2Cheng, Z. et al. Status and characteristics of ambient PM2.5 pollution in global megacities.

Online chemical characterization of NR-PM1 (nonrefractory particulate matter ≤1 μm) has been carried out using an ACSM (Aerosol Chemical Speciation Monitor) at a coastal urban site in Chennai, India. The average mass concentration of NR-PM1 during the campaign was 30.4 ± 28.3 μg/m3 (arithmetic mean ± standard deviation) with organics accounting for a major fraction of ∼47.4% followed by sulfate (∼33.3%). Back trajectory analysis and STILT model simulations enabled the identification of a relatively clean period with prevailing air masses from ocean. During this period, the average NR-PM1 mass concentration was 7.1 ± 2.8 μg/m3, which is ∼5 times lower than that of the rest of the campaign (with air masses sampled from both continent and ocean) (33.3 ± 29.1 μg/m3). This reduction was primarily attributed to the dilution of local primary emissions due to cleaner marine influx. Comprehensive source apportionment for the organic fraction was performed using Positive Matrix Factorization (PMF). While equal contributions of primary (∼49%) and secondary (∼51%) organic factors were observed for the rest of the campaign, more oxidized-oxygenated organic aerosol (MO-OOA) factor dominated the OA and accounted for ∼82% of the total OA mass during the clean period. Simultaneously, during the clean period a significant increase in the fraction of organic liquid water was observed. We studied the effect of marine influx on the enhanced secondary organic aerosol (SOA) fraction. In brief, our results demonstrate the significance of marine winds and meteorological conditions on the chemical composition and ambient aerosol mass burden at a coastal site. Further, this study emphasizes that marine influx can cause the dilution in local pollution and can demonstrate distinct chemical composition with impacts on local aerosol properties.

The megacity of Delhi is located within the Indo-Gangetic Plain (IGP) and is one of the major sources of anthropogenic air pollution. It is a continental metropolitan area in a large valley south of the Himalayas, which causes the air masses to be constrained within the IGP (Figure 1). The air masses are vulnerable to high levels of particulate matter emissions from the megacity all year round (Bhandari et al., 2020), since it is a fast-growing urban agglomeration (Jain et al., 2016;Paul et al., 2021). During winter, a strong radiative thermal inversion causes the megacity to be enveloped by a shallow planetary boundary layer (PBL) in the nighttime, resulting in high relative humidity (RH) and aerosol mass burden (Arun et al., 2018;Murthy et al., 2020). The cold, humid and polluted conditions coupled with low wind speeds make the landlocked atmosphere conducive to fog and haze formation (Dhangar et al., 2021;Dumka et al., 2019;Ojha et al., 2020). Moreover, the aerosols in Delhi have enhanced water uptake ability as reported in the companion study by Gunthe et al. (2021), which can facilitate multiphase processes for formation of aerosols and thereby cause drastic visibility deterioration.

Aerosol Liquid Water Content (ALWC), a ubiquitous component of atmospheric aerosols, modulates atmospheric chemistry through aerosol surface reactions and reduces the atmospheric visibility. However, the complex dependency of ALWC on aerosol chemistry and relative humidity (RH) in the Indian region remains poorly characterized. Here, we combine available measurements of aerosol chemical composition with thermodynamic model, ISORROPIA2.1, to reveal a comprehensive picture of ALWC in fine mode aerosols during the winter season over the Indian continental region. The factors modulating ALWC are primarily dependent on the RH, such that the effect of aerosol dry mass and hygroscopicity are significant at high RH while the effect of hygroscopicity significantly reduces with decreasing RH. ALWC is observed to display a sharp non‐linear rise, beyond a critical value of ambient RH dependent on the particle hygroscopicity. Further analysis by coupling Weather Research Forecasting‐Chem simulation with ISORROPIA2.1 revealed significant spatial heterogeneity in ALWC over India, strongly associating with regions of high aerosol mass loading and RH. The Indo‐Gangetic Plain is consequently observed to be a hotspot of higher ALWC, which explains the prevalent conditions of haze and smog during winter in this region. Our findings re‐emphasize that high aerosol mass resulting from intense pollution is vital in modulating aerosol–climate interaction under favorable meteorological conditions. Observations suggest the need for localized pollution control strategies, directed at the reduction in aerosol emissions of specific chemical composition observed to contribute to the enhancement in PM through an increase in ALWC during wintertime in the region.

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