Ozone production rates as a function of NOx abundances and HOx production rates in the Nashville urban plume

Abstract

Tropospheric O 3 concentrations are functions of the chain lengths of NO x (NO x ≡ NO + NO 2) and HO x (HO x ≡ OH + HO 2 + RO 2) radical catalytic cycles. For a fixed HO x source at low NO x concentrations, kinetic models indicate the rate of O 3 production increases linearly with increases in NO x concentrations (NO x limited). At higher NO x concentrations, kinetic models predict ozone production rates decrease with increasing NO x (NO x saturated). We present observations of NO, NO 2, O 3, OH, HO 2, H 2 CO, 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 HO x production rate (PHOx). These observations provide quantitative evidence for the response of PO3 to NO x. For high PHOx (0.5 < PHOx < 0.7 ppt/s), O 3 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 RO 2 + HO 2 reaction for the classes of RO 2 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 RO 2 + HO 2 → ROOH + O 2, is reduced by a factor of 3–12. Alternatively, these peroxides could be rapidly photolyzed under atmospheric conditions making them at best a temporary HO x reservoir. This result implies that O 3 production in or near urban areas with similar hydrocarbon reactivity and HO x production rates may be NO x saturated more often than current models suggest.