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Nocturnal chemistry in the atmospheric boundary layer plays a key role indetermining the initial chemical conditions for photochemistry during thefollowing morning as well as influencing the budgets of O3 andNO2. Despite its importance, chemistry in the nocturnal boundary layer(NBL), especially in heavily polluted urban areas, has received littleattention so far, which greatly limits the current understanding of theprocesses involved. In particular, the influence of vertical mixing onchemical processes gives rise to complex vertical profiles of variousreactive trace gases and makes nocturnal chemistry altitude-dependent. Theprocessing of pollutants is thus driven by a complicated, and not wellquantified, interplay between chemistry and vertical mixing.In order to gain a better understanding of the altitude-dependentnocturnal chemistry in the polluted urban environment, a field studywas carried out in the downtown area of Phoenix, AZ, in summer 2001.Vertical profiles of reactive species, such as O3, NO2,and NO3, were observed in the lowest 140 m of the tropospherethroughout the night. The disappearance of these trace gas verticalgradients during the morning coincided with the morning transitionfrom a stable NBL to a well-mixed convective layer. The verticalgradients of trace gas levels were found to be dependent on bothsurface NOx emission strength and the vertical stability ofthe NBL. The vertical gradients of Ox, the sum of O3and NO2, were found to be much smaller than those of O3and NO2, revealing the dominant role of NO emissions followedby the O3+NO reaction for the altitude-dependence of nocturnalchemistry in urban areas. Dry deposition, direct emissions, andother chemical production pathways of NO2 also play a role forthe Ox distribution. Strong positive vertical gradients ofNO3, that are predominantly determined by NO3 loss nearthe ground, were observed. The vertical profiles of NO3 and thecalculated vertical profiles of its reservoir species(N2O5) confirm earlier model results suggesting complexvertical distributions of atmospheric denoxification processesduring the night

atmospheric chemistry and physics

Vertical profiles of O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; and NO&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt; chemistry in the polluted nocturnal boundary layer in Phoenix, AZ: I. Field observations by long-path DOAS