Measurement report: Hygroscopicity of Size-Selective Aerosol Particles at Heavily Polluted Urban Atmosphere of Delhi: Impacts of Chloride Aerosol 

Abstract. Recent studies reveal that wintertime chloride emission in the Delhi region is crucial in governing enhancement to theoretically calculated aerosol hygroscopicity and aerosol-bound liquid water to trigger Delhi's fog episodes. Here, we reported that the high volume fractional contribution of ammonium chloride into aerosol governs the high aerosol hygroscopicity and associated liquid water content based on the experimental data first time in Delhi. The episodically high chlorides bonded with excess ammonia present in Delhi's atmosphere, which could lead to haze and fog formation under high relative humidity in the region. Therefore, our study suggests that controlling the plastic-contained waste, open burning, and e-waste industrial chloride emission could significantly minimize Delhi’s heavily polluted haze/fog events. The high chloride (H-Cl) period was observed significantly (p<0.05) higher hygroscopicity (0.35 ± 0.06) compared to high biomass burning (H-BB) (0.18 ± 0.04), high hydrocarbon-like organic aerosol (H-HOA) (0.17 ± 0.05), and relatively cleaner period (0.27 ± 0.07).

In this study, we present the measurement results of bulk aerosol composition of non-refractory PM1 from ACSM and size-resolved (Nucleation, Aitken, and Accumulated mode particles) hygroscopic growth factor and associated hygroscopicity parameter at 90 % relative humidity (RH) measured using H-TDMA at Delhi Aerosol Supersite (DAS) first time. The hygroscopic parameter (κ_{H-TDMA_ 90%}) was significantly (p<0.05) enhanced with the size of the particles. The observed κ_{H-TDMA_90%} ranged from .00 to 0.11 (0.03 ± 0.02), 0.05 to 0.22 (0.11 ± 0.03), 0.05 to 0.30 (0.14 ± 0.04), 0.05 to 0.41 (0.18 ± 0.06), and 0.05 to 0.56 (0.22 ± 0.07) for 20, 50, 100, 150, and 200 nm aerosol particles, respectively. The Inorganic-to-organic aerosol ratio in aerosol modulated mainly the aerosol hygroscopicity. In addition, the accumulation mode particle's hygroscopicity was regulated potentially by the volume fraction of NH₄Cl and OA in aerosol particles. Interestingly, our results reveal that the daytime flattening pattern of accumulation aerosol particles in diurnal variation is potentially due to counter the effect of increment of (NH₄)₂SO₄ and NH₄NO₃ and decrement of NH₄Cl and OA in aerosol particles.