Vertical profiles of NO₃, N₂O₅, O₃, and NOx in the nocturnal boundary layer: 2. Model studies on the altitude dependence of composition and chemistry

Recent field observations in urban areas have shown that trace gases, such as O 3 , NO 2 , and NO 3 , develop distinct vertical concentration profiles at night. Because nocturnal chemistry can change the gas‐phase and particulate composition in the urban boundary layer considerably, it is important to understand the mechanisms that lead to the change of trace gas levels with altitude. The quantification of the altitude dependence of chemical processes leading to the removal of volatile organic carbons (VOC) and NO x are crucial to assess the influence of nocturnal chemistry on ozone formation during the following day. We present a one‐dimensional chemical transport model developed to study the interaction between chemistry and vertical transport in the nocturnal boundary layer. The model reproduces the general features found in field observations, such as positive O 3 and NO 3 gradients. The slow upward transport of NO and VOC emitted near the ground and the simultaneously occurring chemistry, in particular the reactions of NO with O 3 and NO 3 , are found to control the vertical structure of the chemistry of NO x , NO 3 , N 2 O 5 , and VOC. In the case of NO 2 and O 3 , dry deposition is also significant. The model results show that vertical transport of N 2 O 5 plays an important role, and is often the main source of NO 3 radicals near the ground. Chemical steady state calculations of the concentrations of NO 3 and N 2 O 5 , as they have been used in the past, are therefore not representative in cases with significant vertical fluxes of N 2 O 5 . The vertical gradient of the oxidation rate of NO 2 implies that the removal of NO x occurs predominately in the upper nocturnal boundary layer (NBL). Our study shows that observations at one altitude and chemical box models are often insufficient to accurately describe the chemistry in NBL.