Seasonal variation of vertical distribution of aerosol single scattering albedo over Indian sub-continent: RAWEX aircraft observations

Atmospheric Environment 125 (2016) 312–323

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Seasonal variation of vertical distribution of aerosol single scattering albedo over Indian sub-continent: RAWEX aircraft observations S. Suresh Babu a,∗, Vijayakumar S. Nair a, Mukunda M. Gogoi a, K. Krishna Moorthy b a b

Space Physics Laboratory, Vikram Sarabhai Space Centre, Thiruvananthapuram, India Indian Space Research Organization Head Quarters, Bangalore, India

h i g h l i g h t s •

Altitude profiles of aerosol SSA and its seasonality over Indian landmass. Enhancement in free tropospheric aerosol absorption in spring. Near-surface loading peaks during winter and decreases in spring. Pre-monsoon aerosols are more absorptive than winter.

• • •

a r t i c l e

i n f o

Article history: Received 13 February 2015 Received in revised form 13 September 2015 Accepted 15 September 2015 Available online 16 September 2015 Keywords: Aerosol absorption Scattering coefficients Single scattering albedo

a b s t r a c t To characterize the vertical distribution of aerosols and its seasonality (especially the single scattering albedo, SSA) extensive profiling of aerosol scattering and absorption coefficients have been carried out using an instrumented aircraft from seven base stations spread across the Indian mainland during winter 2012 and spring/pre-monsoon 2013 under the Regional Aerosol Warming Experiment (RAWEX). Spatial variation of the vertical profiles of the asymmetry parameter, the wavelength exponent of the absorption coefficient and the single scattering albedo, derived from the measurements, are used to infer the source characteristics of winter and pre-monsoon aerosols as well as the seasonality of free tropospheric aerosols. The relatively high value of the wavelength exponent of absorption coefficient over most of the regions indicates the contribution from biomass burning and dust aerosols up to lower free tropospheric altitudes. A clear enhancement in aerosol loading and its absorbing nature is seen at lower free troposphere levels (above the planetary boundary layer) over the entire mainland during spring/pre-monsoon season compared to winter, whereas concentration of aerosols within the boundary layer showed a decrease from winter to spring. This could have significant implications on the aerosol heating structure over the Indian region and hence the regional climate. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction The atmospheric aerosols over the south and southeast Asia has been the topic scientific interest because of projected regional climate impacts as well as the implications of these fine particles on the hydrological cycle, air quality and health of the millions of people living in this region (Lau et al., 2008; Lawrence and Lelieveld, 2010). The regional radiative forcing due to aerosolradiation and aerosol-cloud interactions are assessed (through observations and modelling) to be significantly higher than that of the other climate forcing agents (for eg: greenhouse gases) over

∗

Corresponding author. E-mail address: [email protected] (S. Suresh Babu).

http://dx.doi.org/10.1016/j.atmosenv.2015.09.041 1352-2310/© 2015 Elsevier Ltd. All rights reserved.

this region (Ramanathan, 2003). The sustained efforts since the first regional compilation by Mani et al. (1969) and the subsequent systematic measurements made from a network of multiinstrumented observatories under the Aerosol Radiative Forcing over India (ARFI) project, and the extensive field campaigns from INDOEX (Indian Ocean Experiment) (Ramanathan et al., 2001) to ICARB (Integrated Campaign for Aerosols, gases and Radiation Budget) (Moorthy et al., 2009) have revealed significant amount of aerosol loading over the Indian sub-continent and surrounding oceans (Lawrence and Lelieveld, 2010), which has been depicting a steady increasing trend (Babu et al., 2013). South Asian aerosols show significant seasonality in association with the monsoon circulation; anthropogenic aerosols dominate during winter and natural aerosols dominate during summer seasons (Ramanathan et al., 2001; Moorthy et al., 2007a; Vinoj et al., 2014). Several studies

