Size-segregated sugar composition of transported dust aerosols from Middle-East over Delhi during March 2012

Abstract

During March 20–22, 2012 Delhi experienced a massive dust-storm which originated in Middle-East. Size segregated sampling of these dust aerosols was performed using a nine staged Andersen sampler (5 sets of samples were collected including before dust-storm (BDS)), dust-storm day 1 to 3 (DS1 to DS3) and after dust storm (ADS). Sugars (mono and disaccharides, sugar-alcohols and anhydro-sugars) were determined using GC–MS technique. It was observed that on the onset of dust-storm, total suspended particulate matter (TSPM, sum of all stages) concentration in DS1 sample increased by >2.5 folds compared to that of BDS samples. Interestingly, fine particulate matter (sum of stages with cutoff size <2.1μm) loading in DS1 also increased by >2.5 folds as compared to that of BDS samples. Sugars analyzed in DS1 coarse mode (sum of stages with cutoff size >2.1μm) samples showed a considerable increase (~1.7–2.8 folds) compared to that of other samples. It was further observed that mono-saccharides, disaccharides and sugar-alcohols concentrations were enhanced in giant (>9.0μm) particles in DS1 samples as compared to other samples. On the other hand, anhydro-sugars comprised ~13–27% of sugars in coarse mode particles and were mostly found in fine mode constituting ~66–85% of sugars in all the sample types. Trehalose showed an enhanced (~2–4 folds) concentration in DS1 aerosol samples in both coarse (62.80ng/m3) and fine (8.57ng/m3) mode. This increase in Trehalose content in both coarse and fine mode suggests their origin to the transported desert dust and supports their candidature as an organic tracer for desert dust entrainments. Further, levoglucosan to mannosan (L/M) ratios which have been used to predict the type of biomass burning influences on aerosols are found to be size dependent in these samples. These ratios are higher for fine mode particles, hence should be used with caution while interpreting the sources using this tool.