Vertical profiles of nocturnal boundary layer chemistry in polluted urban environments: Field observations and model studies

Nocturnal chemistry in the atmospheric boundary layer determines initial chemical conditions for morning photochemistry and influences the budgets of O3 and NO2. Despite its importance, chemistry in the nocturnal boundary layer (NBL), especially in heavily polluted urban areas, has received surprisingly little attention so far. In particular, the influence of vertical mixing on chemical processes leads to complex vertical profiles of reactive species and makes NBL chemistry altitude-dependent. The processing of pollutants is thus driven by a complicated, and not well understood, interplay between chemistry and vertical mixing.; To gain a better understanding of NBL chemistry in urban environments, a field study was carried out in the downtown area of Phoenix, AZ. Vertical profiles of reactive species such as O3, NO2, and NO 3 were observed in the lowest 140 m of the troposphere. The disappearance of vertical profiles during the morning coincided with the transition from a stable NBL to a well-mixed convective layer. The vertical profiles were dependent on both surface NOx emissions and the vertical stability of the NBL. The analysis of Ox (the sum of O3 and NO 2) vertical distribution reveals the dominant role of the O3+NO reaction for the vertical variations of NBL chemistry in typical urban areas. Dry deposition, direct emissions, and other chemical pathways also play a role in some circumstances. Strong positive NO3 vertical gradients are predominantly determined by NO3 loss processes and the vertical distribution of the reservoir species (N2O5). The altitude-dependent NO3-N2O5 chemistry suggests complex vertical distributions of atmospheric denoxification, which is critical for nocturnal Ox loss. A 1-D chemical transport model was applied to study these vertical profiles and the relevant chemical processes. Model results agree well with the general features of observed profiles, showing its applicability for describing the altitude-dependent NBL chemistry and predicting the initial atmospheric conditions for photochemistry in typical urban areas. The NBL Ox budget and the ultimate impact on O3 levels are discussed based on field observations and model results. Although the nighttime O x loss dominates the O3 reduction in clean areas, the NO x accumulation in a polluted urban NBL is found to have a strong impact on morning O3 levels.