Ozone production rates as a function of NOx abundances and HOx production rates...

Thornton, J. A., P. J. Wooldridge, R. C. Cohen, M. Martinez, H. Harder, W. H. Brune, E. J. Williams, J. Roberts, F. Fehsenfeld, S. R. Hall, R. E. Shetter, B. P. Wert, and A. Fried (2002), Ozone production rates as a function of NOx abundances and HOx production rates in the Nashville urban plume, J. Geophys. Res., 107, NO. D12, doi:10.1029/2001JD000932.

Tropospheric O3 concentrations are functions of the chain lengths of NOx (NOx NO + NO2) and HOx (HOx OH + HO2 + RO2) radical catalytic cycles. For a fixed HOx source at low NOx concentrations, kinetic models indicate the rate of O3 production increases linearly with increases in NOx concentrations (NOx limited). At higher NOx concentrations, kinetic models predict ozone production rates decrease with increasing NOx (NOx saturated). We present observations of NO, NO2, O3, OH, HO2, H2CO, actinic flux, and temperature obtained during the 1999 Southern Oxidant Study from June 15 to July 15, 1999, at Cornelia Fort Airpark, Nashville, Tennessee. The observations are used to evaluate the instantaneous ozone production rate (PO3) as a function of NO abundances and the primary HOx production rate (PHOx). These observations provide quantitative evidence for the response of PO3 to NOx. For high PHOx (0.5 < PHOx < 0.7 ppt/s), O3 production at this site increases linearly with NO to ~500 ppt. PO3 levels out in the range 500–1000 ppt NO and decreases for NO above 1000 ppt. An analysis along chemical coordinates indicates that models of chemistry controlling peroxy radical abundances, and consequently PO3, have a large error in the rate or product yield of the RO2 + HO2 reaction for the classes of RO2 that predominate in Nashville. Photochemical models and our measurements can be forced into agreement if the product of the branching ratio and rate constant for organic peroxide formation, via RO2 + HO2 ! ROOH + O2, is reduced by a factor of 3–12. Alternatively, these peroxides could be rapidly photolyzed under atmospheric conditions making them at best a temporary HOx reservoir. This result implies that O3 production in or near urban areas with similar hydrocarbon reactivity and HOx production rates may be NOx saturated more often than current models suggest.

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