The regional picture of the altitude variations of the CN, CCN (SS = 0.1 %) and AR (SS = 0.1 %) for the pre-onset phase is shown in Fig. 4. The most striking feature is that there is a steady increase in the AR with altitude in the western and central IGP, and the altitude variations are much weaker in central India and almost absent in the eastern IGP, indicating a difference in the aerosol properties across the IGP. Below 2 km the CN concentrations are highest in the central IGP (where the anthropogenic emissions are higher), closely followed by the eastern IGP (BBR) and western IGP (JPR), with the lowest values in central India (NGP). The CN concentrations above 2 km are comparable at all the stations except BBR, where the CN concentrations were high due to the presence of a thick elevated layer in the 2.5 to 3.5 km region. The main factor controlling the CCN activation at low SS (∼ 0.1 %) is the hygroscopicity of the particles (Meng et al., 2014), which is determined by its chemical composition. Gunthe et al. (2009) found that particles with diameters ∼ 50 and ∼ 200 nm respectively had hygroscopicity parameters 0.1 and 0.2 respectively.

The altitude variations of CCN concentrations are not as well defined as those of CN. Below 1.5 km the CCN concentrations were highest in the eastern IGP (except for a sharp drop in the concentrations near 0.5 km), followed by LCK, JPR and NGP. CCN concentrations increase in the 1.5 to 2.5 km range in all data except those from NGP. Above 2.5 km, the magnitudes of CCN at various locations differ widely (though with an overall decreasing trend with altitude), with the highest concentrations at BBR followed by LCK. The lowest concentrations were observed at NGP, where the values were comparable to JPR in the 3 to 3.2 km altitude range; measurements above 3.2 km are not available for JPR.

The AR (at SS = 0.1 %) clearly indicates two features: (a) the presence of more CCN-active aerosols at higher altitudes in the western and central IGP as well as central peninsula, which is not seen over the eastern IGP (b) over the most anthropogenically impacted central IGP, two different aerosol types with a less CCN-active layer below ∼ 2 km, and a more CCN-active layer above leading to a change in the CCN / CN ratio in the altitude region 2 to 3 km. The vertical distribution of the concentrations and the AR of aerosols had more distinct regionally varying patterns above 3 km. The increase in the activation ratio above 3 km observed in our study has not been reported earlier. The regional variations in the CCN concentrations above 3 km were found to have an east-to-west gradient, with the highest values in the east. To understand the peculiar behaviour of AR over the eastern IGP with two elevated peaks in the CN and CCN (near 1 and 3 km) and a nearly steady and low value of AR, we examined the air-mass back trajectories in Fig. 5. These clearly reveal that the altitude region of 1.5 to 3 km was under the influence of distinct advection pathways. One path favours the advection of more hygroscopic particles (as evidenced by Fig. 3a and b) from the north-western region (magenta colour in Fig. 5), at around 3 km altitude, while the other encounters less CCN-active aerosols being lofted from close to the ground, from sources in the south-west (central IGP), as it approached BBR (red line in Fig. 5). The presence of less hygroscopic particles (probably freshly emitted BC) in the lower altitudes < 1 km is seen from Fig. 3a and d (around the central IGP and central peninsula). The mixing of these two types of particles in the altitude region of 1.5 to 2.5 km seems to contribute to the elevated peaks in CN and CCN as well as the near-steady value of AR in this region, in contrast to the increase seen at other locations. From an independent airborne measurement of CCN characteristics (again as a part of SWAAMI, using an Indian aircraft) during the first week of June, a week prior to our measurements, Jayachandran et al. (2020a) observed almost similar features in the eastern IGP, with the CCN concentrations (at 0.4 % supersaturation) below 1 km being comparable to that in the central IGP, while in the 1 to 3 km range the CCN concentrations were higher compared to the central IGP. The complex interplay of different advected species appear to be responsible for this, at least partly. The results by Jayachandran et al. (2020a) are however limited to the changes happening within the boundary layer and fail to completely capture the elevated aerosol layers which we found to exist even above 3 km.

