US researchers have made substantial headway in answering one of the most vexing air pollution questions; the relationship between pollutant emissions and the acid concentrations in precipitation occurring downwind of the emission sources, known as acid rain.

Gene Likens and his research team, based at the Institute of Ecosystem Studies, New York, have analysed data on precipitation chemistry collected between 1963 and 2000 from the Hubbard Brook Experimental Forest (HBEF) in the White Mountains of New Hampshire. They studied the relationship between pollutant emissions of SO₂ and NO_x and the resultant concentrations of SO₄^2- and NO₃^- in the precipitation. This relationship is expected to have important policy and ecological implications. 

Samples of bulk deposition have been collected and analysed for chemical content on a weekly basis since 1963, using an open plastic funnel and reservoir system for rain and a plastic bucket system for snow. It has been difficult, however, to relate these data to emissions because the Environmental Protection Agency has regularly changed its methodology for deriving emission data for SO₂ and NO_x. 

Likens and colleagues have overcome this problem and found a clear and strong relationship between SO₂ emissions and the SO₄^2- concentrations in bulk precipitation for the HBEF. A weak statistical relationship was also apparent for emissions of NO_x and concentrations of NO₃^-.

The relationships they have found are robust, demonstrating that careful, long-term measurements from a single deposition collection site provide a sound and reasonable approach for monitoring trends in air pollution.

Joanne Bell