A Wintertime Impact of Dilution on Transformation Rate in the Planetary Boundary Layer

Abstract

The effects of dilution on photochemical reactions of trace gases released from spontaneous combustion processes near open cast coalmines in winter have been examined with a view to assessing the impact of meteorology on atmospheric chemical reactions. Daytime observed gas concentrations vary significantly due to dilution and transformation at average relative humidity of 48.9 ± 4.3 % and temperature of 14.4 ± 0.6 °C. Dilution occurred during the day with rise in wind speed. This yielded dilution rate constants of 1.0 × 10-3 min-1 for CO, 2.5 × 10-3 min-1 for NO, 1.9 × 10-3 min-1 for NO2, 8.4 × 10-4 min-1 for H2S, 9.8 × 10-4 min-1 for SO2 and 1.6 × 10-3 min-1 for O3. H2S and SO2 were observed to exhibit very close resemblance in diurnal variations. The average daytime and night time concentrations of the observed H2S were 31.5 ± 13.4 ppb and 171.7 ± 15.9 ppb and 31.6 ± 13.2 ppb and 128.5 ± 18.1 ppb for SO2. Chemical reaction rates were evaluated over a temperature range between 0 and 20 °C and mean surface pressure of 850.4 ± 2.7 mbar for the trace inorganic gases. Over this temperature range and surface pressure, transformation rates of SO2 and H2S are 0.023 %·h-1 and 0.008 %·h-1 respectively. The pre-exponential rate constants of the gaseous species are kH2S = 6.65 × 10-3 L·μg–1·min –1, kSO2 = 1.67 × 10-3 L·μg–1·min–1, kNO2 = 4.7 × 10-3 min–1, kNO = 1.27 × 10-2 min–1 and kO3 = 5.49 × 10-1 L·μg–1·min–1. The energy of activation of the gaseous species are EA / R was approximately 317 K for H2S, 696 K for SO2, 258 K for NO2, 292 K for NO and 1684 K for O3.

Keywords: Coalmining, combustion, dilution, meteorology, photo-oxidation, trace gas