For a detailed assessment of aerosol physiochemical properties, an SMPS (TSI, Shoreview, MN), ACSM (Aerodyne Research, Billerica MA), and aethalometer (Magee Scientific Model AE33, Berkeley, CA) were operated at the Indian Institute of Technology (IIT) Delhi in Block 5, at a height of nearly 15 m, as a part of the DAS campaign. This sampling site in New Delhi was free from any source activity, except for a road, located 150 m away. The IIT campus is cleaner than the rest of the city. However, it lies in the heart of the city, and the outskirts of the campus experience fresh traffic influx. The IIT campus allows only limited access to vehicles and therefore has less traffic compared to the city in general. A temperature-controlled room was used to carry out the measurements. Two separate and thermally insulated sampling lines (3/4 in. outer-diameter stainless-steel tubes) with flows of 3 and 2 L min-1 equipped with PM1 cyclone, in line with a water trap and a Nafion membrane diffusion dryer (Magee Scientific Sample Stream Dryer, Berkeley, CA), were used for (1) an SMPS and ACSM in conjunction with a flow controller and (2) an aethalometer, respectively. A brief description of the instruments is given below. A detailed description of the instruments is given in Gani et al. (2019) and Bhandari et al. (2020).

The SMPS was comprised of a differential mobility analyser (DMA; TSI 3081), an electrostatic classifier (TSI 3080), an X-ray aerosol neutralizer (TSI 3088), and a water-based condensation particle counter (CPC; TSI 3785). The ambient air was sampled in the size range 12–560 nm, with a time difference of 135 s between two scans. The sheath-to-aerosol flow ratio was 4:1, and the total flow drawn by the CPC was 1 L min-1. The two dominant modes (Aitken and accumulation) are well captured within this size range. The ACSM sampled the inlet air at a flow rate of 0.1 L min-1 with a time resolution of ∼1 min. The calibration procedures and data processing are discussed in Gani et al. (2019). The ACSM collected time-resolved NRPM1 (non-refractory PM1) based on species that volatilize by 600 ∘C and included NO3-, Cl-, SO42-, NH4+, and organics.

The composition data presented in this work were collected in the DAS study. PMF (positive matrix factorization) analysis was conducted on the 15 months in the dataset. As a result, biomass-burning organic aerosol (BBOA) could be resolved as a separate factor only in spring 2018. This inability to resolve primary organic aerosol (POA) to separate factors, namely hydrocarbon-like organic aerosol (HOA) and BBOA, was attributed to the unit mass resolution of the instrument (Bhandari et al., 2020, and references therein). Owing to the lack of explicit BBOA and HOA separation in all seasons, the Ng et al. (2010) compilation of profiles was analysed, combined with the profiles identified in spring in Delhi. It was observed that spring 2018 profiles fell within the bounds of the uncertainty of the Ng et al. (2010) compilation. Thus, Ng et al. (2010) reference profiles were utilized for source attribution of each cluster. While factor profiles can differ across the world, taking regionally relevant profiles together with those usually employed as reference profiles for PMF analysis likely accounts for this variability. As a part of the analysis conducted here, the mean strength at the relevant m/z (s) (m/z 57 and 60) and the standard deviation (SD) of the profiles at these m/z (s) were utilized in the analysis.

Organic aerosols were qualitatively segregated by comparing the m/z ratios of f57 and f60 with the reference profiles of BBOA (f57: 0.0337±0.00884, f60: 0.025±0.00521), HOA (f57: 0.0838±0.00378, f60: 0.00227±0.00214), and oxygenated organic aerosol (OOA) (f57: 0.00997±0.00786, f60: 0.00571±0.00349), as reported by Ng et al. (2010). This was done by first calculating the cluster means of f57 and f60 for each cluster (Table 2). This was followed by an evaluation of residuals. Residuals represent the deviation of the cluster means from the reference profiles. The HOA, BBOA, and OOA residuals (RHOA, RBBOA, and ROOA) were then calculated based on cluster means of f57 and f60 (CMf57 and CMf60), with respect to the corresponding means of reference profiles (RMf57 and RMf60), as given below for HOA in Eq. (1): 1RHOA=(CMf57_HOA-RMf57_HOA)2+(CMf60_HOA-RMf60_HOA)2‾. The reference residuals for HOA, BBOA, and OOA (RRef_HOA, RRef_BBOA, RRef_OOA) were then calculated using standard deviations of reference profiles (SDf57_Ref_HOA, SDf57_Ref_BBOA, SDf57_Ref_OOA), as given below for HOA in Eq. (2): 2RRef_HOA=(SDf57_Ref_HOA)2+(SDf60_Ref_HOA)2. The residuals of the cluster means were then compared with the reference residuals as per the six conditions described in detail in Sect. S1 in the Supplement and classified as HOA, BBOA, OOA, or mixed.

