Occurrence of atmospheric nitrous acid in the urban area of Beijing (China).

The atmospheric concentrations of nitrous acid (HONO) have been measured during two field campaigns in the winter and summer of 2007 at Beijing (China). The results were discussed from the standpoint of temporal and diurnal variations and meteorological effects. The daily average HONO concentrations were in the range of 0.03-2.91ppb and didn't show temporal variation between the winter and summer periods. The temporal trends seemed to be largely affected by meteorological conditions. HONO concentrations showed very typical diurnal variations during intensive winter and summer periods. Nitrogen oxides were key precursors of HONO formation and the HONO/NO2 values were higher than those reported for direct emission (<1%), indicating the prevalence of secondary chemical HONO formation on direct emission during both periods. We used a pseudo steady state approach (PSS), which included homogeneous and heterogeneous reactions and direct emission, explaining on average about 83% and 48% of the observed HONO levels during the intensive winter and summer periods, respectively. The daytime unknown HONO production was on average 2.58ppbh(-1) during the summer period. The HNO3 and fine particulate NO3(-) photolysis contributed weakly as HONO source. Including these sources in the PSS calculation, we explained about 53% of the observed HONO levels. The results showed that heterogeneous JNO2 dependent processes on aerosol and ground surfaces, involving NO2 as HONO precursor, were HONO sources during the summer measurements.