Formation of sulfuric and nitric acid in the atmosphere during long-range transport

A simple photochemical model has been used to simulate the formation of sulfuric and nitric acid during long-range transport through the atmosphere. Comparisons have been made with observations of sulfate and nitrate in precipitation at various distances from the source areas in northern Europe.

Both observations and model calculations indicate that HNO₃ is formed at a faster rate than H₂SO₄ and that the long-range transport of HNO3 is thus somewhat less pronounced than that of H₂SO₄. Mainly because of the common dependence of the oxidation of SO₂ and NO_x on the concentration of the OH radical, the concentration of NO_x has a significant influence on the rate of formation of H₂SO₄: A higher emission of NO_x tends to reduce the levels of OH and H₂O₂ close to the source area thereby delaying and decreasing the transformation of SO₂ to H₂SO₄.

Because of the interactions of the chemical species, the dependence of the concentrations on emission rates is not linear. Our model suggests that the concentrations of H₂SO₄ at travel distances up to a few tens of hours should have increased significantly less over the last 20 years than the rates of emission of SO₂. This also seems to be brought out by observations of sulfate in precipitation.