It is well known that the CCN concentration strongly depends on the number size distribution and chemical composition of aerosols, while the supersaturation spectra depend on the aerosol number size distribution (Fitzgerald, 1973). If ammonium sulfate and adipic acid with a size of 100 nm are considered, the former can be activated at 0.15 % SS, while the latter can only be activated at 0.27 % SS (Hings et al., 2008; Zhang et al., 2012). As such, we examined the changes in the chemical composition of aerosols at BBR, from concurrent measurements by other investigators aboard the same flight (Brooks et al., 2019a), who have reported a sharp increase in the concentration of organics near 1 km (Brooks et al., 2019a; Fig. 11c; B957 PM), where our observations show a sharp increase in CCN concentration and a weak increase in AR, which otherwise remained featureless (nearly steady at ∼ 0.08) in the entire altitude region. Moving to higher altitudes, measurements by Brooks et al. (2019a, same figure) have shown a large increase in the concentration of sulfates, NH4, organics and BC in the altitude region of 2.5 to 3 km, where our observations show a broad peak in CN and CCN concentration but with no perceptible impact on AR. Residential sectors are the main source of organics in the region, while brick kilns (which use coal and lignite) emit large amounts of sulfates (Pandey et al., 2014). We hypothesize that though the increase in the concentrations of sulfates and organics in this region was favourable for an increase in AR, the simultaneous increase in BC (hydrophobic) prevented any conspicuous impact on AR. This is also supported by the observations by Jayachandran et al. (2020a) of low CCN efficiency associated with high concentrations of BC based on independent measurements made a week prior to our measurements. While Jayachandran et al. (2020a) showed that the presence of BC can reduce the activation efficiency of aerosols in the boundary layer, we found that this also happens at higher altitudes. Consequently, though CN and CCN increase in line with the increase in the concentration of precursor gases (Brooks et al., 2019a), AR remains nearly unaffected. This did not happen for the peak around 1 km, because of the lower concentration of BC at that altitude (Brooks et al., 2019a). Extending the above role of chemistry to the central and western IGP regions, we recall that Brooks et al. (2019a) showed that within the boundary layer, the concentration of organics (43 %) exceeded the concentration of sulfates (29 %) in the submicron mass over the central IGP, whereas in the western (JPR/JDR) and eastern (BBR) IGP, sulfate was the dominant species, contributing 44 %, followed by organics (30 %). However, this distinction was confined within the boundary layer, above which sulfates dominated throughout the IGP region. This implies that in the central IGP, local emissions contributed significantly at the lower altitudes. This, at least partly, accounts for the increase in the AR above the boundary layer seen in both the western and central IGP seen in Figs. 3a and b and 4c. The low activation within the boundary layer, in the central IGP, appears to be at least partly associated with the high concentrations of BC (which is hydrophobic in nature when freshly emitted), emitted by the local sources. Similar observations are also reported by Jayachandran et al. (2020a) from near-concurrent measurements at Varanasi in the central IGP (close to LCK) aboard another aircraft about 1 week prior to our measurements. Thus, we see that along with advection, the changes in the chemistry (concentration of precursors) also have a significant role in producing the observed spatial variation of the altitude profile of CCN characteristics and AR across the IGP prior to onset of monsoon.

During the pre-monsoon period, the entire IGP is extremely hot, with temperatures routinely above 40 ∘C, going as high as 48 ∘C during the peak. The resulting strong convective mixing distributes local surface-based emissions deep into the planetary boundary layer (PBL), which itself is deep (going to 2 km or more). Above the PBL, however, long-range transport has a strong influence on the altitudinal distribution. We examined the 3 d back trajectories of the air mass reaching LCK (central IGP) and JPR (western IGP) in Figs. S3 to S6. The figures clearly show that while long-range transport had a negligible role below 1 km, it influenced significantly above 2 km. This lends further support to our inference that in the central IGP the local emission of fresh hydrophobic particles (like BC) is responsible for the low AR within the PBL (< 2 km), and the long-range transported aerosols including dust, discussed in more detail below, lead to the increase in AR and a subsequent increase in CCN (despite the decrease in CN) above 2 km. Jayachandran et al. (2020a) do not cover the transport of aerosols above the boundary layer, which we found were more hygroscopic and more amenable to CCN activation compared to the boundary layer aerosols.