The ACSM data were used to calculate κ as per the following mixing rule in Eq. (3) (Petters and Kredenweis, 2007): 3κ=∑iϵiκi, where ϵi and κi represent the volume fractions and individual hygroscopicity parameters of the various components. The inorganics were represented by (NH4)2SO4, NH4Cl, and NH4NO3. The organic κi was taken as 0.1 (Gunthe et al., 2009, 2011; Dusek et al., 2010; Rose et al., 2011). κi values were taken as 0.61 for (NH4)2SO4, 1.02 for NH4Cl, and 0.67 for NH4NO3 (Sullivan et al., 2009; Petters and Kredenweis, 2007). The density values to estimate the volume fraction of the inorganic constituents were taken as 1770 kg m-3 for (NH4)2SO4, 1519 kg m-3 for NH4Cl, and 1720 kg m-3 for NH4NO3 (Haynes et al., 2014). The density of organics was taken as 1500 kg m-3 (Bougiatioti et al., 2009). κ for BC was taken as zero, as reported in several studies (Hong et al., 2014; Leng et al., 2014; Wu et al., 2013).

It should be noted that we assumed that the κ calculated from NRPM1 data of ACSM represents the bulk hygroscopicity parameter in the absence of size-resolved measurements and is a limitation of this work. The difference due to the assumption cannot be accounted for and should be investigated in the future. However, it is reported that for κ>0.1, CCN closures within 20 % can be achieved assuming bulk composition and internal mixing (Wang et al., 2010). Temperature, relative humidity (RH), and the calculated κ were then used to calculate the critical diameter (Dc) from the multi-component κ-Köhler theory (Bhattu and Tripathi, 2015). The temperature and RH data are available from the RK Puram site (∼ 3–4 km aerial distance from the measurement site) maintained by the Central Pollution Control Board (CPCB), India. NCCN was then estimated by integrating the size distribution obtained from SMPS above Dc. The CCN estimates were obtained for the supersaturation (SS) of 0.1 %, 0.15 %, 0.2 %, 0.35 %, 0.4 %, 0.5 %, 0.6 %, 0.7 %, 0.75 %, 0.8 %, 0.85 %, and 1 %. However, for the sake of detailed analysis, 0.1 %, 0.4 %, and 0.8 % were chosen: 0.1 % represents the condition when the effect of chemical composition is expected to be the highest, 0.4 % represents the condition for convective clouds, and 0.8 % represents a high-supersaturation state, when almost all aerosols tend to get activated as CCN.

To characterize the air masses, the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used (Draxler and Rolph, 2003) to determine the major pathways of aerosols reaching Delhi. The 5 d back-trajectory analysis was done at the receptor site at a height of 500 m. The cluster analysis was then performed seasonally to identify the cluster mean trajectories per season. These mean trajectories were then again re-clustered to identify three main clusters based on the directions of the mean cluster trajectories: the Arabian Sea (AS) branch (16.5 % of total trajectories), the Bay of Bengal (BB) branch (13 % of total trajectories), and the southern Asian (SA) branch (70.5 % of total trajectories). The BB branch was further classified as B (54 %) and B.reg (45 %), where B represents the air masses reaching the sea, while B.reg represents the air masses that aligned towards reaching the Bay of Bengal but did not hit the sea. The SA branch was partitioned into L (17.5 %), R1 (54 %), R2 (18 %), and R3 (11 %). L represents the local trajectories originating within India, mainly from Delhi, Punjab, and Haryana. R1 represents trajectories coming from Pakistan and Afghanistan. R2 represents trajectories originating from Iran. R3 is representative of all trajectories beyond these, including a portion of South Africa, the Mediterranean Sea, and Turkey. The seasonal clusters for the winter, spring, summer, and monsoon of the year 2017 and winter and spring of the year 2018 are shown in Fig. S1 in the Supplement. The re-clustering is shown in Fig. 1. All the chemical speciation data from ACSM and size distribution data from SMPS were then categorized as per the classification discussed above and used in the following discussion.

