MAX-DOAS measurements of ClO, SO2 and NO2 in the mid-latitude coastal boundary layer and a power plant plume

Abstract

Remote sensing techniques have been preferred for measurements of atmospheric trace gases because they allow direct measurement without pre-and/or post-treatment in the laboratory. UV-visible absorption measurement techniques have been used for ground-based remote sensing of atmospheric trace species. The multi-axis differential optical absorption spectroscopy (MAX-DOAS) technique, one of the remote sensing techniques for air quality measurement, uses scattered sunlight as a light source and measures it at various elevation angles by sequential scanning with a stepper motor. Ground-based MAX-DOAS measurements were carried out to investigate ClO, SO2 and NO 2 levels in the mid-latitude coastal boundary layer from 27 May to 9 June, 2005, and SO2 and NO2 levels in fossil fuel power plant plumes from 10 to 14 January 2004. MAX-DOAS data were analyzed to identify and quantify ClO, SO2 and NO2 by utilizing their specifi c structured absorption features in the UV region. Differential slant column densities (dSCDs) for ClO, SO2 and NO2 were as high as 7.3 × 1014, 2.4 × 1016 and 6.7 × 10 16 molecules/cm2 (with mean dSCDs of 2.3 × 10 14, 8.0 × 1015 and 1.2 × 1016 molecules/cm 2), respectively, at a 3°elevation angle in the coastal boundary layer during the measurement period. Based on the assumption that the trace gases were well mixed in the 1 km height of the boundary layer, estimates of the mean mixing ratios of ClO, SO2 and NO2 during the measurement period were 8.4 (±4.3), 296 (±233) and 305 (±284) pptv, respectively. MAX-DOAS measurement of the power plant plumes involved making vertical scans through multiple elevation angles perpendicular to the plume dispersion direction to yield cross-sectional distributions of ClO, SO2 and NO2 in the plume in terms of SCDs. Mixing ratios based on the estimated cross-sections of the plumes were 15.5 (ClO), 354 (SO2) and 210 (NO2) ppbv in the plumes of the fossil fuel power plant. Keywords: Air pollution, chlorine monoxide, DOAS, remote sensing © 2008 Springer Science+Business Media B.V.