Statistics and meteorology of air pollution episodes over the South African Highveld based on satellite-model datasets

Abstract

he meteorology of air pollution episodes on South Africa?s Highveld was studied using OMI and AIRS satellite estimates, MERRA2 reanalysis model products and in-situ weather data. Surface-layer SO2 and NO2 display high concentrations during winter (May-July) and provide a focus for statistical analysis of monthly and daily time series. Highveld area averaged monthly model SO2 was temporally correlated with boundary layer height (-0.76) and temperature lapse rate (+0.65) in the period 1980-2015, but relationships with winds were weak. Daily Highveld area satellite NO2 was related to dewpoint temperature (-0.59) and exhibited pulsing in the range 7-24 days in the period 2005-2015. High concentrations of these short-lived locally-generated air pollutants were found over and southeast of Johannesburg due to urban and industrial emissions.The spatial regression of daily NO2 onto regional sea level air pressure fields in May-July 2005-2015 revealed the slow eastward movement of an anticyclone. At the climate timescale, Pacific La Nina conditions favored an increase of May-July SO2 concentrations when SST in the equatorial Atlantic were warmer than normal. The meteorological pattern underlying the highest ranked air pollution event of 18-25 July 2008 was characterized by sharp anticyclonic curvature of low level winds that induce subsidence, and consequently a stable lapse rate and low dewpoint temperature (-5C). The wind vorticity exerted a stronger influence on dispersion than the surface divergence. This new understanding will underpin better air quality forecasts over the South African Highveld.