Abstract:
A DOAS system has been developed and deployed to determine horizontal concentrations of such pollutants as NO2, SO2 and O3 in Bologna, Italy and Tema, Ghana. Measurements were conducted in Italy during the winter season of March 2005 and in Ghana during the dry harmattan season in January 2008 and raining seasons in July and August 2008. Their average line-of-sight concentrations were determined by measuring the scattered solar radiation through three optical windows in the zenith and two horizontal directions. The retrieval method was based on DOAS and a radiative transfer model. The slant column of NO2 was retrieved within 4360-4600 Å and SO2 and O3 within 3120-3270 Å and 3100-3300 Å respectively. In Italy concentration values of NO2 (in μgm-3) were in the range 28.21 ≤ NO2 ≤ 55.51 while in Ghana they were: 0.16 ≤ NO2 ≤ 35.48, 0.43 ≤ SO2 ≤ 159.43, 0.03 ≤ O3 ≤ 764.29 for measurements in the dry season and 0.02 ≤ NO2 ≤ 25.71, 0.15 ≤ SO2 ≤ 166.37, 1.46 ≤ O3 ≤ 688.84 for measurements in the raining season. NO2 values were usually high during mornings, reduced towards the afternoon, due to dissociation, and increased again towards sundown. Gradual increases in SO2 and O3 concentrations from morning to evening indicated a build-up due to industrial and vehicular activities. Results compared with in-situ measurements carried out by the Environmental Protection Agencies in Italy and Ghana as the first method of validation showed good agreement with a correlation coefficient greater than 0.8. This DOAS technique presents an attractive approach for ground-based measurements of atmospheric trace gases in selected directions.
