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. Nitrous acid (HONO), an important precursor of the hydroxyl radical (OH),has long been recognized as of significance to atmosphericchemistry, but its sources are still debated. In this study, we conductedcontinuous measurement of HONO from November 2017 to November 2018 at theSORPES station in Nanjing of eastern China. The yearly average mixing ratioof observed HONO was 0.69±0.58  ppb, showing a larger contribution toOH relative to ozone with a mean OH production rate of 1.16 ppb h −1 . Toestimate the effect of combustion emissions of HONO, the emitted ratios ofHONO to NOx were derived from 55 fresh plumes ( NO∕NOx  > 0.85), with a mean value of 0.79 %. During the nighttime, the chemistry ofHONO was found to depend on RH, and heterogeneous reaction of NO2 onan aerosol surface was presumably responsible for HONO production. The averagenighttime NO2 -to-HONO conversion frequency ( CHONO ) was determinedto be 0.0055±0.0032  h −1 from 137 HONO formation cases. Themissing source of HONO around noontime seemed to be photo-induced, with anaverage Punknown of 1.04 ppb h −1 , based on a semi-quantitativeHONO budget analysis. An over-determined system of equations was applied toobtain the monthly variations in nocturnal HONO sources. Besides theburning-emitted HONO (accounting for about 23 % of the total concentration), thecontribution of HONO formed heterogeneously on ground surfaces to measured HONO was anapproximately constant proportion of 36 % throughout the year. The soilemission revealed clear seasonal variation and contributed up to 40 % ofobserved HONO in July and August. A higher propensity for generating HONO onaerosol surfaces occurred in severe hazes (accounting for 40 % of the total concentration inJanuary). Our results highlight ever-changing contributions of HONO sourcesand encourage more long-term observations to evaluate the contributions fromvaried sources.

atmospheric chemistry and physics

Semi-quantitative understanding of source contribution to nitrous acid (HONO) based on 1 year of continuous observation at the SORPES station in eastern China