Highly significant impact of mineral dust on aerosol hygroscopicity at New Delhi

All conventional hygroscopicity parameter (kappa) estimates are based on ammonium salts being the dominant salts, besides sea salt at marine locations. This grossly underestimates kappa for highly polluted cities like New Delhi which is subjected to high mineral dust. To validate this hypothesis, the impact of mineral dust on kappa was quantified following two mathematical approaches based on Mann-Kendall's test which differed in the sequence followed for compound formation. This leads to a remarkably high increment in kappa, on an average of 57% and 63.5% during the day and night respectively, for the period Dec 2012-2014 and is based on the detailed chemical speciation of PM2.5 data. The inter-comparison of kappa obtained from various analytical methods challenge the traditional way of estimating kappa by including the chemical composition of a few dominant species and ignoring the others without paying any heed to the dependence of kappa on Temperature (T) and Relative Humidity (RH). An increase in the ratio of water-soluble organic carbon to organic carbon lessened the impact of mineral dust on kappa. The insolubility of calcium salts also decreased the effect of mineral dust, but at the same time lead to MgCl2 formation which augmented the estimated kappa. The highest obtained kappa was 0.82 (on 23/7/2014 during the night). The impact of mineral dust was also found to be significant for two other Indian (Durg and Kanpur) and Chinese (Xian and Beijing) cities for both PM1 and PM2.5 regimes. The high value of kappa and impact of mineral dust on kappa can be crucial in Cloud Condensation Nuclei (CCN) estimation at polluted sites subjected to mineral dust and thus impact precipitation quantification by Global Climatic models (GCMs), lead to visibility deterioration and affect the indirect radiative forcing.

Automated summary

The traditional method of estimating hygroscopicity parameter (kappa) underestimates the value for highly polluted cities with mineral dust, while detailed chemical analysis shows a significant increase in kappa. The impact of mineral dust on kappa has crucial implications for Cloud Condensation Nuclei (CCN) estimation and precipitation quantification by Global Climatic models.