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An effective and commonly used technique for studying the sources, photochemistry, and even the “photochemical age” of trace species is to examine ratios of hydrocarbons by assuming the ratio is independent of transport processes. We present results from mesoscale model calculations that suggest a significant effect by atmospheric mixing on the ratio. We also show that the photochemical age of an air mass derived from the ratio of hydrocarbons is a function of both photochemistry and atmospheric transport. Without additional information, it is not possible to derive a unique value for the age of an air mass from hydrocarbon ratios alone.

Abstract. This paper summarizes measured photodissociation quantum yields for acetone in the 290-320 nm wavelength region for pressures and temperatures characteristic of the upper troposphere. Calculations combine this laboratory data with trace gas concentrations obtained during the NASA and NOAA sponsored Stratospheric Tracers of Atmospheric Transport (STRAT) field campaign, in which measurements of OH, HO2, odd-nitrogen, and other compounds were collected over Hawaii, andwest of California during fall and winter of 1995/1996. OH and HO 2 concentrations within 2 to 5 km layers just below the tropopause are -50% larger than expected from 03, CH 4, and H20 chemistry alone. Although not measured during STRAT, acetone is inferred from CO measurements and acetone-CO correlations from a previous field study. These inferred acetone levels are a significant source of odd-hydrogen radicals that can explain a large part of the discrepancy in the upper troposphere. For lower altitudes, the inferred acetone makes a negligible contribution to HOx (HO+HO2), but influences NOy partitioning. A majorfractionofHOx production by acetone is through CH20 formation, and the HOx discrepancy can also be explained by CH 20 levels in the 20 to 50 pptv range, regardless of the source.

Measurements of NO, NO2, HNO3, particulate nitrate, peroxyacetyl nitrate (PAN), O3, and total reactive odd nitrogen (NOy) were made in the nonurban troposphere during the summer and fall of 1984. The field site was located near Niwot Ridge, Colorado, at an elevation of 3 km. NOy was measured by catalytic reduction to NO, followed by the detection of NO with a chemiluminescence instrument. The other species were measured with conventional techniques. The data and interpretation presented focus primarily on the relationships between a measurement of NOy and concurrent measurements of the individual species, as examined through ratio and correlation plots. Through the separate display of daytime and nighttime data, the plots provide insight into the photochemical nature of the individual species. In addition, the composition of NOy is addressed through a comparison of the measured NOy level with that found for the sum of the measured component species. The NOy level systematically exceeded the sum level, with the difference being larger in the summer than in the fall. The presence of organic nitrate species other than PAN is proposed as one way to account for the observed difference.

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