Chemical and dynamical processes affecting the composition of the tropical Pacific troposphere.

摘要

How long-range transport and chemistry affect the composition of the remote tropical Pacific troposphere is analyzed using observations from the Pacific Exploratory Mission to the Tropics A (PEM-Tropics A), conducted in September--October 1996, and PEM-Tropics B, conducted in March--April 1999, in conjunction with a global three-dimensionalal model with assimilated meteorology.; Considerable biomass burning influence was observed above the South Pacific during PEM-Tropics A. The model reproduces the long-range transport of biomass burning effluents from southern Africa and South America in the westerly subtropical flow. Meteorological conditions in 1996 were particularly favorable for this transport. Biomass burning dominates the supply of nitrogen oxides (NO x = NO + NO2) in the lower troposphere over the South Pacific (through long-range transport and decomposition of peroxyacetyl nitrate), but lightning dominates in the upper troposphere. Observations in the upper troposphere indicate low ratios of nitric acid (HNO3) to NOx, and a chemical imbalance in the NOx budget. The model reproduces these observations and attributes them to the subsidence of primary NOx injected by lightning in the uppermost troposphere, rather than any fast chemistry recycling HNO3 to NOx.; Biomass burning in southeast Asia contributed most of the carbon monoxide (CO) enhancements observed in the free troposphere over the northern tropical Pacific during PEM-Tropics B. Fossil fuel combustion in Europe and Asia caused most of the observed CO enhancements in the boundary layer over the north Pacific; the European influence dominated north of 35°N. North American pollution made little contribution to CO anywhere over the Pacific. Circulation of Eurasian industrial pollution around the North Pacific High and into the trade winds produced a "river of pollution" flowing in the lower troposphere towards the western equatorial Pacific. Elevated CO concentrations observed in the upper troposphere over the southeastern Pacific provide evidence for interhemispheric exchange through a narrow region of westerlies. This westerly duct was the most important pathway for interhemispheric exchange during PEM-Tropics B.; The sources of NOx to the tropical Pacific troposphere during PEM-Tropics B are examined. The model reproduces the spatial distribution of the observations despite underestimating NO, HNO3, HNO 3/NOx, peroxyacetyl nitrate (PAN), and PAN/NOx above the South Pacific. Decreasing the simulated scavenging of HNO3 in deep convection improves the simulation of NO, HNO3, and HNO3/NOx. The simulation of PAN and PAN/NO x is corrected by setting acetone, which the model underestimates, to match the PEM-Tropics B observations. In contrast, fixing the model to match observed acetaldehyde, which is also underestimated, causes severe overestimates of PAN and PAN/NOx over the tropical Pacific, indicating that our current understanding of NOy chemical cycling is incompatible with the acetaldehyde observations.