The presence of oxygenated organic compounds in the troposphere strongly influences key atmospheric processes. Such oxygenated species are, for example, carriers of reactive nitrogen and are easily photolysed, producing free radicals-and so influence the oxidizing capacity and the ozone-forming potential of the atmosphere-and may also contribute significantly to the organic component of aerosols. But knowledge of the distribution and sources of oxygenated organic compounds, especially in the Southern Hemisphere, is limited. Here we characterize the tropospheric composition of oxygenated organic species, using data from a recent airborne survey conducted over the tropical Pacific Ocean (30 degrees N to 30 degrees S). Measurements of a dozen oxygenated chemicals (carbonyls, alcohols, organic nitrates, organic pernitrates and peroxides), along with several C2-C8 hydrocarbons, reveal that abundances of oxygenated species are extremely high, and collectively, oxygenated species are nearly five times more abundant than non-methane hydrocarbons in the Southern Hemisphere. Current atmospheric models are unable to correctly simulate these findings, suggesting that large, diffuse, and hitherto-unknown sources of oxygenated organic compounds must therefore exist. Although the origin of these sources is still unclear, we suggest that oxygenated species could be formed via the oxidation of hydrocarbons in the atmosphere, the photochemical degradation of organic matter in the oceans, and direct emissions from terrestrial vegetation.

[1] Continuous CO measurements were obtained at Cheeka Peak Observatory (CPO, 48.3°N, 124.6°W, 480 m), a coastal site in Washington state, between 9 March 2001 and 31 May 2002. We analyze these observations as well as CO observations at ground sites throughout the North Pacific using the GEOS-CHEM global tropospheric chemistry model to examine the seasonal variations of Asian long-range transport. The model reproduces the observed CO levels, their seasonal cycle and day-to-day variability, with a 5-20 ppbv negative bias in winter/spring and 5-10 ppbv positive bias during summer. Asian influence on CO levels in the North Pacific troposphere maximizes during spring and minimizes during summer, ranging from 91 ppbv (44% of total CO) to 52 ppbv (39%) along the Asian Pacific Rim and from 44 ppbv (30%) to 24 ppbv (23%) at CPO. Maximum export of Asian pollution to the western Pacific occurs at 20°-50°N during spring throughout the tropospheric column, shifting to 30°-60°N during summer, mostly in the upper troposphere. The model captures five particularly strong transpacific transport events reaching CPO (four in spring, one in winter) resulting in 20-40 ppbv increases in observed CO levels. Episodic long-range transport of pollutants from Asia to the NE Pacific occurs throughout the year every 10, 15, and 30 days in the upper, middle, and lower troposphere, respectively. Lifting ahead of cold fronts followed by transport in midlatitude westerlies accounts for 78% of long-range transport events reaching the NE Pacific middle and upper troposphere. During summer, convective injection into the upper troposphere competes with frontal mechanisms in this export. Most events reaching the NE Pacific lower troposphere below 2 km altitude result from boundary layer outflow behind cold fronts (for spring) or ahead of cold fronts (for other seasons) followed by low-level transpacific transport.

Abstract. A large number of oxygenated organic chemicals (peroxyacyl nitrates, alkyl nitrates, acetone, formaldehyde, methanol, methylhydroperoxide, acetic acid and formic acid) were measured during the 1997 Subsonic Assessment (SASS) Ozone and Nitrogen Oxide Experiment (SONEX) airborne field campaign over the Atlantic. In this paper, we present a first picture of the distribution of these oxygenated organic chemicals (Ox-organic) in the troposphere and the lower stratosphere, and assess their source and sink relationships. In both the troposphere and the lower stratosphere, the total atmospheric abundance of these oxygenated species (ZOx-organic) nearly equals that of total nonmethane hydrocarbons (ZNMHC), which have been traditionally measured.

[1] Extensive chemical characterization of ozone (O 3 ) depletion events in the Arctic boundary layer during the TOPSE aircraft mission in March-May 2000 enables analysis of the coupled chemical evolution of bromine (BrO x ), chlorine (ClO x ), hydrogen oxide (HO x ) and nitrogen oxide (NO x ) radicals during these events. We project the TOPSE observations onto an O 3 chemical coordinate to construct a chronology of radical chemistry during O 3 depletion events, and we compare this chronology to results from a photochemical model simulation. Comparison of observed trends in ethyne (oxidized by Br) and ethane (oxidized by Cl) indicates that ClO x chemistry is only active during the early stage of O 3 depletion (O 3 > 10 ppbv). We attribute this result to the suppression of BrCl regeneration as O 3 decreases. Formaldehyde and peroxy radical concentrations decline by factors of 4 and 2 respectively during O 3 depletion and we explain both trends on the basis of the reaction of CH 2 O with Br. Observed NO x concentrations decline abruptly in the early stages of O 3 depletion and recover as O 3 drops below 10 ppbv. We attribute the initial decline to BrNO 3 hydrolysis in aerosol, and the subsequent recovery to suppression of BrNO 3 formation as O 3 drops. Under halogen-free conditions we find that HNO 4 heterogeneous chemistry could provide a major NO x sink not included in standard models. Halogen radical chemistry in the model can produce under realistic conditions an oscillatory system with a period of 3 days, which we believe is the fastest oscillation ever reported for a chemical system in the atmosphere.