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have addressed the implications of these aerosols on summer monsoon (Ramanathan et al., 2005; Lau et al., 2006; Vinoj et al., 2014). However, heterogeneous processes associated with aerosol– climate interactions are still uncertain over this region. During the last two decades, there have been numerous field campaigns, laboratory experiments and continuous observations from ground based and space borne instruments to characterize the extensive and intensive aerosol properties for the accurate estimation of aerosol radiative forcings (Ramanathan et al., 2001; Moorthy et al., 2009; Lawrence and Lelieveld, 2010). In general, aerosol optical depth, single scattering albedo and aerosol phase function are the most important aerosol parameters essential for understanding the aerosol-radiation interactions (Direct effect). Even though the accuracy of the estimation of aerosol optical depth has significantly improved in the past decade due to the expansion of network observatories and improvements in the satellite based observation techniques, the uncertainties in the estimation of single scattering albedo, which decides the sign of the aerosol direct radiative forcing, remain unaddressed for a quite long time (Heintzenberg et al., 1997). This arises mainly from the technical challenges in measuring the aerosol absorption continuously from the surface or space. The quantification of aerosol absorption is essential not only for the assessment of direct radiative forcing, but also for the aerosol-cloud (semi-direct effect) and aerosol-cryosphere (snow albedo reduction due to black carbon deposition) interactions (Nair et al., 2013a,b). Even though there exist continuous measurements of columnar aerosol properties and surface level aerosol parameters from ARFI network, the lack information on the vertical distribution of aerosols, especially aerosol absorption, have hindered providing observational evidence to various hypotheses put forward by numerical modelling (Lau et al., 2006). However, this is very important because the effects of a given aerosol layer strongly depends on its altitude in the atmosphere and position with respect to clouds. For example a given layer of absorbing aerosols can produce higher warming, if it is located higher in the atmosphere due to the exponentially decreasing air density with altitude. Recognizing the importance of three dimensional information of aerosol properties and realizing the scarcity of such measurements over South Asia and especially over Indian mainland, Regional Aerosol Warming Experiment (RAWEX) has been conceived with its main objectives including (i) establish high altitude aerosol observatories for the continuous measurements of free tropospheric aerosols (Babu et al., 2011a) (ii) high altitude balloon experiments for vertical profiles of black carbon mass measurements (Babu et al., 2011b), and (iii) effect of black carbon deposition in snow albedo over Himalayas (Nair et al., 2013a). In the backdrop of the outcomes from these experiments, and to further investigate the vertical distribution of aerosol properties as well as the seasonality in their spatial variations, extensive campaign-mode measurements were carried out over the Indian mainland using an instrumented aircraft during the winter of 2012 and spring/pre-monsoon season of 2013. This paper presents the details of this campaign and the results of the characteristics of SSA derived from these measurements, perhaps providing this information for the first time over the entire mainland.

313

ter phase) and May, 2013 (spring/pre-monsoon phase). Measurements were made from 7 base stations (Hyderabad, Nagpur, Lucknow, Patna/Ranchi, Jaipur, Jodhpur and Dehradun) as shown in Fig. 1. Due to foggy and low visibility conditions during the winter phase, measurements at Patna were called off at the last moment and the aircraft moved to the nearby airport, Ranchi for measurements. Table 1 shows the details of location, time period and instruments operated during the campaign. Over the mainland, Hyderabad and Nagpur are representatives of central India (CI), which are urban locations with relatively arid climate; Lucknow and Patna/Ranchi represent the Indo Gangetic Plain (IGP), the most populated and industrialized region of India; Jaipur and Jodhpur represent the west India (WI), which are close to Thar desert and exhibit mostly dry climate; and Dehradun representing the Himalayan foothills (HF). In the present study we have used a shrouded solid diffuser inlet (University of Hawaii) to sample the ambient air at near-isokinetic conditions for all the instruments aboard the aircraft. The inlet is fitted under the fuselage of the aircraft opening into the flow as the aircraft flies. Based on laboratory calculations, a volumetric flow rate of 70 LPM (litres per minute) has been used and the aircraft flight speed was maintained around 300 km h−1 during sampling. The aerosol sampling onboard aircraft has inherent uncertainties associated with the sampling efficiency especially for particles above 1 μm. McNaughton et al. (2007), have reported (during DC-8 Inlet characterization Experiment (DICE)) that the ‘University of Hawaii (UH) inlet (used in the present study)’ can be used very effectively for sampling particles below 4 μm and they have reported very good association between the scattering coefficient measured using the UH inlet system and reference measurements. Based on inlet characterization experiments, McNaughton et al. (2007) and Huebert et al. (2004) have shown that solid diffuser inlet can be used for aerosol sampling in the sub-micron size range (50% passing efficiency aerodynamic diameters is 5 μm) efficiently. The profiling at each station, consisted of three vertical sorties, made during consecutive days following a staircase pattern as shown in Fig. 2. All the sorties were made around noon time, so that the aerosols are well-mixed within the atmospheric boundary layer by thermal convections which of course is stronger

