The vertical structure of OH‐HO2‐RO2 chemistry in the nocturnal boundary layer: A one‐dimensional model study

Abstract

Elevated OH and peroxy radical levels have recently been observed in the nocturnal boundary layer (NBL). Despite the possible importance of OH for the gas phase and particulate composition, the source of these ROx radicals at night is currently unclear. To investigate the influence of vertical mixing on nocturnal ROx chemistry, calculations with a one‐dimensional chemical transport model were performed. The model predicts distinct vertical profiles for all ROx radical species during the night, with a pronounced RO2 maximum aloft and maxima of HO2 and OH closer to the ground. We conceptualize our results by distinguishing three chemical regimes in the NBL: (1) In the unreactive ground layer, which only forms at high NO emissions and strong vertical stabilities, OH chemistry is suppressed by high NO levels. (2) The upper layer, located in the upper NBL, is decoupled from the NO emissions at the ground. Ozonolysis of volatile organic compounds (VOCs) leads to the formation of ∼105 molecules cm−3 of OH. The RO2 maximum develops in the lower part of the upper layer by the elevated O3/NO3 + VOC reaction rates. (3) In the reactive mixing layer, in the height interval between the upper layer and the ground layer, RO2 and HNO4 (as HO2 reservoir) from the upper layer are mixed with NO from the ground layer. The active radical propagation chain leads to distinctive maxima of HO2 and OH (up to several 106 molecules cm−3) in this layer. The OH radicals in the reactive mixing layer can contribute up to 43% to the nocturnal VOC oxidation, illustrating that OH chemistry can be important even at night. Our model simulations show that vertical transport can redistribute ROx radicals in the NBL, acting as an important radical source in the lowest few meters of the atmosphere.