Snowmelt Leads to Seasonal Nitrous Acid Formation Across Northwestern China

Abstract

Abstract 28 snowpack samples were collected across northern Xinjiang, northwestern China in January 2018. 16 of these liquid snowmelt samples, 15 with pH < 6.8 and one with the highest nitrate anion (NO3−) concentration, were selected to investigate the photochemical gaseous nitrous acid (HONO) production rate (P(HONO)(g)), which was observed to range from 4.0 to 180.9 ppt min−1 at room temperature. Surprisingly, the ratio of HONO production rate and NO3− concentration (P(HONO)(g)/[NO3ˉ]) was significantly higher than those obtained from the photolysis of aqueous NO3− solutions with similar pH, due to the complex organic compounds present in the snowmelt samples, enhancing HONO formation or the coexistence of other photochemical HONO sources. We also found that P(HONO)(g) was highly correlated with the pH of snowmelt samples, which were significantly lower in rural/remote areas than those from urban/industrial sites, suggesting a significant influence of anthropogenic activities on pH and P(HONO)(g) of snowmelt.