Grab samples for nonmethane hydrocarbons (NMHCs) were collected from three sites: daily at Alert, Northwest Territories (82.5° N, 62.3° W) from January 21 to April 19, daily at an ice floe 150 km north of Alert from April 9 to 25, and on an aerial survey conducted in April over the Arctic archipelago. Insitu measurements of n‐butane and i‐pentane were also made hourly at Alert from April 2 to April 15. During the dark period (January to March), C2‐C6 hydrocarbon concentrations correlated with those of methane. Concentrations declined gradually from January to April, consistent with removal by HO radicals. On the other hand, during low‐ozone periods in April an additional decrease in NMHC concentrations and change in distribution were observed. Concentration changes of alkanes were correlated to Cl atom reaction rate constants. Acetylene displayed a greater change in concentration than predicted from chlorine kinetics, possibly indicating additional removal by Br atoms. The Br atom concentration derived from the depletion of acetylene can account for the low‐ozone concentrations periodically observed at Alert. The estimated Cl atom concentration is too small to be a significant loss mechanism for ozone. Thus the data from Alert and the ice floe site provide evidence for Cl and Br atom chemistry during the ozone depletion episodes observed at polar sunrise.

Abstract. The concept of ozone production efficiency (OPE) per unit NOx is based on photochemical models and provides a tool with which to assess potential regional tropospheric ozone control strategies involving NOx emissions reductions. An aircraft study provided data from which power plant emissions removal rates and measurement-based estimates of OPE are estimated. This study was performed as part of the Southern Oxidants Study-1995 Nashville intensive and focuses on the evolution of NOx, SO2, and ozone concentrations in power plant plumes during transport. Two approaches are examined. A mass balance approach accounts for mixing effects within the boundary layer and is used to calculate effective boundary layer removal rates for NOx and SO2 and to estimate net OPE. Net OPE is more directly comparable to photochemical model results than previous measurement-based estimates. Derived net production efficiencies from mass balance range from 1 to 3 molecules of ozone produced per molecule of NOx emitted. A concentration ratio approach provides an estimate of removal rates of primary emissions relative to a tracer species. This approach can be combined with emissions ratio information to provide upper limit estimates of OPE that range from 2 to 7. Both approaches illustrate the dependence of ozone production on NOx source strength in these large point source plumes. The dependence of total ozone production, ozone production efficiency, and the rate of ozone production on NOx source strength is examined. These results are interpreted in light of potential ozone control strategies for the region.

[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.