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, SO 2 and NO 2 levels in the mid-latitude coastal boundary layer from 27 May to 9 June, 2005, and SO 2 and NO 2 levels in fossil fuel power plant plumes from 10 to 14 January 2004. MAX-DOAS data were analyzed to identify and quantify ClO, SO 2 and NO 2 by utilizing their specifi c structured absorption features in the UV region. Differential slant column densities (dSCDs) for ClO, SO 2 and NO 2 were as high as 7.3 × 10 14 , 2.4 × 10 16 and 6.7 × 10 16 molecules/cm 2 (with mean dSCDs of 2.3 × 10 14 , 8.0 × 10 15 and 1.2 × 10 16 molecules/cm 2 ), respec- tively, 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, SO 2 and NO 2 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, SO 2 and NO 2 in the plume in terms of SCDs. Mixing ratios based on the estimated cross-sections of the plumes were 15.5 (ClO), 354 (SO 2 ) and 210 (NO 2 ) ppbv in the plumes of the fossil fuel power plant.