Air-pollution-related hydrocarbon emissions from vegetation are much higher than expected over the African savanna (flat tropical grasslands), while those coming from the rain forests are somewhat lower than prior estimates, according to scientists. The National Science Foundation (NSF)-funded research team is mapping natural and human-caused trace gas emissions across the African continent in a project called EXPRESSO, the Experiment for Regional Sources and Sinks of Oxidants.
Because of biomass burning, smog levels over Africa often approach those of a high-pollution day in a major city, says Alex Guenther, a scientist at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado.
Scientists have gathered and analyzed data from previously unsampled remote regions to gain a better understanding of Africa's influence on the composition of the earth's atmosphere.
"Africa's impact on tropical and global air chemistry is considerable, but we haven't had good measurements until now," explains Guenther. "Because the study area includes terrain representative of sub-Saharan Africa, we can map emissions over the entire continent for the first time."
Huge stretches of African savanna and rain forest are burned each fall and winter for agricultural and territorial purposes. The fires produce large amounts of hydrocarbons and nitrogen oxides (NOx). These react in the presence of sunlight to produce low-altitude ozone and other smog-like products. Satellite pictures show that the plumes of ozone stretch, at times, as far as South America. While stratospheric ozone shields us from ultraviolet radiation, closer to the earth this molecule damages forests and crops; destroys nylon, rubber, and other materials; and injures or kills living tissue. Ozone is a particular threat to people who work or exercise outdoors or who suffer from respiratory problems.