The NOx, toluene, and benzene data inventory for the entire campaign was taken from the CPCB for RK Puram, whenever available. The data were not available for branch B. To determine the photochemical ageing of aerosols, toluene and benzene concentrations were used to calculate the life (in hours), as per Nault et al. (2018) in Eq. (4): 4t=-1OH×ktoluene-kbenzene×ln⁡tolueneitbenzeneit-ln⁡tolueneiobenzeneio, where [OH]=1.5×106 molecules cm-3 (Nault et al., 2018), ktoluene=2.3×10-12exp⁡(-190/T), and kbenzene=1.8×10-12exp⁡(340/T) (Atkinson et al., 2006) are the rate constants for each aromatic compound, tolueneio=1.85 and benzeneio=2.31. [OH] is not constant and varies considerably temporally and spatially, but due to the unavailability of data of its variation for Delhi, it was assumed constant for ageing calculation.

Out of the three main branches, the SA branch was the most anthropogenically contaminated, followed by BB and AS branches, as indicated by the mean NRPM1 mass concentrations: 125.2±91.6, 45.9±23.3, and 32.5±20.6 µg m-3, respectively (Fig. 2a). The total NRPM1 loading for the SA branch followed the sequence: NRPM1(L)<NRPM1(R3)<NRPM1(R1)<NRPM1(R2). Amongst the SA branches, L was associated with the lowest organic (52.8±40.6 µg m-3) and inorganic (42.1±33.1 µg m-3) content, while R2 had the maximum organic (85.4±59.8 µg m-3) and inorganic (57.9±47.5 µg m-3) content. A summary of the overall characteristics is given in Table 1. The prominent sources for the SA air mass include metal processing industries (Haryana and Delhi NCR), coke and petroleum refining (Punjab), thermal power plants (Pakistan, Punjab, and NCR Delhi), agricultural-residue burning (Punjab and Haryana), soil dust (Pakistan, Punjab) (Jaiprakash et al., 2017), and coal mines in Pakistan, where non-ideal burning of (NH4)2SO4 occurs (Chakraborty et al., 2015).

When comparing the BB branches, total NRPM1 for B (41.9±20.8 µg m-3) was slightly less than that for B.reg (47.3±25.4 µg m-3), and this can be attributed to the fact that the B.reg air mass does not travel over water (originates adjacent to the coastline) but is subject to its influence, while the B air mass travels over water and is therefore cleaner. B.reg had a slightly higher inorganic and organic content than that of B.

The relatively higher abundance of aerosols of BB over AS can be attributed to both the sources and the pathways of air masses. In terms of source, the Bay of Bengal is more anthropogenically impacted than the Arabian Sea, as concluded by the ICARB campaign (Kalapureddy et al., 2009). Previous studies (Nair et al., 2008a, b; Moorthy et al., 2008) reported higher aerosol number concentration (NCN), as well as black carbon (BC) concentration, over the Bay of Bengal than over the Arabian Sea, in all size ranges within the marine boundary layer as well as the vertical column. The BB air mass travelled over the Indo-Gangetic Plain (IGP), and the AS air mass travelled across western India and the desert region of Rajasthan. Based on previous emission estimates (Habib et al., 2006), the emission fluxes from fossil fuel dominate the aerosol burden over the IGP. The aerosol over the IGP is largely composed of inorganic oxidized matter (IOM), including fly ash from coal-fired power plants and mineral matter from open crop waste burning (Habib et al., 2006). The AS air mass travels over western India and brings pollution from both fossil fuel combustion and desert dust (Habib et al., 2006).

Mass closure between SMPS size distribution data and the sum of ACSM species together with BC was achieved (R2=0.83) as detailed in our parallel paper (Gani et al., 2019). The NRPM1 species (NH4+, Cl-, NO3-, SO42-, POA, and OOA) and BC varied significantly for the different air masses, both in terms of the mass of species (Fig. 2c) and the diurnal patterns (Fig. 3), leading to different aerosol chemistry and chemical reactions. A summary of the average mass of each species for all air masses is detailed in Table 1. In brief, both POA and OOA, followed by NO3-, SO42-, and Cl-, dominated the PM composition for the SA air mass, while OOA, followed by SO42- and OOA, was dominant for BB and AS air masses. High chloride was a special feature of the SA air mass which was not apparent in the other two branches.