2. Instrumentation and campaign details Exhaustive measurements of aerosol properties (concentration, size distribution, and coefficients of scattering and absorption) were carried out during the RAWEX campaign using an instrumented aircraft (Beechcraft, B200) of National Remote Sensing Centre (NRSC), Hyderabad during November–December, 2012 (win-

Fig. 1. Base stations from which aircraft sorties were carried out during winter (2012) and spring (2013) seasons. Hyderabad and Nagpur are representatives of central India, Jodhpur and Jaipur represent West India, Lucknow and Patna/Ranchi represent Indo Gangetic Plain and Dehradun represent Himalayan foothills.

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S. Suresh Babu et al. / Atmospheric Environment 125 (2016) 312–323 Table 1 Details of the RAWEX aircraft experiment. Region

Central India Nagpur Northwest India Jodhpur Indo-Gangetic Plain Patna Ranchi Himalayan foothills a

Base station details

Dates

Instruments operated in all sorties

Station name

Lat (°N)

Lon (°E)

Winter 2012

Spring/Pre-monsoon 2013

Hyderabad 21.2 Jaipur 26.3 Lucknow 25.6 23.4 Dehradun

17.4 79.1 26.9 73.0 26.8 85.1 85.3 30.3

78.5 3–5 Dec 75.8 17–19 Dec 80.9 – 27–29 Dec 78.4

18–19 Nov 01 Dec 30 Apr, 01–02 May 22–24 Dec 18–20 May 8–10 Dec 09–12 May – 12–15 Dec

25–27 Apr 22–24 May 05–07 May

Aethalometer, Nephelometer, PhotoAcoustic Soot Spectrometera , GPS, Aerodynamic particle sizera .

14–16 May

The data from the PhotoAcoustic Soot Spectrometer, Aerodynamic particle sizer are not discussed in this paper.

lengths at a sampling rate of 1 min. To correct the inherent biases of the instrument due to angular and wavelength non-idealities, we have used the correction scheme proposed by Anderson and Ogren (1998) using the spectral information of scattering coefficients (Nair et al., 2009). The wavelength dependence of scattering coefficients (σ sca ) is parameterized using a relationship similar to the Angstrom equation used for aerosol optical depth–wavelength relationship.

σsca (λ ) = σ0 λ−αsca

Fig. 2. Typical staircase pattern followed for measuring the vertical distribution of aerosol properties onboard the aircraft. Measurements were carried out for 4 h spending ∼30 min in each level.

and deeper during the spring season. Considering the aircraft endurance (4 h) and the ceiling altitude permissible for unpressurized mode of operation (∼4 km), six altitude levels have been chosen for every sortie, with about thirty minutes of sampling at each level. To ensure the iso-kinetic flow and stability in the measurements, we have excluded the first five minute data after attaining each level from the further analysis. As such, the average sampling period in each level varied between 20 and 25 min. The measurements includes spectral scattering coefficients (σ sca ) using integrating nephelometer (TSI 3653, USA), spectral absorption coefficient (β aeth ) using Aethalometer (AE 33, Magee scientific, USA), made aboard the aircraft are used in this study. Spectral scattering coefficients were measured using a 3wavelength (450, 550 and 700 nm) integrating nephelometer (Model 3563 of TSI, USA) onboard the aircraft. The instrument design, theory, calibration and uncertainties are well documented in Anderson et al. (1996) and its application in field experiments over India, in earlier campaigns have been described by Nair et al. (2009). Following the standard protocol, the instrument was calibrated prior to the campaign using CO2 as high-span gas and air as low-span gas and the stability of the calibration parameters were ensured after the campaign. The instrument aspirates ambient air at a flow rate of 16 LPM from the common air inlet. Each measurement cycle of the nephelometer include total (7°– 170°) and back (90°–170°) scattering coefficients at three wave-

