HR: 0830h
AN: A51E-0725    [PDF]
TI: Vertical variations of boundary layer chemistry in the urban environment of Boston
AU: * Wang, S
EM: shw@atmos.ucla.edu
AF: Department of Atmospheric Sciences University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90095-1565 United States
AU: Geyer, A
EM: andreas@atmos.ucla.edu
AF: Department of Atmospheric Sciences University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90095-1565 United States
AU: Stutz, J
EM: jochen@atmos.ucla.edu
AF: Department of Atmospheric Sciences University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90095-1565 United States
AB: Boundary layer chemistry in urban areas is often strongly influenced by surface emissions of NO$_{x}$ and VOCs. Vertical mixing in combination with chemical transformations leads to distinctive vertical profiles of reactive trace gases. Chemistry, therefore, varies with altitude in the lowest 50 m, in particular in the stable nocturnal boundary layer. The influence of vertical mixing on chemistry in the lowest part of the boundary layer is often not accurately described, and a better understanding is needed to improve our ability to model urban air pollution. During the NEAQS/NAOPEX field campaign in July-August, 2002, vertical distributions of NO$_{2}$, O$_{3}$, HONO, HCHO, NO$_{3}$, and several other trace gases were measured in the lowest 10 - 50 m of the atmosphere with a long-path differential optical absorption spectroscopy instrument in a residential area in Boston, MA. Various meteorological parameters were also monitored at the same location. Clear vertical trace gas concentration gradients were observed. In particular, during nights with strong stabilities, nocturnal boundary layer chemistry was clearly altitude dependent. Positive gradients of O$_{3}$ and NO$_{3}$ developed as a result of the fast chemical reactions with NO emitted at the surface. Negative NO$_{2}$ and HONO gradients point to the vertical variation of the heterogeneous NO$_{2}$-HONO conversion process, suggesting the importance of heterogeneous chemistry at the ground and building walls. With the transition from the stable boundary layer to a convective layer after sunrise, the vertical profiles regressed. However, vertical gradients of HCHO and O$_{3}$ were observed in the afternoon on most days of this study. The observations suggest that ground-based emissions and fast photochemistry can lead to vertical profiles of trace gases in an urban boundary layer even during daytime. The vertical stratification of boundary layer chemistry, which depends on many factors including mixing strength, fast photochemistry and the emission strength, will be discussed.
DE: 0345 Pollution--urban and regional (0305)
DE: 0365 Troposphere--composition and chemistry
SC: Atmospheric Sciences [A]
MN: 2003 Fall Meeting