Abstract. In the present work we deal with emissions originating from combustion processes containing SO₂, NO and NO₂, emitted into the atmosphere by a localized source. We present a method for measuring the NO₂-formation rate (due to conversion of NO to NO₂) from measurements of SO₂- and NO₂-slant column densities across different plume sections, under the (usually justified) assumption that the SO₂-flux is constant. The advantages of the proposed method are that the measurements can be performed from an arbitrary location, without explicit reference to the wind speed and direction in the plume. We present results of ground based DOAS-measurements of SO₂ and NO_x emissions from an oil refinery located in the northern part of Montevideo Bay and give recommendations for future applications of the method.