(1)

where α sca and σ 0 are wavelength exponent and the y intercept respectively and λ is wavelength in micrometers. The wavelength exponent (α sca ) has been widely used for qualitatively inferring the dominant size range of the aerosol distribution. Anderson et al. (1996) have reported that the uncertainties in the scattering coefficient measurements by nephelometer are ∼±10%. The Aethalometer (model AE 33 of Magee Scientific, USA) measures the attenuation of light passing through a quartz fibre filter tape before and after the aerosol loading on the filter, at seven wavelengths 370, 470, 520, 590, 660, 880 and 950 nm. This difference in attenuation (ATN) is used to estimate the black carbon mass concentration using a manufacturer defined mass specific absorption cross section of 16.6 m2 g−1 at 880 nm (Hansen et al., 1984). The set flow rate and time base of the instrument for the RAWEX measurements are 6 LPM and 3 min respectively. The aethalometer data has been corrected for the change in flow rate with altitude using Moorthy et al. (2004). The spectral attenuation measurements made using the aethalometer is used to estimate the aerosol absorption coefficients (β aeth ).

βaeth (λ ) =

A ATN(λ ) V t

(2)

where A is the area of the loading spot, V is the flow rate and

t is the time base. The major uncertainties associated the estimation of absorption coefficients using filter based optical attenuation techniques are multiple scattering and shadowing effects. We have used the correction scheme proposed by Arnott et al. (2005) to correct the absorption coefficient using the simultaneous measurements of scattering coefficients from nephelometer (Nair et al., 2008). Using the wavelength dependence of scattering coefficients (α sca ), the nephelometer values are extra/interpolated to the aethalometer wavelengths. Arnott et al. (2005) have shown that the corrected values are very close to the independent and simultaneous measurements made using PASS with an accuracy of ±20%. It is also known that there are pressure and flow integrity issues, especially if these instruments are flown in pressurized conditions. Since RAWEX measurements were carried out in

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an unpressurized condition (ambient and cabin pressure remained the same) and the ceiling altitude never exceeded 650 hPa (corresponding to 3–3.5 km agl), the leak and pressure integrity issues have been only secondary in nature. However, one important issue in using Aethalometer onboard aircrafts is to ensure steady sample flow. We have frequently monitored the total flow and Aethalometer flow rates externally to ensure that instrument values are accurate. In addition, since it is very difficult to maintain a constant flow during takeoff, landing, ascending and descending from one level to other, we have excluded the first 5 min data after attaining each level from the further analysis. In the present study, measurements were carried out at the ambient humidity conditions. Several studies have showed that relative humidity has significant influence on scattering coefficient and moderate/negligible impact on single scattering albedo depending upon the hygroscopic growth factor of aerosol absorption (Carrico et al., 2003). Generally, the absolute magnitude of relative humidity during spring and winter over the central, north and western part of India are mostly below 60%. Due to technical difficulties we could not include a humidogram (one standard RH nephelometer and nephelometer with humidity scanning mode) onboard the aircraft during RAWEX. In addition, the information on the vertical distribution of the growth function is non-existing over the subcontinent which is essential in order to convert the scattering coefficient at standard RH to ambient. We therefore decided to measure the aerosol scattering coefficients at ambient RH. It should be noted that the temperature and relative humidity of the sampled air might have modified from the ambient conditions due to the inherent issues associated with the measurements onboard aircraft (Wang et al., 2003). In addition, Porter et al. (1992) have mentioned that non-adiabatic heat transfer between the sampling tube and cabin air leads to heat loss (gain) in sampled air at lower (higher) altitudes. Unfortunately we don’t have simultaneous measurements of ambient meteorological parameters to compare with the sampled air inside the aircraft. However, we have used the radiosonde observations made simultaneously with the aircraft measurements at Nagpur during pre-monsoon season to qualitatively understand the change in relative humidity of sampled air compared to the ambient. It is found that RH values are reduced by ∼10% from that of the radiosonde value (40 ± 5%) due to the increase in temperature during the onboard sampling. Due to very low humidity values encountered during the RAWEX period (mostly less than 60%) and slow cruising speed of aircraft, the effect of heating of air parcel and associated decrease in RH might not have contributed significantly to the measured scattering coefficients. All the instruments onboard the aircraft were synchronized with a global positioning system (GPS) time prior to the each sortie and geo-location of the scientific data has been done after the campaign using high resolution (1 s) GPS measurements. Daily mean altitude profiles of the parameters were made by averaging the data for each altitude level and mean vertical profile for each station were estimated from the 3 individual daily mean profiles made during consecutive days at each station. It should be noted that due to power constrains, we could not make surface measurements at all the stations prior to the aircraft sortie, so the lowest altitude values may not be representative of the surface values. On an average, all the instruments were operational throughout the campaign. 3. Transport pathways To understand the transport pathways of aerosols over the Indian region during spring and winter, we have calculated isentropic airmass back-trajectories for 5 days over each base station using HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectory) model (Draxler and Hess, 1998) at 1, 2 and 3 km alti-

