Vertically increased NO radical in the nocturnal boundary layer.

In the nocturnal boundary layer, nitrate radical (NO) has an important contribution to atmospheric chemistry through oxidation of nitrogen oxides and hydrocarbons. Vertical distributions of NO, O and NO 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 NO, O, and NO display a single peak (up to 65.0 ppbv, 196.8 ppbv and 317.5 pptv, respectively) in time. O and NO mixing ratios generally increased against heights, which is opposite to NO, suggesting the contribution of O to NO production at higher altitude. According to the correlation coefficients between NO production rates (P) and NO or O levels, P was sensitive to NO mixing ratio at higher altitude but to O near the ground. Averaged NO lifetimes (τ) of lowest, middle, upper and highest layer intervals were 104, 118, 164 and 213 s, respectively, which indicates τ increase against height and explains why NO mixing ratios are larger at higher altitude to some extent. Main control factors of NO removal changed from gas-phase reactions to NO hydrolysis with height increase. When relative humidity (RH) exceeded 70% or PM level exceeded 50 μg·m, τ was almost less than 300 s with mixing ratio lower than 70 pptv. The clear negative dependence of τ on RH and PM reveals the influencing factors on indirect loss. Under polluted conditions, vertical profiles of NO, O and NO 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 NO gradient. Vertically increased NO radicals may imply the formation of nitrate aerosols and further increase the nitrate content in high- altitude particulate matter.