO (Huang et al., 2017).…”

Elucidating HONO formation mechanism and its essential contribution to OH during haze events

“…The overview of daytime (8:00-16:00), nighttime (18:00-06:00), and daily mean concentrations of HONO, NO2, NO, and PM2.5 during four episodes investigated here and the previously reported data during winter haze events are summarized in table S4. During the four pollution episodes, the average values of HONO, NO, NO2, and PM2.5 were 2.52 ± 1.61 ppb, 35.14 ± 30.27 ppb, 32.23 ± 7.43 ppb, 100 ± 69 μg/m 3 , respectively, which were at an intermediate level compared with previous studies in China [46][47][48] and other countries such as India 8,9 . The ratio of HONO to NOX (i.e., the conversion frequency from NOX to HONO), was around 0.03 among previously reported winter haze events in China before 2018 15,[43][44][45]49 .…”