Vertically increased NO3 radical in the nocturnal boundary layer

Abstract

In the nocturnal boundary layer, nitrate radical (NO3) has an important contribution to atmospheric chemistry through oxidation of nitrogen oxides and hydrocarbons. Vertical distributions of NO2, O3 and NO3 were measured by four differential optical absorption spectroscopy instruments at meteorological tower in Beijing from June 1 to July 22, 2019. The results show the mean diurnal variations of NO2, O3, and NO3 display a single peak (up to 65.0 ppbv, 196.8 ppbv and 317.5 pptv, respectively) in time. O3 and NO3 mixing ratios generally increased against heights, which is opposite to NO2, suggesting the contribution of O3 to NO3 production at higher altitude. According to the correlation coefficients between NO3 production rates (PNO3) and NO2 or O3 levels, PNO3 was sensitive to NO2 mixing ratio at higher altitude but to O3 near the ground. Averaged NO3 lifetimes (τNO3) of lowest, middle, upper and highest layer intervals were 104, 118, 164 and 213 s, respectively, which indicates τNO3 increase against height and explains why NO3 mixing ratios are larger at higher altitude to some extent. Main control factors of NO3 removal changed from gas-phase reactions to N2O5 hydrolysis with height increase. When relative humidity (RH) exceeded 70% or PM2.5 level exceeded 50 μg·m−3, τNO3 was almost less than 300 s with mixing ratio lower than 70 pptv. The clear negative dependence of τNO3 on RH and PM2.5 reveals the influencing factors on indirect loss. Under polluted conditions, vertical profiles of NO2, O3 and NO3 varied drastically. Stable atmosphere (low nocturnal boundary layer height and thermal inversion), RH level and RH gradient are the main reason for the evident difference in NO3 gradient. Vertically increased NO3 radicals may imply the formation of nitrate aerosols and further increase the nitrate content in high- altitude particulate matter.