NH4+ was assumed to be the dominant cation based on high aerosol neutralization ratio (ANR) values (mean values ranging from 0.95 to 0.85). The ANR is defined as the normalized ratio of the measured NH4+ concentration to the NH4+ concentration needed for full neutralization of the anions and calculated as per Eq. (5) (Zhang et al., 2007): 5ANR=NH4+measNH4+neut=(NH4+/18)(2×SO42-/96)+(NO3-/62)+(Cl-/35.5). Detailed ANR values are given in Table S1 in the Supplement. ANR values revealed that while AS, B, and L branches were completely neutralized, B.reg, R1, R2, and R3 were only partly neutralized, indicating that minor components of sulfate, chloride, and nitrate may be bound to non-volatile salts such as NaNO3, NaCl, or Na2SO4 or are associated with organics as organosulfates, organochlorides, or organonitrates, evidence for which is shown in a previous DAS study (Bhandari et al., 2020).

To determine the dominant salts, NH4+ ions were neutralized with SO42- ions. The speciation of salts of NH4+ and SO42- was determined by the molar ratio of NH4+ to SO42- ions (R[SO42-]). R[SO42-]>2 is indicative of (NH4)2SO4 , while 1<R[SO42-]<2 indicates a mixture of (NH4)2SO4 and NH4HSO4, and R[SO42-]<1 indicates a mixture of H2SO4 and NH4HSO4 (Nenes et al., 1998; Asa-Awuku et al., 2011; Padró et al., 2012). For Delhi, R[SO42-]>2 was obtained for all branches, indicating that (NH4)2SO4 was present in all branches. Furthermore, the non-sulfate NH4+ ions [ns-NH4+] were calculated, as per [ns-NH4+]=[NH4+]-2×[SO42-]. The r2 values were then determined for the coupling of ns-NH4+ ions with (a) Cl- ions, (b) NO3- ions, and (c) [NO3-+Cl-] ions jointly (Du et al., 2010). All r2 values are detailed in Table S1. This analysis revealed that (NH4)2SO4 is the dominant salt for AS and B branches based on rNH4+/SO42-2 values of 0.78 for AS and 0.75 for BB 0.75. NH4Cl formation for SA was confirmed by a high r2 value (0.90) for ns-NH4+ coupling with Cl-. A similar finding is reported by Bhandari et al. (2020) based on the coupling of the NH4Cl factor with wind direction. Coupling of ns-NH4+ with NO3- revealed a good correlation for B (0.70) and B.reg (0.63) (Fig. S6). In all cases, an increase in r2 for combined NO3-+Cl- as compared to individual ions indicates that both HNO3 and HCl were synchronously neutralized by NH3. If Cl- and NO3- are present in the fine mode, they are expected to be in the form of their respective ammonium salts (Harrison and Pio, 1983). Thus, the dominating salts were (NH4)2SO4 for AS, (NH4)2SO4 and NH4NO3 for the BB air mass, and NH4Cl for SA and its sub-branches.

The organic speciation revealed that AS organics were BBOA; BB (both B and B.reg) organics were a mix of the three; and SA organics were BBOA, wherein L, R1, and R2 organics were BBOA while R3 organics were both HOA and BBOA.

The NOx emissions (in µg m-3) for the SA air mass (96.88±127.22) were the highest, followed by those in BB (38.30±64.79) and then those in the AS (36.71±68.13) air mass. BB is representative of B.reg only, as NOx data for the B air mass were not available. NOx data for AS were also scarce. The SA aerosols exhibited less ageing (4.38±4.49 h) compared to B.reg. (11.58±3.45 h), but both were representative of aged aerosols. Ageing was not calculated for AS due to very little data availability.

