Concentration and sources of atmospheric nitrous acid (HONO) at an urban site in Western China

Highly time-resolved measurements of nitrous acid (HONO) were carried out with a highly sensitive long path absorption photometer (LOPAP) at an urban site of Xi'an in Western China from 24 July to 6 August 2015 to investigate the atmospheric variations, sources, and formation pathways of HONO. The concentrations of HONO vary from 0.02 to 4.3ppbv with an average of 1.12ppbv for the entire measurement period. The variation trends of HONO and NO2 are very similar and positively correlated which, together with the similar diurnal profiles of HONO/NO2 ratio and HONO, suggest the importance of heterogeneous conversion of HONO from NO2. The nocturnal HONO level is governed by heterogeneous formation from NO2, followed by homogeneous formation of NO with OH and then by direct emissions. Further, it is found that the heterogeneous formation of HONO is largely affected by relative humidity and aerosol surface. Daytime HONO budget analysis indicates that an additional unknown source with HONO production rate of 0.75ppbvh−1 is required to explain the observed HONO concentration, which contributes 60.8% of the observed daytime HONO. [Display omitted] •The atmospheric concentrations and variations of HONO were studied in Western China.•Heterogeneous NO2 conversion and NO+OH reaction are main sources of the nocturnal HONO at Xi'an.•The heterogeneous formation of HONO is largely affected by relatively humidity and aerosol surface.•Unknown daytime HONO source dominates the daytime HONO production.