Large‐scale latitudinal and vertical distributions of NMHCs and selected halocarbons in the troposphere over the Pacific Ocean during the March‐April 1999 Pacific Exploratory Mission (PEM‐Tropics B)

Nonmethane hydrocarbons (NMHCs) and selected halocarbons were measured in whole air samples collected over the remote Pacific Ocean during NASA's Global Tropospheric Experiment (GTE) Pacific Exploratory Mission‐Tropics B (PEM‐Tropics B) in March and early April 1999. The large‐scale spatial distributions of NMHCs and C 2 Cl 4 reveal a much more pronounced north‐south interhemispheric gradient, with higher concentrations in the north and lower levels in the south, than for the late August to early October 1996 PEM‐Tropics A experiment. Strong continental outflow and winter‐long accumulation of pollutants led to seasonally high Northern Hemisphere trace gas levels during PEM‐Tropics B. Observations of enhanced levels of Halon 1211 (from developing Asian nations such as the PRC) and CH 3 Cl (from SE Asian biomass burning) support a significant southern Asian influence at altitudes above 1 km and north of 10°N. By contrast, at low altitude over the North Pacific the dominance of urban/industrial tracers, combined with low levels of Halon 1211 and CH 3 Cl, indicate a greater influence from developed nations such as Japan, Europe, and North America. Penetration of air exhibiting aged northern hemisphere characteristics was frequently observed at low altitudes over the equatorial central and western Pacific south to ∼5°S. The relative lack of southern hemisphere biomass burning sources and the westerly position of the South Pacific convergence zone contributed to significantly lower PEM‐Tropics B mixing ratios of the NMHCs and CH 3 Cl south of 10°S compared to PEM‐Tropics A. Therefore the trace gas composition of the South Pacific troposphere was considerably more representative of minimally polluted tropospheric conditions during PEM‐Tropics B.