To determine the presence of biomass burning and traffic emissions, BCwb (wood-burning component) and BCff (traffic component) for all air masses were determined, based on aethalometer data, as per Sandradewi et al. (2008). The contributions of BCwb and BCff are summarized in Table S4. Since fossil fuel sources are active year-round, there was a strong presence of BCff, ranging from 70 % to 86 %. However, biomass burning is only active during certain specific times for short durations and is very prominent in the north-western direction for the SA air masses. It was observed that the more distant air masses exhibited a higher BCwb contribution compared to those originating within proximity. Hence, while L was associated with 13.9 % BCwb, R3 exhibited 29.2 % BCwb. The BCwb contribution for A and BB air masses was 21 %. It can thus be concluded that both biomass burning and traffic emissions are important sources contributing to the chemical composition of the various air masses.

This study provides the first long-term estimation of aerosol hygroscopicity in the PM1 regime. The mean κ was approximately the same for all the air masses, which is ∼0.3 (0.32±0.06 for AS, 0.31±0.06 for BB, and 0.32±0.10 for SA) and in line with the global average value of 0.27±0.21 for continental aerosols (Andreae and Rosenfeld, 2008; Petters and Kreidenweis, 2007; Pöschl et al., 2009; Pringle et al., 2010). Including BC in κ calculations leads to a difference of 10 % in κ on average, shifting the mean κ of 0.32 to 0.29. The BC mass fraction and volume fractions were 10 % and 9 %, respectively. Thus, the change in κ due to the introduction of BC was not significant.

κ varied from 0.13 to 0.77, and there was a difference in the diurnal variation in the hygroscopicity parameter for the various air masses (Fig. 4). A similar finding was observed in China, with a mean κ of 0.3, varying in the range 0.1–0.5 (Rose et al., 2010). Recently, κ of 0.42±0.07 was also reported for PM2.5 for Delhi based on beta attenuation monitor (BAM) measurements of PM2.5 (Wang and Chen, 2019), using an indirect method in the absence of direct measurements. Thus, the dependence of κ on size cannot be underestimated for Delhi and should be dealt with in the future.

Variation in κ with size has been seen in several places in the world; e.g. at 97 % RH, mass growth factors of 6.95 and 9.78 were reported for the size ranges 0.53–1.6 and 1.6–5.1 µm, respectively, on the Slovenian coast (Turšič et al., 2006). Aitken mode κ was 0.25, while the accumulation mode κ was 0.45 for Beijing (Gunthe et al., 2011). An increase in κ for the higher size regime points to the fact that the organic fraction is higher in the smaller size range, while the inorganic fraction increases substantially with size.

In the PM1 size range, while κ of 0.1 indicates secondary organic aerosol, κ varied from 0.01 to 0.8 for biomass-burning aerosols in lab studies (Petters et al., 2009). κ varied from 0.15 to 0.25, with lower values (around 0.16) being observed during the night, when biomass-burning particles prevailed during wintertime in Athens, Greece (Psichoudaki et al., 2018). Thus, κ values for Delhi can represent both secondary formation and biomass burning. This is true for Delhi, which had both POA and OOA in all the air masses, while BBOA was present in AS and SA air masses, as detailed in the preceding sections on chemical properties.

An important observation for all branches is that when the inorganic volume fraction (of dominant salt) increased (Fig. S2), during the times when κ was high (Fig. 4), or when the organic volume fraction decreased (Fig. S2), a dip in Dc (Fig. 6) was seen, implying that a larger size regime was available for activation. The diurnal variation in κ (Fig. 4) more strongly followed the diurnal pattern of the dominant inorganic salts for a cluster (Fig. S2), since the hygroscopicity parameters for inorganic salts are considerably higher than those of organics. Pearson correlation coefficient (r) values between κ and the salt volume fractions revealed that the diurnal patterns of κ were governed dominantly by volume fractions of (NH4)2SO4 (r: 0.85) for AS, moderately by (NH4)2SO4 (r: 0.55) and NH4NO3 (r: 0.49) for BB, and dominantly by NH4Cl for SA air masses. For the two BB branches, κ of the B branch was governed dominantly by (NH4)2SO4 (r: 0.78) and moderately by (NH4)2SO4 (r: 0.57) and NH4NO3 (r: 0.54) for B.reg. For SA air masses, κ of R1, R2, and R3 was governed dominantly by NH4Cl (r values of 0.71, 0.89, and 0.95) and jointly by NH4Cl (r: 0.65) and NH4NO3 (r: 0.73) for L.

