Contributions of different sources to nitrous acid (HONO) at the SORPES station in eastern China: results from one-year continuous observation

Abstract. Nitrous acid (HONO), a reservoir of the hydroxyl radical (OH), has been long-standing recognized to be of significant importance to atmospheric chemistry, but its sources are still debate. In this study, we conducted continuous measurement of HONO from November 2017 to November 2018 at SORPES station in Nanjing of eastern China. The yearly average mixing ratio of observed HONO was 0.69 ± 0.58 ppb, showing a larger contribution to OH relative to ozone with a mean OH production rate of 0.90 ± 0.27 ppb/h. To estimate the effect of combustion emissions of HONO, the emitted ratios of HONO and NOx were derived from 55 fresh plumes (NO / NOx > 0.85), with a mean value of 0.79 %. The well-defined seasonal and diurnal patterns with clear wintertime and early morning concentration peaks of both HONO and NOx indicate that NOx is the critical precursor of HONO. During the nighttime, the chemistry of HONO was found to depend on RH, and heterogeneous reaction of NO2 on aerosol surface was presumably responsible for HONO production. The average nighttime NO2-to-HONO conversion frequency (CHONO) was determined to be 0.0055 ± 0.0032 h−1 from 137 HONO formation cases. The missing source of HONO around noontime seemed to be photo-induced with an average Punknown of 1.13 ± 0.95 ppb h−1, based on a semiquantitative HONO budget analysis. An over-determined system of equations was applied to obtain the monthly variations in nocturnal HONO sources. Except for burning-emitted HONO (approximately 23 % of total measured HONO), the contribution of heterogeneous formation on ground surfaces was an approximately constant proportion of 36 % throughout the year. The soil emission revealed clear seasonal variation, and contributed up to 40 % of observed HONO in July and August. A higher propensity for generating HONO on aerosol surface occurred in heavily polluted period (about 40 % of HONO in January). Our results highlight ever-changing contributions of HONO sources, and encourage more long-term observations to evaluate the contribution from varied sources.