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 4.3 ppbv with an average of 1.12 ppbv for the entire measurement period. The variation trends of HONO and NO 2 are very similar and positively correlated which, together with the similar diurnal profiles of HONO/NO 2 ratio and HONO, suggest the importance of heterogeneous conversion of HONO from NO 2 . The nocturnal HONO level is governed by heterogeneous formation from NO 2 , 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 0.75 ppbv h − 1 is required to explain the observed HONO concentration, which contributes 60.8% of the observed daytime HONO. [ABSTRACT FROM AUTHOR]