Much of the northern hemisphere has been significantly affected by atmospheric pollution. As a result, many scientists and policymakers are interested in relatively pristine regions such as the remote Western Pacific. This interest arises from both the relatively remote nature of the region and because the rapid growth of Asia and the Pacific Rim makes this region particularly sensitive to regional/global changes in tropospheric chemistry resulting from human activity.

To examine the tropospheric chemistry of the Western Pacific region, the Western Pacific Exploratory Missions (PEM-West) were initiated in 1991. The PEM-West experiment was designed as part of the NASA Global Troposphere Experiment (GTE). The project's primary objective was to examine the atmospheric chemistry of ozone and ozone precursors in the Western Pacific region and to establish the effect of anthropogenic influences on tropospheric chemistry. In addition, the project investigated the chemistry and transport of sulfur species throughout the region with the intent of differentiating between oceanic and atmospheric sources. The region also contains several large-scale meteorological features such as the Asian dust storms, the Japan Jet, monsoons and the El Niño-Southern Oscillation (ENSO) phenomenon that make it particularly interesting to study.

The PEM-West missions were separated into two phases designed to capture the impact of seasonal meteorological dynamics of the region. The first phase, carried out in the Fall of 1991, examined the atmospheric chemistry of the region during a period characterized by relatively clean southwesterly flows from the Pacific and over the Asian continent. The second phase, in Spring of 1994, focused on the atmospheric chemistry of the region during the period when the flow patterns are reversed and the Western Pacific region receives air masses that have traveled over the Asian continent. By focusing on these two periods, researchers were able to examine the importance of emissions from the Asian continent and compare tropospheric chemistry during the Spring to the chemistry during the more pristine Fall conditions.

The experiment combined data from both aircraft flights and ground stations distributed through the Western Pacific region. The data obtained during the experiment included measurements of ozone and a suite of ozone precursors which included the nitrogen oxides NO and NO₂ (NO_x), carbon monoxide (CO), methane (CH₄) and the non-methane hydrocarbons (NMHC). In addition measurements of sulfur species and radiatively important species such as nitrous oxide (N₂O), CH₄, and carbon dioxide (CO₂) were made.

The PEM-West campaign allowed researchers to evaluate the distribution of trace gases and aerosols in the remote troposphere while also allowing a quantitative evaluation of the vertical transport of trace atmospheric species from the Asian continent and Pacific Rim to the atmosphere over the Pacific Ocean. The results of the experiment suggest that the Pacific is not as pristine as many researchers had previously thought. Even in the upper levels of the troposphere during the Phase A portion of the experiment, researchers found evidence of pollution from surface emissions over Asia and perhaps the whole of the northern hemisphere. CO concentrations were found to reach twice those observed in the Southern Hemisphere and the upper troposphere was established as the major ozone production regime in the area. Researchers also found that typhoons were very effective in transferring lower tropospheric trace species up to the upper troposphere. Transport of stratospheric air into the troposphere appears to be significantly slower in the Fall than in the Spring.

During Phase B of the experiment, lightning was established as the major source of NO_x in the upper troposphere. In addition, winds behind cold fronts were found to sweep large quantities of pollutants from Asia as far as Alaska. Anthropogenic signatures were found in both reactive species and radiatively important species. Interestingly, strong correlations were found between pairs of greenhouse gases such as in CH₄ and N₂O and in CH₄ and CO₂. During Phase A, however, an inverse correlation between CH₄ and CO₂ was found. These correlations can be used to gain information about continental sources and sinks.

Sulfur concentrations at altitudes as high as 10 km show very significant anthropogenic influences. As much as two-thirds of the sulfur dioxide above 8 km seems to be anthropogenically derived and probably most is from Asian emissions with a lesser contribution from Europe and perhaps even North America.

The impact of anthropogenic pollutants is also significant on a local to regional level through the effects of aerosols. Aerosols can act to directly affect climate through the scattering of incoming solar radiation back to space and the absorption of solar radiation. There are also indirect effects which are less well understood but may include the behavior of aerosols as cloud condensation nuclei (CCN). Most of the aerosol effects on climate tend to counteract warming due to the greenhouse gases. The magnitude of the radiative effects is not well understood.

Using a case study for the eastern U.S., it is possible to examine the potential local to regional effects of aerosols. Using simulations for a high pressure system in the Eastern U.S., aerosols resulted in a 0.9° K decrease in surface temperature near noon, as well as decreases in the height of the planetary boundary layer, latent heat flux and sensible heat flux. These changes may be negligible with respect to day-to-day weather but may be very significant for long term regional to global climate change. Anecdotal evidence suggests that aerosol effects in China could be very important and could result in significant human health impacts. Additional study will be needed to fully understand the regional and global effects of aerosols but studies such as PEM-West provide an excellent opportunity to examine the effects of large land regions on tropospheric chemistry.