Glen W. Sachse

Ozone and aerosol distributions and air mass characteristics over the South Pacific during the burning season

In situ and laser remote measurements of gases and aerosols were made with airborne instrumentation to establish a baseline chemical signature of the atmosphere above the South Pacific Ocean during the NASA Global Tropospheric Experiment (GTE)/Pacific Exploratory Mission-Tropics A (PEM-Tropics A) conducted in August-October 1996. This paper discusses general characteristics of the air masses encountered during this experiment using an airborne lidar system for measurements of the large-scale variations in ozone (O3) and aerosol distributions across the troposphere, calculated potential vorticity (PV) from the European Centre for Medium-Range Weather Forecasting (ECMWF), and in situ measurements for comprehensive air mass composition. Between 8°S and 52°S, biomass burning plumes containing elevated levels of O3, over 100 ppbv, were frequently encountered by the aircraft at altitudes ranging from 2 to 9 km. Air with elevated O3 was also observed remotely up to the tropopause, and these air masses were observed to have no enhanced aerosol loading. Frequently, these air masses had some enhanced PV associated with them, but not enough to explain the observed O3 levels. A relationship between PV and O3 was developed from cases of clearly defined O3 from stratospheric origin, and this relationship was used to estimate the stratospheric contribution to the air masses containing elevated O3 in the troposphere. The frequency of observation of the different air mass types and their average chemical composition is discussed in this paper.

On the origin of tropospheric ozone and NOx over the tropical South Pacific

The budgets of ozone and nitrogen oxides (NOx = NO + NO2) ill the tropical South Pacific troposphere are analyzed by photochemical point modeling of aircraft observations at 0-12 km altitude from the Pacific Exploratory Mission-Tropics A campaign flown in September- October 1996. The model reproduces the observed NO2/NO concentration ratio to within 30% and has similar success in simulating observed concentrations of peroxides (H202, CH3OOH), lending confidence in its use to investigate ozone chemistry. It is found that chemical production of ozone balances only half of chemical loss in the tropospheric column over the tropical South Pacific. The net loss is 1.8 x 1011 molecules cm -2 S 1. The missing source of ozone is matched by west- erly transport of continental pollution into the region. Independent analysis of the regional ozone budget with a global three-dimensional model corroborates the results from the point model and reveals the importance of biomass burning emissions in South America and Africa for the ozone budget over the tropical South Pacific. In this model, biomass burning increases average ozone concentrations by 7-8 ppbv throughout the troposphere. The NOx responsible for ozone produc- tion within the South Pacific troposphere below 4 km can be largely explained by decomposition of peroxyacetylnitrate (PAN) transported into the region with biomass burning pollution at higher altitudes.

A general model of how fire emissions and chemistry produce African/oceanic plumes (O3, CO, PAN, smoke) in TRACE A

A full-chemistry simulation of the Great African Plume gives one example of a broad conceptual model of the intercontinental pollution of the tropical middle troposphere by lofted biomass burning plumes. This two-dimensional idealization “calibrated” by carbon monoxide distributions links conventional estimates of burning emissions to oceanic concentrations of pollutants. This paper makes use of GRACES, a modular photochemical simulation system, in two forms. The results of the chemically intensive two-dimensional form, using idealized winds, mixing, deposition, and rainout, match the general concentration patterns of a three-dimensional GRACES model study of CO during the TRACE A/SAFARI period of October 1992 (reported separately). The study highlights the importance of simulating the vertical and diurnal variation of the planetary boundary layer and cloud activity. These correlate temporally with the intensity of tropical agricultural burning. We emphasize one situation, the drift northward and eastward of pollution into the interocean convergence region, where it rises by small-scale motions and rides out westward in the lower midtroposphere (<5 km). These effects help set in place large strata of enhanced CO, ozone, and other pollution over the equatorial Atlantic Ocean. Overall, our comparisons of simulations with the TRACE A data on the cycling of CO, NOx, and O3 in the tropical atmosphere suggest substantial agreement of current emission estimates and atmospheric concentrations. In certain regions, ozone is simulated slightly below observed levels. The striking major disagreements are in NOy, (total reactive nitrogen) and HNO3, which are intimately related to CO and O3; this suggests that current theory omits at least one fundamental process.

Atmospheric sampling of Supertyphoon Mireille with NASA DC-8 aircraft on September 27,1991, during PEM-West A

The DC-8 mission of September 27, 1991, was designed to sample air flowing into Typhoon Mireille in the boundary layer, air in the upper tropospheric eye region, and air emerging from the typhoon and ahead of the system, also in the upper troposphere. The objective was to find how a typhoon redistributes trace constituents in the West Pacific region and whether any such redistribution is important on the global scale. The boundary layer air (300 m), in a region to the SE of the eye, contained low mixing ratios of the tracer species 03, CO, C2H6, C2H2, C3H8, C6H6 and CS2 but high values of dimethylsulfide (DMS). The eye region relative to the boundary layer, showed somewhat elevated levels of CO, substantially increased levels of 03, CS2 and all nonmethane hydrocarbons (NMHCs), and somewhat reduced levels of DMS. Ahead of the eye, CO and the NMHCs remained unchanged, 03 and CS2 showed a modest decrease, and DMS showed a substantial decrease. There was no evidence from lidar cross sections of ozone for the downward entrainment of stratospheric air into the eye region; these sections show that low ozone values were measured in the troposphere. The DMS data suggest sub- stantial entrainment of boundary layer air into the system, particularly into the eye wall region. Estimates of the DMS sulphur flux between the boundary layer and the free tro- posphere, based on computations of velocity potential and divergent winds, gave values of about 69 gg S m -2 d-x7faveraged over a 17.5 o grid square encompassing the typhoon. A few hours aer sampling with the DC-8, Mireille passed over Oki Island, just to the north of Japan, producing surface values of ozone of 5.5 ppbv. These 03 levels are consistent with the low tropospheric values found by lidar and are more typical of equatorial regions. We suggest that the central eye region may act like a Taylor column which has moved poleward from low latitudes. The high-altitude photochemical environment within Typhoon Mireille was found to be quite active as evidenced by significant levels of measured gas phase H202 and CH300H and model-computed levels of OH.