High volume fractions of (NH4)2SO4 and NH4NO3 may be attributed to SOx, NOx, and NH3 emissions due to power plant emissions and traffic. In the SA sub-branches (Fig. 4), the spike in κ during the early morning (07:00–08:00) exhibited the sequence κ(R3)>κ(R2)>κ(R1)>=κ(L), and the lower spike in late evening (18:00–22:00) exhibited the sequence R3>R2<R1<L and is attributed to NH4Cl formation. This implies that during the morning, R3 aerosols were most hygroscopic, while L aerosols were least hygroscopic, while after 09:00, L aerosols were most hygroscopic and R3 aerosols were the least hygroscopic. The flatter curve of κ can be attributed to two factors: (a) the chloride contribution of distant trajectories decreased very steeply with time compared to the local emissions, and (b) κ of L was also supplemented substantially by NH4NO3. Thus, the source activities by which the chemical properties of aerosols are shaped impacted the hygroscopicity parameter tremendously. This consequently impacted the size regime of aerosols available for activation and is discussed in the following section.

CCN number concentration (NCCN) for SA (22 526 ± 13 439) was higher compared to that in BB (12 526 ± 5626) and AS (11 089 ± 6650), where values (cm-3) are given at 0.4 % SS. Amongst the SA sub-branches, NCCN followed an increasing order, as NCCN(L) (18 810 ± 9434) < NCCN(R3) (20 469 ± 10 580) < NCCN(R2) (23 736 ± 13 739) < NCCN(R1) (24 053 ± 14 743), while for the B branches, the order of increase in NCCN (cm-3) was NCCN(B) (11 699 ± 4900) < NCCN(B.reg) (14 088 ± 6506) at 0.4 % SS. Correspondingly, the activated fractions (af) followed the sequence af(SA) (0.70±0.15) < af(BB) (0.64±0.17) < af(AS) (0.55±0.18), wherein for SA sub-branches, af(R3) (0.65±0.16) < af(R2) (0.694±0.16) < af(L) (0.692±0.13) < af(R1) (0.71±0.15), and for BB, af(B.reg) (0.62±0.16) < af(B) (0.65±0.18) at 0.4 % SS. Mean NCCN and af for all branches at 0.1 %, 0.4 %, and 0.8 % SS are detailed in Tables 3 and 4, respectively. The total number concentrations (NCN in cm-3) followed the sequence NCN(AS) (20 558 ± 9654) < NCN(BB) (20 864 ± 9731) < NCN(SA) (31 406 ± 15 168), and for SA, NCN(L) (27 009 ± 11 651) < NCN(R3) (30 974 ± 12 223) < NCN(R1) (32 772 ± 16 475) < NCN(R2) (33 371 ± 14 989), and NCN(B) (19 025 ± 7704) < NCN(B.reg) (24 333 ± 11 956). Mean NCN values for all branches are listed in Table S2.

High values of NCCN for Delhi are consistent with other polluted regions in the world. The relevant statistics for two highly polluted sites, namely Beijing and Kanpur, are presented by Gunthe et al. (2011) and Bhattu and Tripathi (2015), respectively. NCCN was 7660±3460 and 900–27 000 (in cm-3) at 0.46 % SS and in the range 0.18 %–0.6 % SS, respectively. The high NCCN is consistent with the high NCN. Correspondingly, NCN was 16 800 ± 9100 and ∼ 50 000 (cm-3). Even though high number concentrations of CCN and condensation nuclei (CN) have been reported, the af was not seen to be so high. The af in Beijing was 0.54±0.23 and 0.66±0.23 at 0.46 % SS and 0.86 % SS, respectively. For Kanpur, af was reported as ∼ 0.018–0.54 for 0.18 %–0.60 % SS. However for Delhi, the af ranged from 0.19 for AS, at 0.1 % SS, to 0.86 for R1, at 0.8 % SS, implying that even at low SS, a considerably large number of particles were activated, and at high SS, almost all particles reached the activated state. It should be noted here that the statistics for Beijing and Kanpur correspond to the range (3–900 nm) and (14.6–680 nm), while the estimates for Delhi are given in the (10–560 nm) range. This finding is also consistent with Wang and Chen (2019), which states that for Delhi, activation of a 0.1 µm particle requires SS ∼0.18±0.015 %, compared with ∼0.3 % for Beijing; 0.28 %–0.31 % for Asia, Africa, and South America; and ∼0.22 % for Europe and North America. The high activated fractions of aerosol can impact the precipitation patterns in Delhi and may be responsible for the short, intense precipitation events and decrease in overall rainfall. However, no study to date has validated this growing trend with CCN measurements or estimates, and this needs to be investigated in the future.

