An observational study of nitrous acid (HONO) in Shanghai, China: The aerosol impact on HONO formation during the haze episodes.

Continuous HONO measurement was conducted to study the formation features of HONO during the haze episodes at Shanghai, China. The HONO concentration ranged from 0.26 to 5.84ppb and averaged at 2.31ppb during the measurement period. The HONO concentration during the haze episode (P1), the haze-fog episode (P2) and the clean period (P3) were 2.80, 2.35 and 1.78ppb, respectively. Heterogeneous conversion of NO2 was the dominate pathway for nocturnal HONO formation, and the heterogeneous conversion efficiency of NO2 to HONO was closely associated with the PM2.5 concentration. The averaged heterogeneous conversion rate of NO2-to-HONO (CHONO) during the pollution periods (P1+P2) was 1.58×10-2h-1, higher than that during the clean period (P3) (0.93×10-2h-1), suggesting the higher conversion potential of NO2 to HONO during the pollution episodes. The daytime unknown HONO production rate (Punknown) in the pollution period was 2.98ppb/h, higher than 1.78ppb/h in the clean period. Further, aerosol played a role in Punknown during the transformation of the clean period to the pollution period. At a single particle scale, transmission electron microscopy (TEM) images revealed that most of the particles during P1 and P2 were agglomerated, whereas the particles collected from P3 were uniformly distributed and showed simple morphologies. The number percentage of the S/N-bearing particles during P1 (34%) and P2 (27%) were higher than that during P3 (20%). In addition, particles contained more internally mixed nitrates during P1 and P2 than those during P3, suggesting more intense heterogeneous conversion of NO2 to HONO on particle surfaces during the pollution episodes. In the present study, the averaged HONO/NOx ratio (5.60%), especially during P1 (7.80%) and P2 (7.50%) was much higher than that assumed global averaged value of 2.0%, suggesting a potentially important role for the HONO chemistry in Shanghai. This study provides new insights into the HONO formation mechanism in the atmosphere characterized by high fine particle level.