Project at a Glance

Title: Gas-phase formation rates of nitric acid and its isomers under urban conditions.

Principal Investigator / Author(s): Okumura, Mitchio; Sander, Stanley P

Contractor: California Institute of Technology

Contract Number: 03-333

Research Program Area: Atmospheric Processes

Topic Areas: Chemistry & Reactivity


Ozone formation in urban smog is controlled by a complex set of reactions which includes radical production from photochemical processes, catalytic cycles which convert NO to NO2, and termination steps that tie up reactive intermediates in long-lived reservoirs. The reaction OH + NO2 + M ¨HONO2 + M (1a) is a key termination step because it transforms two short-lived reactive intermediates, OH and NO2, into relatively long-lived nitric acid. Under certain conditions (low VOC/NOx), ozone production in polluted urban airsheds can be highly sensitive to this reaction, but the rate parameters are not well constrained. This report summarizes the results of new laboratory studies of the OH + NO2 + M reaction including direct determination of the overall rate constant and branching ratio for the two reaction channels under atmospherically relevant conditions:

OH + NO2 + M --- > HONO2 + M (1a)

--- > HOONO + M (1b)

where the overall rate constant is k1 = k1a + k1b and the branching ratio is k1b/k1a. Measurements of the pressure dependence of the OH + NO2 bimolecular rate coefficient k1 have been performed in N2, O2, Air and He (50-900 Torr) at 298 K and in Air at 315 K. Measurements of the HOONO/HONO2 branching ratio have been made over the range 50-750 Torr at 298 K. Theoretical calculations of the integrated intensities and correction factors for the HOONO integrated absorbance have been completed. The results of these three studies have been combined to provide fitted fall-off parameters for k1a and k1b over the range of pressures studied at 298 K and 315 K, and recommendations have been made for incorporations in atmospheric chemistry models.

For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753

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