The af and NCCN for all air masses increased as expected with an increase in supersaturation. The variation in CCN and the activated fraction with SS are shown in Figs. 5 and S3. The figures clearly show that even though NCCN for SA was far greater compared to BB and AS, the activated fractions were fairly close. B.reg had higher NCCN but a close af compared to B. Similarly, L had the lowest NCCN among all SA branches but the highest af. These features elucidate that many factors are at play and impact NCCN and af differently. To determine the governing parameters impacting both NCCN and af, the diurnal patterns of NCCN, af, and Dc at 0.1 %, 0.4 %, and 0.8 % SS; NCN, NAitken, and NAccumulation; κ; and geometric mean diameter (GMD) are shown in Figs. 6, S4, and S5.

The NCCN curve for SA showed a sharp diurnal feature which was not as prominent for the other two. However, at noon, at 0.4 % and 0.8 % SS, while NCCN dipped for SA, it rose for AS and BB (Fig. 6). Furthermore, with the increase in supersaturation, the dip in CCN of SA increased such that (a) at 0.1 % SS, SACCN>ASCCN, BBCCN; (b) at 0.4 % SS, SACCN≅ASCCN, BBCCN; and (c) at 0.8 % SS, SACCN<BBCCN and ≅ASCCN. The explanation for this observation is that at 0.1 % SS, the CCN was governed by the accumulation mode, but at 0.4 % and 0.8 % SS, it was governed more by the Aitken mode. It is the Aitken mode that dominantly governs the total number concentration, and hence it can be said that CCN is governed by CN at a higher SS and by the accumulation mode at low SS. This is because as supersaturation increases, Dc increases. At low SS, Dc is high (Dc>100 nm), almost always at a 0.1 % SS, for all branches, as shown in Fig. 6d; hence the size distribution that is integrated to get the CCN involves the accumulation mode only. At 0.4 % SS, the Dc was around 40–47 nm; therefore a considerable fraction of the Aitken mode and accumulation mode was available for activation. At 0.8 % SS, the Dc was ∼ 25–30 nm; therefore, the contribution of the Aitken mode further increases and the accumulation mode is also available as usual. These findings are also true for BB branches (B and B.reg) and SA branches (L, R1, R2, and R3), as shown in Fig. S4. It is the low value of Dc relative to other places that is responsible for high CCN. As explained in Sect. 3.4, Dc is largely associated with κ. The Dc at other places such as Kanpur varied from 50 to 200 nm, for SS ranging from 0.18 to 0.60 (Bhattu and Tripathi, 2015), compared to which Dc for Delhi is lower (17–142 nm, for SS ranging from 0.1 % to 0.8 %), implying that a larger regime is available for activation. It is pertinent to mention here that the dip in CCN for SA at midday and the peak at the same time for AS and BB can be attributed to the following. (a) The dip in NAccumulation for SA at midday was much more prominent compared to AS and BB. (b) At midday, the NAitken for SA also decreased, while it increased for AS and BB. Thus, the dip in NCCN was strengthened by the simultaneous dip of both the Aitken and accumulation modes, while the peak in NCCN for AS and BB was a manifestation of the dominant peak in Aitken modes. Similar features were also exhibited for sub-branches of BB and SA. A deeper insight reveals that the dip in number concentration at midday for SA was most aptly seen in the diurnal pattern of POA (which is the most dominant NRPM1 species) and, to quite a good extent, in other NRPM1 species barring the SO42- ion. Similarly, the peak in the Aitken mode for AS and BB can be attributed to NH4+, SO42-, and OOA concentrations (the dominating species in the respective branches). Thus, the source activities and trajectory pathways impact CCN concentration at the receptor site.