Examining the size distributions, it is well known that during the pre-monsoon, accumulation-mode and coarse-mode particles dominate the aerosol volume size distribution respectively within and above the PBL in the central IGP, due to the influence of the prevailing wind, flowing from the arid, dusty regions in the north-west of India (Gautam et al., 2011). The accumulation-mode number concentration was greatest within the boundary layer, coinciding with high organic aerosol loading. However, in the western IGP, coarse-mode (dust) aerosols (emitted locally) prevailed at all heights. Concurrent measurements by Brooks et al. (2019b) have shown that the BC within the boundary layer was not coated thickly (rather freshly emitted), but above the boundary layer the BC had thick coating (aged BC) in both the central and western IGP (LCK and JPR). The relatively larger particles above the boundary layer are better amenable for activation compared to the smaller particles within the boundary layer. Observations from the central Himalayan site, Nainital, located 1958 m above sea level (Dumka et al., 2015, 2021), revealed that aged and coated BC aerosols (consequently bigger in size), transported to this location from the IGP, were more hygroscopic compared to freshly emitted BC aerosols. In the western IGP the CN and CCN variations go hand in hand with a high correlation coefficient of 0.94. The AR is high and comparable to the values in the central IGP. The increase in AR with altitude is also observed in the dust-dominated western IGP, similar to the central IGP. Though freshly emitted dust is hydrophobic, they become CCN-active when mixed with other species like sulfates and nitrates (Kelly et al., 2007). It may also be noted that the air mass arriving at this region has considerable overpass above the Arabian Sea, gathering moisture and allowing the aerosols to mix with marine aerosols, thus enhancing their hygroscopicity (Fig. S3). The relation of the observed changes in AR with the chemical composition, BC mixing state and size distribution changes are further investigated in the coming sections also for the observations during monsoon. The low values of CN and CCN near NGP (central peninsula) might have been partly due to the rain in the vicinity prior to our measurements. The AR has a weak altitudinal variation with values increasing above 3 km, indicating a different type of aerosol, but other measurements are not available to further investigate the observed changes.

Summarize the pre-onset scenario.

Elevated aerosol layers are present throughout the IGP region, above 2 km, and are identified by large increases in the number concentrations of CN and CCN. However, the AR in these elevated layers did not show sharp changes, implying that the sources are more or less homogeneous, at all altitudes within a region, except perhaps in the central IGP.

The above characteristics of CCN are re-examined during the active phase of monsoon, based on the FAAM data collected during Phase 3 of the campaign, covering the different sub-regions of the IGP (east, central and west – BBR, LCK, and JPR) and AMD during 2–7 July. By this date the monsoon is established over the entire Indian region (Figs. 1 and 2). The mean profiles of CN and CCN concentrations were estimated using 14 profiles for LCK, 11 profiles for JPR and 2 profiles each for the other two stations as detailed in Table 1. The variation of the mean CN concentration, CCN concentration and the AR for all the above four locations are presented in Fig. 6a–d following the same methodology as in Fig. 3.

The changes from prior to onset of monsoon to the active phase of monsoon are clearly depicted in Fig. 7, where all three parameters are compared. The most conspicuous features revealed by Figs. 6 and 7 are the following.

Very large reduction in the concentrations of CN and CCN during the active phase (from the values which prevailed prior to the onset) at all altitudes and across the entire IGP, with concentrations of CCN dropping more dramatically than the CN.

During Phase 3, there was an additional profiling on 7 July in western India, over AMD, south of the western IGP. The mean features, shown in Fig. 6d, are (a) a decrease in CN concentrations up to about 2 km and a nearly steady profile above up to about 5 km and (b) a monotonic decrease in CCN concentration and AR from close to the surface to the highest altitude (5 km), somewhat resembling the pattern seen in the western IGP but with a stronger altitudinal variation of AR.

While the CN concentrations are almost the same in the western stations (JPR and AMD) at all altitudes, the CCN concentrations are higher in AMD (Fig. 6b and d), indicating a more CCN-active aerosol. This is attributed to the stronger advection of marine aerosols from the Arabian Sea, bringing in more hygroscopic aerosols to AMD (Fig. S7).

With a view to furthering the above understanding of the spatial and vertical variation in CCN characteristics and the changes with respect to the monsoon activity, we have examined the CCN spectra (variation of CCN concentration as a function of supersaturation) using the measured data. It is well established that the ability of an aerosol to be CCN is a function of both the hygroscopicity and size distribution of aerosols (Twomey and Wojciechowski, 1969; Hegg et al., 1991; Khain, 2009; Jefferson, 2010). Following Twomey (1959) and Cohard et al. (1998), we have parameterized the CCN spectra using a power-law relation. 1NCCN=CSSk, where NCCN is the number concentration of CCN at a particular supersaturation (SS), and C and k are empirical coefficients. Lower values of k imply quick activation of CN even at low SS and are generally associated with more hygroscopic and coarse-mode aerosols (such as sea spray). On the other hand, higher values of k mean more activation only at higher SS, typical of less hygroscopic and fine-mode anthropogenic aerosols (Hegg et al., 1991; Jefferson, 2010). The altitude variation of k will have implications for aerosol–cloud interaction through hygroscopicity and size distribution of aerosols (for example, Raga and Jonas, 1995).

