Highly Significant Impact of Mineral Dust on Aerosol Hygroscopicty at New Delhi

The hygroscopicity parameter (k) is a measure of the water uptake ability of the aerosols primarily depends on size and chemical composition. All conventional k estimates are based on ammonium salts being the most dominant salts in the atmosphere with sea salt (NaCl) being considered at marine locations. However, this consideration in evaluating hygroscopicity for highly polluted cities such as New Delhi which is subjected to a huge amount of mineral dust leads to a gross underestimation of k. To verify this hypothesis, we consider filter-based measurements of PM_2.5 and its detailed chemical characterization from Dec 2012- Dec 2014 for New Delhi, separately for day and night. The overall k was calculated using four different methods based on ammonium salts. The methods M_a, M_b, M_c and M _d were based on (NH₄)₂SO₄ only as the dominant salt, inorganic and organic fractions, ion pairing scheme and E-AIM model respectively. These methods yielded a k of 0.13±0.05, 0.30±0.06, 0.34±0.11 and 0.57± 0.01for day. The night-time k was evaluated as 0.14±0.06, 0.31±0.06, 0.32±0.09 and 0.48±0.19 respectively. To quantify the impact of mineral dust, two approaches have been proposed in this work based on Man-Kendall's test including all possible hygroscopic salts of mineral dust. Out of these M1 was based on single iteration and M2 was based on multiple iterations. These two methods consequently resulted in k of 0.39±0.10 and 0.55±0.06 for day and 0.55±0.13 and 0.62± 0.01for night respectively. On comparing M2 (the most appropriate method to quantify hygroscopicity due to most detailed inclusion of chemical composition) to M_a (used as the reference method to quantify mineral dust) seasonally, the increment in k was in the range 0.39-0.53, which is remarkably high. The highest obtained was 0.86 (on 2/8/2014 during night), when k from M_a was 0.14, thus changing hugely by a factor of 5. The impact of mineral dust was further checked for Indian cities (Durg and Kanpur) and Chinese cities (Xi'an and Beijing), and k showed an increase on inclusion of mineral dust for both PM₁ and PM_2.5. This important finding can be crucial in CCN estimation at polluted sites subjected to mineral dust and thus can impact precipitation quantification by GCMs, lead to visibility deterioration and affect the indirect radiative forcing, thus indicating that the contribution of mineral dust to aerosol hygroscopicity cannot be ignored.