Long-term tropospheric ozone trends: A critical review

In this review we attempt to critically evaluate the availability of reliable tropospheric ozone measurements suitable for long-term trend analysis. The focus is on large-scale changes deduced from measurements, which are used for comparison with numerical simulations of the tropospheric ozone cycle. These are required to quantify the influence of anthropogenic ozone precursor emission changes on climate. Long-term tropospheric ozone measurements show that ozone over Europe has increased by more than a factor of two between World War 11 and the early 1990s which is consistent with the large increase in anthropogenic ozone precursor emissions in the industrialized world. However, the further increase in background ozone over Europe and North America since the early 1990s cannot be solely explained by regional ozone precursor changes because anthropogenic ozone precursor emissions decreased in the industrialized countries as consequence of air pollution legislation. Measurements also indicate large increases in ozone in the planetary boundary layer over the tropical Atlantic since the late 1970s, which have been attributed to large increases in fossil fuel related emissions. Measurements at southern midlatitudes, which are limited in number, show a moderate increase in tropospheric ozone since the middle of the 1990s.