Chemical Equator Divides Hemispheres

Earth's polluted Northern Hemisphere is separated from its cleaner Southern Hemisphere by a sharp gradient in the troposphere known as a chemical equator, according to observations by Jacqueline F. Hamilton at the University of York, in England, and colleagues. Scientists previously thought that the atmospheric boundary between the hemispheres was formed solely by a meteorological feature known as the Intertropical Convergence Zone (ITCZ). This cloudy belt, where the trade winds from each hemisphere meet, also acts as a barrier for pollution transport. The new observations show that the chemical and meteorological boundaries can become decoupled. Hamilton's team used sensors on an airplane to measure carbon monoxide, ozone, aerosols, and nonmethane hydrocarbons. They detected the 50-km-wide chemical equator over the north coast of Australia during the study, while satellite imagery showed the ITCZ over the central part of the country. The ability to distinguish between chemical and meteorological boundaries is expected to help improve global pollution-transport models. The research will be published in an upcoming issue of the Journal of Geophysical Research: Atmospheres.