Emissions of volatile chemicals control the hydroxyl radical (OH), the atmosphere's main cleansing agent, and thus the production of secondary pollutants. Accounting for all of these chemicals can be difficult, especially in environments with mixed urban and forest emissions. The first direct measurements of the atmospheric OH reactivity, the inverse of the OH lifetime, were made as part of the Southern Oxidant Study (SOS) at Cornelia Fort Airpark in Nashville, TN in summer 1999. Measured OH reactivity was typically 11 s(-1). Measured OH reactivity was 1.4 times larger than OH reactivity calculated from the sum of the products of measured chemical concentrations and their OH reaction rate coefficients. This difference is statistically significant at the 1sigma uncertainty level of both the measurements and the calculations but not the 2sigma uncertainty level. Measured OH reactivity was 1.3 times larger than the OH reactivity from a model that uses measured ambient concentrations of volatile organic compounds (VOCs), NO, NO2, SO2, and CO. However, it was within approximately 10% of the OH reactivity from a model that includes hydrocarbon measurements made in a Nashville tunnel and scaled to the ambient CO at Cornelia Fort Airpark. These comparisons indicate that 30% of the OH reactivity in Nashville may come from short-lived highly reactive VOCs that are not usually measured in field intensive studies or by US EPA's Photochemical Assessment Monitoring Stations.

[1] OH and HO 2 mixing ratios and total OH reactivity were measured together with photolysis frequencies, NO x , O 3 , many VOCs, and other trace gases during the midsummer 1999 SOS campaign in Nashville, Tennessee. These measurements provided an excellent opportunity to study OH and HO 2 (collectively called HO x ), and their sources and sinks in a polluted metropolitan environment. HO x generally showed the expected diurnal evolution, with maxima around noon of up to about 0.8 pptv of OH and 80 pptv of HO 2 during sunny days. Overall, daytime observed OH and HO 2 were a factor of 1.33 and 1.56 times modeled values, within the combined 2s instrument and model uncertainties. The chain length of HO x , which is determined from the ratio of the measured total OH reactivity that cycles OH to the total HO x loss, was on average 3-8 during daytime and up to 3 during nighttime, in general agreement with expectations. However, differences occurred between observed HO x behavior and expectations from theory and models. First, HO 2 was greater than expected during daytime when NO mixing ratios were high; ozone production did not decrease as expected when NO was greater than 2 ppbv. Ozone production determined by the imbalance of the NO x photostationary state, which was almost twice that from HO 2 , also shows this dependence on NO. Second, the calculated OH production rate, which should equal the measured OH loss rate because OH is in steady state, is instead less than the measured OH loss rate by (1-2) Â 10 7 molecules cm -3 s -1 , with low statistical significance during the day and high statistical significance at night. Third, surprisingly high OH and HO 2 mixing ratios were often observed during nighttime. The nighttime OH mixing ratio and the HO 2 /OH ratio cannot be explained by known reaction mechanisms, even those involving O 3 and alkenes. Because instrument tests have failed to reveal any instrument artifacts, more exotic chemicals or chemistry, such as OH adducts or other radicals that fall apart into OH inside the instrument, are suspected.

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[1] The difficulties in validating aerosol optical depth from spaceborne lidars such as the Geoscience Laser Altimeter System (GLAS) on board IceSat and the lidar on board the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite with ground-based instruments are discussed. Because observations are often not collocated, matching errors, which increase with separation distance, confound the validation of instrumental errors. These matching errors can be assessed by comparing the aerosol total-column optical depth measured in a swath by the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on board the Terra satellite with point Aerosol Robotic Network (AERONET) Sun photometer measurements at different separation distances. In particular, the relationship of the correlation of the two sensors' aerosol total-column optical depth observations with increasing spatial separation is determined. The use of back trajectories to reduce the loss in correlation with increasing spatial separation is then evaluated. Matching errors are found to increase faster over land than over water sites, with the correlation dropping by 20% in 200 km over the land sites and 500 km over the ocean sites. Constraining the area over which the MODIS aerosol optical depth is calculated to within 30°azimuth of the average back trajectory only improved the correlation for a site where long-range transport of aerosols often occurs.

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