Absorption spectrum of ambient aerosol and its correlation with size distribution in specific atmospheric conditions after a red mud accident

Abstract

Approximately one month after a red sludge industrial accident, a high concentration of atmospheric aerosol was reported in the affected area. This increased aerosol load originated mainly from the fugitive dust emitted from the red sludge sediment, representing a serious threat to public health. In a study presented here, a recently developed four-wavelength photoacoustic spectrometer (4λ-PAS), which simultaneously measures optical aerosol absorption in the near infrared, visible and ultra-violet spectral ranges with high temporal resolution, was operated in the centre of the sludge hit area. Based on the results of the photoacoustic absorption measurements of temporal changes in light absorption and their correlations with other aerosol variables, including size distribution, were carried out in special atmospheric conditions (including with the presence of "atypical" red mud particles). Correlations between segregated absorption Ångström exponents (AAEs) and specific size ranges were tabulated, and the resulting data were used to draw conclusions about the reasons for the temporal changes to the measured quantities. The absorption behavior of red mud particles was determined in UV, VIS, and near IR spectral regions under laboratory conditions. The absorption spectrum of red mud aerosol was found to be atypical (i.e., it did not follow a power law), and this was used as a selective parameter for its identification in ambient bulk. Each measurement day, at around noon, dust from the red sludge sediment was identified; while during the rest of the day atmospheric aerosol composition was dominated by carbonaceous aerosols, originating most probably from home heating and/or from traffic. In-situ number concentration and size distribution measurements confirm this interpretation of the measurements obtained with 4λ-PAS. © Taiwan Association for Aerosol Research.