Observations of N₂O₅ and ClNO₂ at a polluted urban surface site in North China : high N₂O₅ uptake coefficients and low ClNO₂ product yields

Abstract

Dinitrogen pentoxide ( N₂O₅) and its heterogeneous uptake product, nitryl chloride (ClNO₂), play important roles in the nocturnal boundary layer chemistry. To understand the abundances and chemistry of N₂O₅ and ClNO₂ in the polluted urban atmosphere in North China, field measurements were conducted by deploying a chemical ionization mass spectrometer in urban Ji'nan in September 2014. The observed surface N₂O₅ concentrations were relatively low, with an average nocturnal value of 22 pptv, although the source of NO₃ was rather strong, i.e., the NO₂ and O₃ were at very high levels. The N₂O₅ concentration peaked in the early evening, which was associated with thermal power plant plumes and residual O₃. Nocturnal N₂O₅ was lost very rapidly, mainly through heterogeneous reactions on aerosol surfaces. The estimated N₂O₅ uptake coefficient was in the range of 0.042–0.092, among the highest values obtained from ground based field measurements. The fast heterogeneous reaction of N₂O₅ on high loadings of aerosols generated relatively high levels of ClNO₂, with an average nocturnal concentration of 132 pptv. Despite the rich chloride content in aerosols, the ClNO₂ product yield was low, 0.014 and 0.082 in two nighttime cases, much lower than the calculated values from the experiment-derived parameterization. The suppressed chlorine activation in polluted urban atmospheres was possibly associated with the reduced hygroscopicity, solubility, and activity of chloride in complex ambient aerosols.