315

tudes above mean sea level. Fig. 3 shows the three level trajectories corresponding to the RAWEX measurement days over (i) central Indian stations (Hyderabad and Nagpur), (ii) Himalayan foothills (Dehradun), (iii) Indo Gangetic Plain (Lucknow, Ranchi, Patna) and (iv) Western India (Jodhpur and Jaipur) during winter and spring seasons. It is found that, spring time aerosols over the Indo Gangetic Plain are significantly influenced by the airmasses traversing over the west Asian region. Jaipur and Jodhpur (west India) experiences west Asian airmasses during spring and northwestern influence during Winter. The airmasses over the central Indian region mostly trace back to within the Indian mainland during both the seasons and some of the lower free tropospheric trajectories originate from the northern Arabian Sea. Trajectories over the Himalayan region are influenced by the airmasses from the south of Dehradun (western part of the country) during winter and from the west of Dehradun during spring. Except over western Indian stations and some of the Indo Gangetic Plain trajectories at 3 km amsl, most of the trajectories are confined within the Indian subcontinent during winter season. In contrast to the earlier observations, the dust transport from the west Asian regions during the spring season was not significant over the Indian region during the RAWEX period, except over Indo Gangetic Plain. 4. Results and discussions 4.1. Vertical distribution of scattering coefficients The mean vertical distribution of scattering coefficients at 550 nm measured over each base station is shown in Fig. 4 as four panels corresponding to central India (CI), Himalayan Foothills (HF), Indo Gangetic Plain (IGP) and western India (WI). The magnitude of scattering coefficients within the shallow boundary layer during winter were greater than 100 Mm−1 and values decreased to less than 1 Mm−1 above 3 km at most of the stations, except at Dehradun. In contrast to this winter pattern, scattering coefficients decreased gradually with altitude during spring/pre-monsoon season. As the seasons progressed from winter to spring, there is an enhancement by a factor 10 to 20 times in the magnitude of scattering coefficients above 3 km altitude. This pattern is seen in almost all the stations, except at the Himalayan foothills. In fact, the scattering values are comparable over Himalayan regions during both the seasons. RAWEX observations clearly indicated the presence of high aerosol loading along the Himalayan foothills during winter season. This could be attributed to the mountainous topography of the region as a possible candidate to confine the horizontal dispersion of pollutants and the low level inversions in trapping the pollutants during winter. However, this observation needs other supporting evidences and should be explored in detail. The vertical distribution of scattering coefficients measured in each individual region (typically 6 profiles corresponding to measurements over two base stations) shows good consistency during spring season indicating the influence of synoptic scale processes in contrast to the role of local sources as depicted as large variability in the aerosol properties during winter. The high aerosol loading at lower free troposphere during spring was observed at IGP followed by central India and western India, supporting the latitudinal and longitudinal gradients in aerosol loading above the boundary layer reported by Moorthy et al. (2009) using aircraft measurements. Earlier measurements of vertical profiles of scattering coefficients over Indian region have been limited to those from ICARB 2006 experiments (Ramachandran and Rajesh, 2008) and Winter ICARB 2009 (Sreekanth et al., 2011). Based on the measurements during March–May, 2006, Ramachandran and Rajesh (2008) have shown steady vertical profiles of scattering coefficients up to 3 km altitude.

