Reconstructed light extinction coefficients of fine particulate matter in rural Guangzhou, Southern China

Abstract

A one-year campaign was conducted to collected PM2.5 samples in the rural area of Guangzhou, the largest megacity in South China, from March 2012 to February 2013. Mass concentration of PM2.5, carbonaceous fractions (i.e., organic carbon (OC) and elemental carbon (EC)) and 6 water-soluble ions were analyzed. Light extinction coefficient (bext) of fine particulate matter was reconstructed using the revised IMPROVE formula at the site. The reconstructed bext was compared with the measured bext converted from visibility. A good correlation was obtained between the two sets of bext with a coefficient of determination (R2) of 0.61 and a slope of 0.99. The average reconstructed bext in the study was 253.7 ± 162.9 Mm–1. The seasonal reconstructed bext was in the order of autumn (319.4 ± 207.2 Mm–1) > winter (269.6 ± 175.5 Mm–1) > summer (219.0 ± 129.3 Mm–1) > spring (193.3 ± 94.9 Mm–1). (NH4)2SO4 (AS) made a dominant contribution to the light extinction budget, accounting for 61.3% (155.6 ± 108.5 Mm–1) annually, with highest in autumn (68.0%) and lowest in winter (55.2%). Organic matter (OM) was the second largest contributor accounting for 20.5% (52.2 ± 42.7 Mm–1) with highest in winter (23.4%) and lowest in spring (18.0%). The relationship between reconstructed bext and measured bext was investigated under the influence of seasonality, visibility and PM2.5 concentration. We found that bext could be reconstructed using revised IMPROVE formula in high PM2.5 days (threshold value of ~60 µg m–3). On other hand, the performance of formula was unsatisfactory for bext reconstruction of in low PM2.5 days, when meteorological conditions could have significant impact on visibility.