The CCN spectra are shown in Fig. 8, where the panels from left to right show the spectra across the IGP from west to east; in each case panels a, b and c represent the conditions just prior to the onset of the monsoon (Phase 1 of the campaign) and panels d, e and f represent the active monsoon (Phase 3) conditions. The spectra are limited only up to SS = 0.4, as the SS range during measurements was restricted in the range 0.1 % to 0.4 %, which makes it possible to make high-resolution measurements even at higher altitudes. The lines are regression fits to Eq. (1). The figure reveals the following.

In general, irrespective of the phase of the monsoon, k values are highest in the central IGP (LCK), though the values during the active phase of the monsoon are lower than the values just prior to the onset of the monsoon. This confirms the prevalence of submicron aerosols with lower hygroscopicity over the central IGP and is also in line with the high concentration of anthropogenic aerosols in that region (denser sources of emissions). This is also supported by the high values of BC (∼ 2 µg m-3) that prevailed over this region, as has been reported by Brooks et al. (2019a) from concurrent measurements and long-term observations from Nainital (Dumka et al., 2021). As the CCN concentrations at higher supersaturations are mainly attributed to accumulation and fine-mode particles (Lance et al., 2013), the higher values of k over the central IGP clearly suggest prevalence of an aerosol system dominated by fine-mode particles during both phases of the monsoon.

The FAAM measurements provided an opportunity to examine the changes occurring in CCN characteristics in the vertical across the IGP during the contrasting phases of the monsoon. As such, we examined the CCN spectra for the free troposphere (3 to 5 km altitude) separately from the spectra for the lower altitudes (less than 3 km). This also facilitates examination of the lower-altitude features with those derived from the measurements aboard the Indian aircraft in the pre-monsoon period, about a week to 10 d prior to Phase 1 of FAAM (Jayachandran et al., 2020a), which was confined only to the lower atmosphere. The panels in Fig. 9 show the results during the active phase of the monsoon, when there was a clear difference in the k values in the upper atmosphere from those in the lower regions. It clearly emerges that over both the locations, aerosols in the upper atmosphere (free troposphere) are more hygroscopic (with lower k values) than those in the boundary layer, where the influence of local emissions would be felt more.

Vertical variations in the values of k have been examined over the Indian region based on a few aircraft measurements in recent years under different campaigns (e.g. Varghese et al., 2016; Jayachandran et al., 2020a). However, our study is the first one focusing on the transformation of CCN characteristics across the phase of the monsoon from prior to its onset to the active phase that followed immediately. One important finding is the significant decrease in the k values (increase in the CCN activity) of aerosols across the IGP during the active phase of the monsoon, from its values just prior to the onset. Despite this feature, there is spatial distinctiveness across the IGP. In both phases of the monsoon, the central IGP with significant anthropogenic activities and associated emissions (from industries, thermal power plants, automobiles, etc.) is less CCN-active, with k values lying in the range (2.07 prior to onset of monsoon and 1.46 during the active phase), while aerosols in the western and eastern IGP are more easily activated. Similar spatial distinctiveness has also been reported by Jayachandran et al. (2020a) during the pre-monsoon period.

Another important finding emerging from our study is the decrease in k values with altitude during the active phase of the monsoon, showing prevalence of more hygroscopic aerosols in the free troposphere. This is in sharp contrast to the results reported for the pre-monsoon period by Jayachandran et al. (2020a), who found a significant increase in k values with altitude across the entire IGP, indicating a decrease in the hygroscopicity and/or increasing dominance of fine and accumulation aerosols at higher altitudes. Low values of k at higher altitudes as seen in our study are also in line with the low values reported by Dumka et al. (2015) from a Himalayan station at 2 km altitude based on measurements during the RAWEX–GVAX campaign. Similar low values of k were also reported by Roy et al. (2017) over the high-altitude (2.2 km) site Darjeeling located in the eastern part of Himalayas in India. In the central peninsula, Jayachandran et al. (2020b) reported smaller k values for the continental air mass compared to marine air mass due to the presence of coarser particles in the continental air mass. The near-flat CCN spectra around BBR are a consistent feature (Jayachandran et al., 2020a) and appear to be typical of coastal regions, where highly hygroscopic and coarse-mode marine aerosols are available in large numbers (Jayachandran et al., 2017). However, it should be kept in mind that the k values depend on the supersaturation range used for its estimation, and in our study it was limited to only 0.4 % in order to extend the measurements to higher altitudes. Similar differences between the near-surface and below-cloud values of k were also reported by Varghese et al. (2016) during the CAIPEEX, who found higher k values (0.72) associated with polluted conditions and low k values (0.25) during clean conditions.