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Abstract. Delhi is a megacity subject to high local anthropogenic emissions andlong-range transport of pollutants. This work presents for the first timetime-resolved estimates of hygroscopicity parameter ( κ ) and cloud condensation nuclei (CCN), spanning formore than a year, derived from chemical composition and size distributiondata. As a part of the Delhi Aerosol Supersite (DAS) campaign, thecharacterization of aerosol composition and size distribution was conductedfrom January 2017 to March 2018. Air masses originating from the Arabian Sea(AS), Bay of Bengal (BB), and southern Asia (SA) exhibited distinctcharacteristics of time-resolved sub-micron non-refractory PM 1 (NRPM 1 ) species, size distributions, and CCN number concentrations.The SA air mass had the highest NRPM 1 loading with high chloride andorganics, followed by the BB air mass, which was more contaminatedthan AS, with a higher organic fraction and nitrate. The primary sourceswere identified as biomass-burning, thermal power plant emissions,industrial emissions, and vehicular emissions. The average hygroscopicity parameter( κ ), calculated by the mixing rule, was approximately 0.3 (varying between0.13 and 0.77) for all the air masses ( 0.32±0.06 for AS, 0.31±0.06 for BB, and 0.32±0.10 for SA). The diurnal variations in κ wereimpacted by the chemical properties and thus source activities. The total,Aitken, and accumulation mode number concentrations were higher for SA,followed by BB and AS. The mean values of estimated CCN number concentration( NCCN ; 3669–28926 cm −3 ) and the activated fraction ( af ;0.19–0.87), for supersaturations varying from 0.1 % to 0.8 %, also showed thesame trend, implying that these were highest in SA, followed by those in BBand then those in AS. The size turned out to be more important than chemicalcomposition directly, and the NCCN was governed by either the Aitkenor accumulation modes, depending upon the supersaturation (SS) and criticaldiameter ( Dc ). af was governed mainly by the geometric meandiameter (GMD), and such a high af ( 0.71±0.14 for the mostdominant sub-branch of the SA air mass – R1 – at 0.4 % SS) has not been seenanywhere in the world for a continental site. The high af was aconsequence of very low Dc (25–130 nm, for SS ranging from0.1 % to 0.8 %) observed for Delhi. Indirectly, the chemical propertiesalso impacted CCN and af by impacting the diurnal patterns of Aitkenand accumulation modes, κ and Dc . The high-hygroscopic nature ofaerosols, high NCCN , and high af can severely impact theprecipitation patterns of the Indian monsoon in Delhi, impact the radiation budget,and have indirect effects and need to be investigated to quantify thisimpact.

Air mass physiochemical characteristics over New Delhi: impacts on aerosol hygroscopicity and cloud condensation nuclei (CCN) formation