Long-term observation of atmospheric nitrous acid (HONO) and its implication to local NO2 levels in Shanghai, China

Long-term observation of atmospheric nitrous acid (HONO) was performed from August 2010 to June 2012 in urban area of Shanghai, China. In total, valid data for nineteen months were obtained. The monthly average HONO mixing ratio varied from 0.60 ppb in February 2011 to 1.20 ppb in December 2010, but no obvious monthly or seasonal variations were observed. The mean mixing ratio of HONO was obtained as 0.92 ± 0.57 ppb, with average HONO mixing ratio of 1.08 ± 0.63 ppb for night-time, and 0.76 ± 0.44 ppb for daytime. It was found that HONO mixing ratio is dependent on NO2 mixing ratio both in daytime and at night. Besides, high HONO level during daytime implicates that strong daytime sources must be active in Shanghai. The ratio of HONO to NO2 varied from 0.3% to 42.3% with a typical error of 1% and more than 99% of them below 17%. When NO2 mixing ratio was above 30 ppb, the HONO/NO2 ratio (HONO/NO2 averaged over 5.0 ppb intervals of NO2 mixing ratio) was nearly constant at 3.3 ± 0.2%. When the NO2 mixing ratio is below 30 ppb, however, the HONO/NO2 ratio appeared to increase as the NO2 mixing ratio decreased. Additionally, HONO/NO2 ratio depends obviously on relative humidity at night, while there is no obvious correlation between them during daytime. The average conversion frequency of HONO in sixteen selected days was about 0.007 h−1 under different NO2 pollution scenarios and higher conversion frequency tends to be occurred at lower NO2 levels.