[1] Airborne measurements of CH 2 O were acquired employing tunable diode laser absorption spectroscopy during the 2001 Transport and Chemical Evolution Over the Pacific (TRACE-P) study onboard NASA's DC-8 aircraft. Above $2.5 km, away from the most extreme pollution influences and heavy aerosol loadings, comprehensive comparisons with a steady state box model revealed agreement to within ±37 pptv in the measurement and model medians binned according to altitude and longitude. Likewise, a near unity slope (0.98 ± 0.03) was obtained from a bivariate fit of the measurements, averaged into 25 pptv model bins, versus the modeled concentrations for values up to $450 pptv. Both observations suggest that there are no systematic biases on average between CH 2 O measurements and box model results out to model values $450 pptv. However, the model results progressively underpredict the observations at higher concentrations, possibly due to transport effects unaccounted for in the steady state model approach. The assumption of steady state also appears to contribute to the scatter observed in the point-by-point comparisons. The measurement-model variance was further studied employing horizontal flight legs. For background legs screened using a variety of nonmethane hydrocarbon (NMHC) tracers, measurement and model variance agreed to within 15%. By contrast, measurement variance was $60% to 80% higher than the model variance, even with small to modest elevations in the NMHC tracers. Measurement-model comparisons of CH 2 O in clouds and in the lower marine troposphere in the presence of marine aerosols suggest rather significant CH 2 O uptake by as much as 85% in one extreme case compared to expectations based on modeled gas phase processes.

[1] During the Tropospheric Ozone Production about the Spring Equinox (TOPSE) aircraft program, ozone depletion events (ODEs) in the high latitude surface layer were investigated using lidar and in situ instruments. Flight legs of 100 km or longer distance were flown 32 times at 30 m altitude over a variety of regions north of 58°between early February and late May 2000. ODEs were found on each flight over the Arctic Ocean but their occurrence was rare at more southern latitudes. However, large area events with depletion to over 2 km altitude in one case were found as far south as Baffin Bay and Hudson Bay and as late as 22 May. There is good evidence that these more southern events did not form in situ but were the result of export of ozone-depleted air from the surface layer of the Arctic Ocean. Surprisingly, relatively intact transport of ODEs occurred over distances of 900-2000 km and in some cases over rough terrain. Accumulation of constituents in the frozen surface over the dark winter period cannot be a strong prerequisite of ozone depletion since latitudes south of the Arctic Ocean would also experience a long dark period. Some process unique to the Arctic Ocean surface or its coastal regions remains unidentified for the release of ozone-depleting halogens. There was no correspondence between coarse surface features such as solid ice/snow, open leads, or polynyas with the occurrence of or intensity of ozone depletion over the Arctic or subarctic regions. Depletion events also occurred in the absence of long-range transport of relatively fresh ''pollution'' within the high latitude surface layer, at least in spring 2000. Direct measurements of halogen radicals were not made. However, the flights do provide detailed information on the vertical structure of the surface layer and, during the constant 30 m altitude legs, measurements of a variety of constituents including hydroxyl and peroxy radicals. A summary of the behavior of these constituents is made. The measurements were consistent with a source of formaldehyde from the snow/ice surface. Median NO x in the surface layer was 15 pptv or less, suggesting that surface emissions were substantially converted to reservoir constituents by 30 m altitude and that ozone production rates were small (0.15-1.5 ppbv/d) at this altitude. Peroxyacetylnitrate (PAN) was by far the major constituent of NO y in the surface layer independent of the ozone mixing ratio.

We characterize the chemical composition of Asian continental outflow observed during the NASA Transport and Chemical Evolution over the Pacific (TRACE‐P) mission during February–April 2001 in the western Pacific using data collected on the NASA DC‐8 aircraft. A significant anthropogenic impact was present in the free troposphere and as far east as 150°E longitude reflecting rapid uplift and transport of continental emissions. Five‐day backward trajectories were utilized to identify five principal Asian source regions of outflow: central, coastal, north‐northwest (NNW), southeast (SE), and west‐southwest (WSW). The maximum mixing ratios for several species, such as CO, C2Cl4, CH3Cl, and hydrocarbons, were more than a factor of 2 larger in the boundary layer of the central and coastal regions due to industrial activity in East Asia. CO was well correlated with C2H2, C2H6, C2Cl4, and CH3Cl at low altitudes in these two regions (r2 ∼ 0.77–0.97). The NNW, WSW, and SE regions were impacted by anthropogenic sources above the boundary layer presumably due to the longer transport distances of air masses to the western Pacific. Frontal and convective lifting of continental emissions was most likely responsible for the high altitude outflow in these three regions. Photochemical processing was influential in each source region resulting in enhanced mixing ratios of O3, PAN, HNO3, H2O2, and CH3OOH. The air masses encountered in all five regions were composed of a complex mixture of photochemically aged air with more recent emissions mixed into the outflow as indicated by enhanced hydrocarbon ratios (C2H2/CO ≥ 3 and C3H8/C2H6 ≥ 0.2). Combustion, industrial activities, and the burning of biofuels and biomass all contributed to the chemical composition of air masses from each source region as demonstrated by the use of C2H2, C2Cl4, and CH3Cl as atmospheric tracers. Mixing ratios of O3, CO, C2H2, C2H6, SO2, and C2Cl4 were compared for the TRACE‐P and PEM‐West B missions. In the more northern regions, O3, CO, and SO2 were higher at low altitudes during TRACE‐P. In general, mixing ratios were fairly similar between the two missions in the southern regions. A comparison between CO/CO2, CO/CH4, C2H6/C3H8, NOx/SO2, and NOy/(SO2 + nss‐SO4) ratios for the five source regions and for the 2000 Asian emissions summary showed very close agreement indicating that Asian emissions were well represented by the TRACE‐P data and the emissions inventory.

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