HR: 1340h
AN: A23A-0774    [Abstracts]
TI: Vertical Profiles of Nocturnal O$_3$-NO$_x$ Chemistry in the Urban Boundary Layer --- Field Observations in Phoenix and the Corresponding Model Studies
AU: * Wang, S
EM: shw@atmos.ucla.edu
AF: Department of Atmospheric and Oceanic Sciences, University of California at Los Angeles, 405 Hilgard Avenue, 7127 Math Sciences Bldg, Los Angeles, CA 90095-1565 United States
AU: Stutz, J
EM: jochen@atmos.ucla.edu
AF: Department of Atmospheric and Oceanic Sciences, University of California at Los Angeles, 405 Hilgard Avenue, 7127 Math Sciences Bldg, Los Angeles, CA 90095-1565 United States
AB: Nocturnal boundary layer (NBL) chemistry in urban areas is strongly influenced by surface NO$_{x}$ emissions. Vertical mixing in combination with chemical transformations leads to distinctive vertical profiles of reactive trace gases. The O$_{3}$-NO$_{x}$ chemistry system, therefore, varies with altitude in the stable NBL. To understand the influence of vertical mixing on nocturnal chemistry and to improve the accuracy of urban air pollution models, vertical distributions of a number of trace gases were measured in the lowest 10-140 m of the atmosphere with a long-path DOAS instrument in downtown Phoenix, AZ in June-July, 2001. Here we present and analyze results from these measurements. Strong positive vertical profiles of O$_{3}$ and NO$_{3}$ and negative vertical profiles of NO$_{2}$, HONO, HCHO and SO$_{2}$ were observed during all nights. The magnitudes of gradients were significantly larger than earlier observations in rural or suburban areas due to higher nighttime ground-level emissions. Vertical profiles of O$_{x}$ (O$_{3}$ + NO$_{2}$) were much lower than those of O$_{3}$ and NO$_{2}$. This shows the dominant role of the reaction of NO with O$_{3}$ in the urban NBL. In all cases, total O$_{x}$ levels decreased gradually throughout the night. An analysis of the NO$_{3}$ production rate reveals complex vertical profiles of this parameter depending on the distribution of both NO$_{2}$ and O$_{3}$. The positive NO$_{3}$ profiles are, however, a consequence of strong emissions of NO and VOCs near the surface. The calculated steady-state N$_{2}$O$_{5}$ distribution further implies vertical variations in the atmospheric loss of O$_{x}$ and NO$_{x}$. To quantitatively analyze our observations, we employed a 1-D chemical transport model. The model results generally agree with the observations. Our calculations reveal that the profiles of O$_{x}$ are controlled by the interplay between, NO$_{x}$ emissions, dry deposition of O$_{3}$, and the initial O$_{3}$ level. NO$_{3}$ and N$_{2}$O$_{5}$ chemistry was found to be responsible for a large part of the ultimate O$_{3}$ and NO$_{x}$ loss at night. Details of the model results will be discussed.
DE: 0345 Pollution--urban and regional (0305)
DE: 0365 Troposphere--composition and chemistry
DE: 0368 Troposphere--constituent transport and chemistry
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting