The first measurements of peroxy (HO 2 +RO 2 ) and hydroxyl (OH) radicals above the arctic snowpack were collected during the summer 2003 campaign at Summit, Greenland. The median measured number densities for peroxy and hydroxyl radicals were 2.2 Â 10 8 mol cm À3 and 6.4 Â 10 6 mol cm À3, respectively. The observed peroxy radical values are in excellent agreement (R 2 ¼ 0:83, M=O ¼ 1:06) with highly constrained model predictions. However, calculated hydroxyl number densities are consistently more than a factor of 2 lower than the observed values. These results indicate that our current understanding of radical sources and sinks is in accord with our observations in this environment but that there may be a mechanism that is perturbing the (HO 2 +RO 2 )/OH ratio. This observed ratio was also found to depend on meteorological conditions especially during periods of high winds accompanied by blowing snow. Backward transport model simulations indicate that these periods of high winds were characterized by rapid transport (1-2 days) of marine boundary layer air to Summit. These data suggest that the boundary layer photochemistry at Summit may be periodically impacted by halogens. r

Abstract. Observations of OH, NO, and actinic flux at the SouthPole surface during December 1998 suggest a surprisingly active photochemical environment which should result in photochemical production of ozone. Long-term South Pole in situ ozone data as well as sonde data also appear to support this conclusion. Other possible factors contributing to ozone variability such as stratospheric influence and the origin of air transported to the South Pole are also explored. Based on box model calculations it is estimated that photochemistry could add 2.2 to 3.6 ppbv/day of ozone to surface air parcels residing on the Antarctic polar plateau. Although the oxidizing potential of the polar plateau appears to be exceptionally high for a remote site, it is unlikely that it has a significant impact on surrounding regions such as the Southern Ocean and the Antarctic free troposphere. These new findings do suggest, however, that the enhanced oxidizing power of the polar plateau may need to be considered in interpreting the chemical history of climate proxy species in ice cores.

We use the NASA GEOS-5 transport model with tagged tracers to investigate the contributions of different regional sources of CO and black carbon (BC) to their concentrations in the Western Arctic (i.e., 50–90° N and 190–320° E) in spring and summer 2008. The model is evaluated by comparing the results with airborne measurements of CO and BC from the NASA Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) field campaigns to demonstrate the strengths and limitations of our simulations. We also examine the reliability of tagged CO tracers in characterizing air mass origins using the measured fossil fuel tracer of dichloromethane and the biomass burning tracer of acetonitrile. Our tagged CO simulations suggest that most of the enhanced CO concentrations (above background level from CH₄ production) observed during April originate from Asian anthropogenic emissions. Boreal biomass burning emissions and Asian anthropogenic emissions are of similar importance in July domain wise, although the biomass burning CO fraction is much larger in the area of the ARCTAS field experiments. The fraction of CO from Asian anthropogenic emissions is larger in spring than in summer. European sources make up no more than 10% of CO levels in the campaign domain during either period. Comparisons of CO concentrations along the flight tracks with regional averages from GEOS-5 show that the along-track measurements are representative of the concentrations within the large domain of the Western Arctic in April but not in July

[1] The first direct in situ measurements of HO 2 NO 2 in the upper troposphere were performed from the NASA DC-8 during the Intercontinental Chemical Transport Experiment-North America 2004 with a chemical ionization mass spectrometer (CIMS). These measurements provide an independent diagnostic of HO x chemistry in the free troposphere and complement direct observations of HO x , because of the dual dependency of HO 2 NO 2 on HO x and NO x . On average, the highest HO 2 NO 2 mixing ratio of 76 pptv (median = 77 pptv, s = 39 pptv) was observed at altitudes of 8-9 km. Simple steady state calculations of HO 2 NO 2 , constrained by measurements of HO x , NO x, and J values, are in good agreement (slope = 0.90, R 2 = 0.60, and z = 5.5-7.5 km) with measurements in the midtroposphere where thermal decomposition is the major loss process. Above 8 km the calculated steady state HO 2 NO 2 is in poor agreement with observed values (R 2 = 0.20) and is typically larger by a factor of 2.4. Conversely, steady state calculations using model-derived HO x show reasonable agreement with the observed HO 2 NO 2 in both the midtroposphere (slope = 0.96, intercept = 7.0, and R 2 = 0.63) and upper troposphere (slope = 0.80, intercept = 32.2, and R 2 = 0.58). These results indicate that observed HO 2 and HO 2 NO 2 are in poor agreement in the upper troposphere but that HO 2 NO 2 levels are consistent with current photochemical theory.

[1] Aircraft measurements of ozone (O 3 ) and its precursors (reactive nitrogen, CO, nonmethane hydrocarbons) were made over the western Pacific during the Transport and Chemical Evolution Over the Pacific (TRACE-P) campaign, which was conducted during February-April 2001. Biomass burning activity was high over Southeast Asia (SEA) during this period (dry season), and convective activity over SEA frequently transported air from the boundary layer to the free troposphere, followed by eastward transport to the sampling region over the western Pacific south of 30°N. This data set allows for systematic investigations of the chemical and physical processes in the outflow from SEA. Methyl chloride (CH 3 Cl) and CO are chosen as primary and secondary tracers, respectively, to gauge the degree of the impact of emissions of trace species from biomass burning. Biomass burning is found to be a major source of reactive nitrogen (NO x , PAN, HNO 3 , and nitrate) and O 3 in this region from correlations of these species with the tracers. Changes in the abundance of reactive nitrogen during upward transport are quantified from the altitude change of the slopes of the correlations of these species with CO. NO x decreased with altitude due to its oxidation to HNO 3 . On the other hand, PAN was conserved during transport from the lower to the middle troposphere, consistent with its low water solubility and chemical stability at low temperatures. Large losses of HNO 3 and nitrate, which are highly water soluble, occurred in the free troposphere, most likely due to wet removal by precipitation. This has been shown to be the major pathway of NO y loss in the middle troposphere. Increases in the mixing ratios of O 3 and its precursors due to biomass burning in SEA are estimated using the tracers. Enhancements of CO and total reactive nitrogen (NO y ), which are directly emitted from biomass burning, were largest at 2-4 km. At this altitude the increases in NO y and O 3 were 810 parts per trillion by volume (pptv) and 26 parts per billion by volume (ppbv) above their background values of 240 pptv and 31 ppbv, respectively. The slope of the O 3 -CO correlation in biomass burning plumes was similar to those observed in fire plumes in northern Australia, Africa, and Canada. The O 3 production efficiency (OPE) derived from the O 3 -CO slope and NO x /CO emission ratio (ER) is shown to be positively correlated with the C 2 H 4 /NO x ER, indicating that the C 2 H 4 /NO x ER is a critical parameter in determining the OPE. Comparison of the net O 3 flux across the western Pacific region and total O 3 production due to biomass burning in JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, D15S12,