HR: 09:00h
AN: A51G-04    [Abstracts]
TI: Interpretation of unexpected high daytime values of nitrous acid (HONO) and its implications for the OH budget in daytime urban environments
AU: * Trick, S
EM: strick@atmos.ucla.edu
AF: University of California Los Angeles, Department of Atmospheric and Oceanic Sciences, 7127 Math Sciences Bldg., 405 Hilgard Avenue, Box 951565, Los Angeles, CA 90095-1565 United States
AU: * Trick, S
EM: strick@atmos.ucla.edu
AF: Ruprecht - Karls Universit\"{a}t Heidelberg, Institut f\"{u}r Umweltphysik, Im Neuenheimer Feld 229, Heidelberg, D - 69120 Germany
AU: Geyer, A
EM: andreas@atmos.ucla.edu
AF: University of California Los Angeles, Department of Atmospheric and Oceanic Sciences, 7127 Math Sciences Bldg., 405 Hilgard Avenue, Box 951565, Los Angeles, CA 90095-1565 United States
AU: Platt, U
EM: ulrich.platt@iup.uni-heidelberg.de
AF: Ruprecht - Karls Universit\"{a}t Heidelberg, Institut f\"{u}r Umweltphysik, Im Neuenheimer Feld 229, Heidelberg, D - 69120 Germany
AU: Stutz, J
EM: jochen@atmos.ucla.edu
AF: University of California Los Angeles, Department of Atmospheric and Oceanic Sciences, 7127 Math Sciences Bldg., 405 Hilgard Avenue, Box 951565, Los Angeles, CA 90095-1565 United States
AB: For decades, the photolysis of nitrous acid (HONO) has been considered to be an important net source of OH radicals only in the early morning hours. The impact of HONO photolysis on the HO$_{x}$ budget was considered to be of minor impor-tance, compared to the photolysis of O$_{3}$ and HCHO, since HONO levels were generally reported to be close to their photostationary state with OH and NO. Recent field measurements by Differential Optical Absorption Spectroscopy (DOAS) at various urban and suburban locations found that under certain circumstances HONO mixing ratios could significantly exceed this photostationary state, reaching $\sim$ 200 ppt at noon. Thus far, these high levels of HONO could not be explained by any model calculations. Consequently, the contribution of HONO to the daytime OH budget in urban environments could not be quantified satisfactorily. Here, we present an analysis of the daytime HONO observations using a 1-dimensional chemical-transport model. Urban scenarios in the model include, for the first time, chemical interactions of trace gases with the urban canopy. Our study shows that the heterogeneous conversion of NO$_{2}$ on the ground, building walls and roofs has a significant impact on daytime HONO levels. Aerosols were found to be of minor importance under all circumstances. In addition, direct emis-sions of HONO also have to be considered. The model results give further evidence that HONO photolysis can act as an important OH source in urban boundary layer throughout the entire day.
DE: 3337 Numerical modeling and data assimilation
DE: 3307 Boundary layer processes
DE: 0317 Chemical kinetic and photochemical properties
DE: 0322 Constituent sources and sinks
DE: 0368 Troposphere--constituent transport and chemistry
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting