Publications

A zero‐dimensional (0‐D) model has been applied to study the sources of hydrogen oxide radicals (HO x = HO 2 + OH) in the tropical upper troposphere during the Pacific Exploratory Mission in the tropics (PEM‐Tropics B) aircraft mission over the South Pacific in March–April 1999. Observations made across the Southern Pacific Convergence Zone (SPCZ) and the southern branch of the Intertropical Convergence Zone (ITCZ) provided the opportunity to contrast the relative contributions of different sources of HO x , in a nitrogen oxide radical (NO x )‐limited regime, in relatively pristine tropical air. The primary sources of HO x vary significantly along the flight track, in correlation with the supply of water vapor. The latitudinal variation of HO x sources is found to be controlled also by the levels of NO x and primary HO x production rates P (HO x ). Budget calculations in the 8‐ to 12‐km altitude range show that the reaction O( 1 D) + H 2 O is a major HO x source in the cloud region traversed by the aircraft, including SPCZ and the southern branch of the ITCZ. Production from acetone becomes significant in drier region south of 20°S and can become dominant where water vapor mixing ratios lie under 200 ppmv. Over the SPCZ region, in the cloud outflow, CH 3 OOH transported by convection accounts for 22% to 64% of the total primary source. Oxidation of methane amplifies the primary HO x source by 1–1.8 in the dry regions.

In situ aircraft measurements of O 3 , CO, total reactive nitrogen (NO y ), NO, and non‐methane hydrocarbons (NMHCs) were made over the western Pacific Ocean and Australia during the Biomass Burning and Lightning Experiment (BIBLE) A and B conducted in August–October 1998 and 1999. Generally, similar features were seen in the BIBLE A and B data in the latitudinal variations of these species in the troposphere from 35°N to 28°S at longitudes of 120°–150°E. The focus of this paper is to describe the characteristics of air masses sampled at 15°N–10°S (tropical Pacific) and 10°S–28°S (over Australia). With the exception of occasional enhancements in reactive nitrogen seen over New Guinea associated with lightning activities, the tropical Pacific region is distinguished from the rest of the region by smaller concentrations of these trace species. This can be explained in terms of the absence of surface sources over the ocean, lack of stratospheric intrusion, and the preclusion of midlatitude air and air from the west due to active convection throughout the troposphere. The median O 3 , CO, NO y , and NO mixing ratios in tropical air above 4 km were about 15–20 parts per billion by volume (ppbv), 60–75 ppbv, 20–100 parts per trillion by volume (pptv), and 5–40 pptv, respectively. Data obtained from PEM‐West A and B conducted in 1991 and 1994 showed similar latitudinal features, although the PEM‐West A values were somewhat elevated due to dominating westerly winds in the lower troposphere associated with El Niño. Over Australia, the levels of O 3 , CO, NO y , NO, and NMHCs were elevated throughout the troposphere over those observed in the tropical Pacific both in 1998 and 1999. The effect from biomass burning that occurred in northern Australia was limited to within the boundary layer because of strong subsidence in the period. Analyses based on 14‐day back trajectories identified free tropospheric air over Australia that originated from Indonesia, the Indian Ocean, Africa, and southern midlatitudes. The levels of O 3 , CO, NO y , and NMHCs in these air masses were much higher than those from the tropical Pacific due to their stronger sources from biomass burning and lightning. These values are compared with those obtained in the South Pacific during PEM‐Tropics A. Effects of biomass burning and lightning are discussed as possible sources of O 3 and its precursors in these air masses.

Data from 10 aircraft programs recorded over the central/eastern North Pacific (180°–120°W, 0°–45°N) were assembled for the purpose of assessing an atmospheric chemical climatology for this region. It represents an early effort at carrying out this task and, thus, it can be expected to undergo many updates in the future. Such a database is useful in both gaining further insights concerning the fundamental processes controlling the chemistry of this region as well as serving as an important baseline by which to evaluate future change. Critical photochemical precursors examined included O 3 , CO, NO x , and H 2 O. In addition, the distribution profiles for select nonmethane hydrocarbons were explored. The precursor data were analyzed according to latitude, pressure altitude, and season of the year. Contrasting the spring and fall measurements, major trends that surfaced in both ozone and CO included observing elevated levels in spring compared to fall, with the largest spring increase occurring at latitudes north of 15°N. Both NO x and H 2 O showed trends quite different from that of CO and O 3 , with an indication of higher mixing ratio levels during the fall season. Variations in precursor distribution patterns within a season will be explained based on climatological flow patterns for the region and the resulting connection to source regions. Where possible, comparisons were performed with other sampling strategies, including ground‐based observations, sonde, and satellite data, and data collected during shuttle missions. Overall, these comparisons revealed a reasonably high level of correspondence between the airborne ensemble data and those collected using other sampling strategies.

The Pacific Exploration of Asian Continental Emission phase B (PEACE‐B) aircraft mission was conducted over the western Pacific during April–May 2002. During several flights, large enhancements of CO greater than 200 parts per billion by volume (ppbv) were observed at altitudes between 5 and 10 km. In this paper, we describe vertical transport mechanisms over east Asia that were responsible for these enhancements, using the European Centre for Medium‐Range Weather Forecasts (ECMWF) meteorological data and infrared cloud data obtained by the Geostationary Meteorological Satellite (GMS)‐5. A case study for the highest CO event shows that it was likely due to vertical transport of pollutants caused by deep cumulus convection along a quasi‐stationary frontal zone, which was formed over central China (around the Yangtze River at 30°N). In the mean meteorological field during the PEACE‐B period, the warm, moist low‐level southerlies converged into the frontal zone, sustaining cross‐frontal temperature and moisture contrasts. Along the frontal zone, the mean vertical motion was distinctively upward, and a subtropical jet aloft was found to transport uplifted air parcels efficiently into the western Pacific. In this study, criteria to identify deep convection are defined using both the ECMWF and GMS data. The results show that convective activity, which was generally high below the subtropical jet, played an important role in producing updrafts over central China. The convective transport resulted mainly from a limited number of episodes, each of which followed the development of a weak cyclonic disturbance. Back trajectories of air parcels sampled at altitudes between 4 and 13 km on board the aircraft during PEACE‐B show that among the air parcels originating from the 800‐hPa level or below, 69% were likely to have undergone convective uplifting. In addition to convection, sloping isentropes often observed along the quasi‐stationary jet axis yielded persistent slow quasi‐adiabatic uplifting of air over the Far East, which was occasionally intensified with a classical warm conveyor belt (WCB) airstream on the passage of migratory cyclonic disturbances. Meteorological conditions during PEACE‐B were thus favorable for the uplifting of boundary layer air influenced by anthropogenic emissions over central China. These results are consistent with the relatively high levels of Halon 1211 (CF 2 ClBr), a good tracer of Chinese anthropogenic emissions, observed in the air parcels that were likely uplifted in the frontal zone.

Aircraft measurements of key reactive nitrogen species (NO, NO 2 , HNO 3 , PAN, PPN, NO 3 ‐, NO y ), C 1 to C 6 hydrocarbons, acetone, O 3 , chemical tracers (C 2 Cl 4 CO), and important meteorological parameters were performed over eastern Canada during July to August 1990 at altitudes between 0 and 6 km as part of an Arctic Boundary Layer Expedition (ABLE3B). In the free troposphere, PAN was found to be the single most abundant reactive nitrogen species constituting a major fraction of NO y and was significantly more abundant than NO x and HNO 3 . PAN and O 3 were well correlated both in their fine and gross structures. Compared to data previously collected in the Arctic/subarctic atmosphere over Alaska (ABLE3A), the lower troposphere (0–4 km) over eastern Canada was found to contain larger reactive nitrogen and anthropogenic tracer concentrations. At higher altitudes (4–6 km) the atmospheric composition was in many ways similar to what was seen over Alaska and supports the view that a large‐scale reservoir of PAN (and NO y ) is present in the upper troposphere over the entire Arctic/subarctic region. The reactive nitrogen budget based on missions conducted from the North Bay site (missions 2–10) showed a small shortfall, whereas the budget for data collected from the Goose Bay operation (missions 11–19) showed essential balance. It is calculated that 15–20 ppt of the observed NO x may find its source from the available PAN reservoir. Meteorological considerations as well as relationships between reactive nitrogen and tracer species suggest that the atmosphere over eastern Canada during summer is greatly influenced by forest fires and transported industrial pollution.

Recent in situ and satellite measurements suggest a contribution of similar to 5 pptv to stratospheric inorganic bromine from short-lived bromocarbons. We conduct a modeling study of the two most important short-lived bromocarbons, bromoform (CHBr3) and dibromomethane (CH2Br2), with the Goddard Earth Observing System Chemistry Climate Model (GEOS CCM) to account for this missing stratospheric bromine. We derive a 'top-down' emission estimate of CHBr3 and CH2Br2 using airborne measurements in the Pacific and North American troposphere and lower stratosphere obtained during previous NASA aircraft campaigns. Our emission estimate suggests that to reproduce the observed concentrations in the free troposphere, a global oceanic emission of 425 Gg Br yr(-1) for CHBr3 and 57 Gg Br yr(-1) for CH2Br2 is needed, with 60% of emissions from open ocean and 40% from coastal regions. Although our simple emission scheme assumes no seasonal variations, the model reproduces the observed seasonal variations of the short-lived bromocarbons with high concentrations in winter and low concentrations in summer. This indicates that the seasonality of short-lived bromocarbons is largely due to seasonality in their chemical loss and transport. The inclusion of CHBr3 and CH2Br2 contributes similar to 5 pptv bromine throughout the stratosphere. Both the source gases and inorganic bromine produced from source gas degradation (Br-y(VSLS)) in the troposphere are transported into the stratosphere, and are equally important. Inorganic bromine accounts for half (2.5 pptv) of the bromine from the inclusion of CHBr3 and CH2Br2 near the tropical tropopause and its contribution rapidly increases to similar to 100% as altitude increases. More than 85% of the wet scavenging of Br-y(VSLS) occurs in large-scale precipitation below 500 hPa. Our sensitivity study with wet scavenging in convective updrafts switched off suggests that Br-y(VSLS) in the stratosphere is not sensitive to convection. Convective scavenging only accounts for similar to 0.2 pptv (4%) difference in inorganic bromine delivered to the stratosphere.