The diurnal pattern for af showed a dip in the midday hours for all the air masses even though NCCN at midday for AS and BB peaked during midday hours. The time of dip in af (more prominent at 0.4 % and 0.8 % SS) occurred earlier for AS compared to that for BB and SA (Fig. 6). This was governed by the time of dip in the GMD of the three branches. The GMD diurnal variation was very similar to af, and the r2 values between GMD and af listed in Table S3 also point to the same result. Thus, even though a dip in Dc should correspond with an increase in af and vice versa, this does not usually happen, as the change in Dc is less compared to the shift in size distribution such that not only the highest number concentration values but also the diameter at which this occurs change, thereby changing the number available for activation. For example, in Fig. 7 two different size distributions for the AS air mass at 02:00 and 11:00 are compared. At 02:00 and 11:00, the following characteristics were noted: (a) GMD02:00: 83.78±16.58 nm (GSD – geometric standard deviation), which is considerably higher than GMD11:00: 47.18±13.01 nm; (b) GSD02:00: 1.69±0.13 nm, which is nearly the same as GSD11:00: 1.62±0.14 nm; (c) Dc02:00: 44.45±3.83 nm, which is slightly higher than Dc11:00: 43.48±2.28 nm (at 0.4 % SS); (d) CN02:00: 15 849 ± 9269 cm-3, which is lower than CN11:00: 25 873 ± 9840 cm-3; (e) CN_Aitken02:00: 9819±5945 cm-3, which is lower than CN_Aitken11:00: 22 376 ± 9693 cm-3; (f) CN_Accumulation02:00: 5729±3684 cm-3, which is higher than CN_Accumulation11:00: 3311±1569 cm-3; (g) af02:00: 0.75±0.12, which is considerably higher than af11:00: 0.38±0.17 at 0.4 % SS; and (h) CCN02:00: 11 595 ± 6710 cm-3, which is higher than CCN11:00: 8946±3899 cm-3 at 0.4 % SS. The af at 02:00 is higher than that at 11:00. A Dc decrease should correspond to a CCN increase, but the magnitude of ΔDc=0.97 nm is very small. The decrease in GMD (ΔGMD=36.6 nm), on the other hand, is very high, with negligible changes in GSD. At 02:00, since Dc was considerably less than GSD, most of the particles are counted for activation. At 11:00, since Dc and GMD were very close, nearly 50 % of particles are not available for activation. The very high Aitken mode concentration at 11:00 (higher than that at 02:00) was not available for activation at both the times. Thus, NCCN here is governed by the accumulation mode, which was higher at 02:00, thus making NCCN higher. In this scenario, NCCN and af go hand in hand. However, there also exists a second possibility; e.g. for the SA branch, NCCN, at 0.4 % SS, at 08:00 is higher than that at 05:00, while the af at 08:00 is lower than that at 05:00 (Fig. 6). NCCN was governed at these times by the Aitken mode, which was higher at 08:00, while af is governed by the GMD, which was lower at 08:00. Therefore, it is established that CCN is governed by CN (dominantly by either the Aitken or accumulation mode as the case may be), while the af is governed by GMD.

At this junction, it is also pertinent to mention how chemical dispersion and parameters governing CCN are interconnected. The standard deviation of κ (σ(κ)) around κ is often used as an estimate of the degree of heterogeneity (chemical dispersion) of particles (Psichoudaki et al., 2018; Lance et al., 2013). The chemical dispersion for all air masses and their sub-branches is shown in Figs. 6, S4, and S5. The chemical dispersion for the SA air mass during the early hours (06:00–08:00) coincided with chloride emissions, and late at night, after 20:00, it coincided with POA and OOA emissions. During the time of high chloride emissions, κ also peaked, since inorganics are associated with high hygroscopicity, while during the late hours, κ dropped due to an increase in organics associated with low hygroscopicity. The diurnal patterns of the activated fraction, GMD, and chemical dispersion were also similar. This implies that higher heterogeneity shifts GMD towards a high value, thereby increasing the available regime for activation, and vice versa. There was no discernible pattern noted for the other air masses.

It is hereby established that aerosol physical and chemical properties, and their time evolution, are tightly linked with each other. The indirect impact of chemical composition on CCN and af is manifested in two ways: (a) NRPM1 species impact the diurnal patterns of Aitken and accumulation modes, which in turn impact CCN, and (b) NRPM1 species impact κ, hence subsequently CCN, by impacting the size regime available for activation.