1993 Tropospheric OH Photochemistry Experiment: A summary and perspective

Abstract

Abstract. The 1993 OH Tropospheric Photochemistry Experiment was performed in the forested Colorado mountains during late summer and early fall 1993. Measurements of the OH radical were made by four different techniques; one pair provided an extensive intercomparison data set. A large number of other species that control OH and HO2 concentrations were also determined. These ancillary variables have been useful both in understanding intercomparison issues and in testing detailed model predictions of OH photochemistry in this environment. The paper places this campaign in historical and scientific perspective; it discusses the results in terms of their contributions both to our ability to measure OH radicals and to our understanding of the pertinent tropospheric photochemistry; and it provides a view toward, and lessons for, future campaigns. We conclude that OH is indeed being measured and that it forms very useful tests of photochemical models but that we do not fully understand the underlying chemistry in this environment.

Kinetics of the OH‐initiated oxidation of isoprene

“…(1) Recent tropospheric field measurements have shown that there is still a significant disagreement between measured OH concentrations and the predictions from regional scale models [Crosley, 1997;McKeen et al, 1997]. Isoprene can be a major OH sink under certain conditions accounting for up to 30% of the OH removal rate [Biesenthal and Bottenheim, 1998 …”

Direct observations of daytime NO3: Implications for urban boundary layer chemistry

“…Today at least four radical species have been found to be important in the boundary layer: OH, NO 3 , O 3 , and, in certain environments, reactive halogen species [e.g., Atkinson, 2000;Crosley, 1997;Finlayson-Pitts and Pitts, 2000;Lelieveld and Dentener, 2000;Platt et al, 2002;Wayne et al, 1995]. For many years, a clear distinction was made between daytime ''photo chemistry'', with OH being the key radical, and nighttime radical chemistry, which is controlled by NO 3 .…”

“…For many years, a clear distinction was made between daytime ''photo chemistry'', with OH being the key radical, and nighttime radical chemistry, which is controlled by NO 3 . Consequently, OH was neglected during the night and NO 3 during the day [e.g., Armerding et al, 1997;Atkinson, 2000;Crosley, 1997;Eisele et al, 1997;Finlayson-Pitts and Pitts, 2000;Fuentes et al, 2000;Kleinman, 2000;Logan et al, 1981;Monks et al, 1998;Platt, 1991;Wayne et al, 1991;Weaver et al, 1996]. In fact, OH and NO 3 measurements were often paused during nighttime and daytime, respectively [e.g., Allan et al, 1999;Brune et al, 1995;Geyer et al, 2001a;Hard et al, 1995;Heintz et al, 1996;Holland et al, 1998;Mount et al, 1997;Platt et al, 1981].…”

New insights on OH: Measurements around and in clouds

“…While its ambient concentration is low (typically <0.3 pptv), its high reactivity makes it the most important tropospheric cleansing agent, responsible for the removal of CH4 and numerous non-methane hydrocarbons [Crosley, 1997]. Through reactions with NO2, 802, and DMS, it is responsible for the production of HNO3 and H2SO4, compounds that are present in acid precipitation and are significant in the formation and growth of aerosols [Calvert et al, 1985].…”