Characteristics of particulate matter over the South African industrialized Highveld
Date
2011-02-25
Authors
Alade, Omotayo Lydia
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Abstract
Atmospheric aerosols through their role in heterogeneous chemistry significantly influence the
earth’s radiation balance and the radiative budget of the Earth’s atmosphere. The aim of this
study is to investigate the characteristics of particulate matter over the industrialised Highveld of
South Africa. Elandsfontein, a supersite surrounded by coal mines, coal-fired power stations and
a petrochemical plant, is situated on the industrialized Highveld. It provides the monitoring
station for the study because it is representative of general conditions of high emissions on the
industrialised Highveld region. Continuous measurements of PM10, black carbon, nitrates,
sulphur dioxide, particulate sulphate, light scattering properties of aerosol by nephelometer, and
meteorological parameters were collected at Eskom’s Elandsfontein air quality monitoring site
from January to December 2005. The temporal behaviors of the atmospheric aerosols are
investigated on diurnal and seasonal time scales. Effects of meteorological parameters such as
temperature, humidity, wind direction, and wind speed on the concentration and properties of the
aerosols are determined. Also identified is the fraction of PM10 on the Highveld composed of
sulphates, nitrates and black carbon. The causes of particularly high particulate concentrations
are discussed. The sources of aerosols are identified and the seasonal effects on the concentration
of aerosols are investigated using pollution roses and wind roses. Average concentrations of
atmospheric aerosols on the industrialized Highveld are highest during late evenings and at night
(18:00-22:00) and lowest during the afternoon (12:00-17:00). Maximum concentrations of
aerosols in the early morning and at night indicate the trapping effects of stable atmospheric
conditions and surface inversions, suggesting that most particulate matter is derived from sources
at the surface or at least from sources below the subsidence inversion layer. Higher humidity in
the morning may also enhance particulate matter mass concentration. Concentrations are at a
minimum during the day due to atmospheric instability and mixing. Concentrations of nitrate are
higher at night especially during winter and are predominantly affected by the same stability
phenomenon. Higher concentrations of nitrate during winter nights may be a result of fossil fuel
combustion at low level e.g. use of coal for space heating and cooking in townships. During the
day NO2 is slowly converted to NO3 by reaction with O3. At night however, the principal
reaction of NO3 is with NO2 to set up equilibrium with N2O5. The subsequent heterogeneous
dissolution of HNO3 and N2O5 into water droplets gives rise to aerosol nitrate. The night time
chemistry of NO3 differs from the daytime behaviour in that it provides a route for conversion of
NOx to HNO3 which could be as high as 50% of the daytime route. Sulphate aerosol and sulphur
dioxide concentrations peak during the day and are lower at night. Higher concentrations during
the day are related to the influence of tall stack emissions from where the sulphur dioxide is
derived. Particulate sulphate concentrations are higher in well aged air masses and are controlled
by the local meteorology as well as the recirculation pattern of atmospheric aerosol over southern
Africa. Particulate concentrations are highest in association with north-westerly winds and
lowest in association with easterly winds, since major industries and power plants are located
north and south of Elandsfontein. Particulate sulphates sources include oxidation of sulphur
dioxide from coal combustion in coal-fired power plants and other industries, and long range air
mass transport of sulphate aerosols. The contribution of black carbon and sulphate to PM10 mass
is highest in spring while the nitrate contribution to PM10 mass is highest in winter. Some
episodes of high particulate matter concentration appear to be related to industrial emissions and
fossil fuel combustion. Other episodes are not related to industrial emissions but are mainly due to suspended dust.