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Fig. 3. Airmass back-trajectories estimated for (a) central India (Hyderabad (HYD) and Nagpur (NGP)), (b) Himalayan foothills (Dehradun (DDN)), (c) Indo Gangetic Plain (Lucknow (LKN), Ranchi (RCH) and Patna (PTN)) and (d) West India (Jaipur (JPR) and Jodhpur (JDH)) during winter 2012 and spring 2013 at 1, 2 and 3 km altitudes above mean sea level.

Winter time enhancement of near-surface aerosol loading over the Indian subcontinent have been established using systematic measurements from various aerosol observatories spread across the country (for eg., Nair et al., 2007; Tripathi et al., 2006; Moorthy et al., 2007a; Kompalli et al., 2014). Some of these studies confirmed that the winter time high in aerosol loading over the Indian region is a near-surface phenomenon mostly driven by the prevailing meteorology assisted by shallow boundary layer depth (0.5– 1 km), weak ventilation and removal processes (Nair et al., 2007; Kompalli et al., 2014; Tripathi et al., 2006). The RAWEX profiles also shows that, the vertical extend of winter time high in aerosol loading is limited to less than a kilometer and particles above three kilometer contribute little to the columnar aerosol loading. It should be noted that the high aerosol loading in the lower part

of the boundary layer has significant adverse effect on the human health and air quality of the region. This study also suggest that future aircraft experiments should supplement with high resolution measurements of vertical profiles of aerosols using tethered balloons, since aircraft measurements have inherent limitations in making high resolution measurements within the boundary layer under low visibility conditions. The spring time enhancement in aerosol loading in the lower free troposphere has been reported based on several recent measurements from different parts of Indian subcontinent. For example, the long-term continuous measurements of BC mass concentration and AOD from Hanle at 4.5 km altitude (Nair et al., 2013a,b; Babu et al., 2011a), BC from NCOP at 5 km (Nair et al., 2013a), Nainital at 2 km (Nair et al., 2013a), and high altitude balloon

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Fig. 4. Vertical distribution of scattering coefficient at 550 nm during winter and spring over (a) central Indian stations (Hyderabad, Nagpur) (b) Himalayan foothill (Dehradun) (c) indo gangetic plain (Lucknow, Patna/Ranchi) and (d) western India (Jaipur, Jodhpur). Note the dramatic increase in scattering coeffiencients in the spring profiles, compared to the winter profiles above 1.5 km.

measurements over Hyderabad (Babu et al., 2011b) have reported spring time high in aerosol loading above the boundary layer. The current investigation of scattering coefficients from RAWEX aircraft experiment also support the inferences made using the mountainbased observations and also implies that the spring time enhancement is not limited to small regions, but is a rather a regional phenomenon. Several studies have suggested the advection of mineral dust aerosols from west Asia or Thar Desert over to the Indian region during the pre-monsoon season (Beegum et al., 2008; Singh et al., 2004; Moorthy et al., 2007b). This would have significant

implications on regional climate, when viewed along with the elevated heat pump (EHP) hypothesis of Lau et al. (2006) (Vinoj et al., 2014). To further understand the vertical heterogeneity in aerosol loading in association with seasonal transformation, the surface level scattering coefficients at 550 nm wavelength and columnar AOD at 500 nm wavelength from various stations (Trivandrum, Anantapur and Dibrugarh and Delhi) for winter and spring period are given in Table 2. Since the long-term measurements of surface and columnar aerosols were not available from the RAWEX base

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stations or stations close to that, we have compiled all the available data in this region for the analysis. It is seen that the near surface aerosol concentration (scattering coefficients, BC mass concentration or total mass concentration) decrease continuously from winter to summer at most of the stations (Soni et al., 2010; Kompalli et al., 2014), whereas columnar AOD values remains same or slightly increases from winter to spring suggesting the increase in aerosol load above the boundary layer, which compensate the decrease of aerosol loading at the surface (Lodhi et al., 2013). The contrast in the seasonal variation of near surface and columnar aerosol loading clearly indicates the role of advection of aerosols above the boundary layer and vertical transport of aerosols during spring, aided by the increased convective mixing and the prevailing anti-cyclonic conditions. Even though there are several studies on the near surface and columnar aerosol loading over the Indian region, the discrepancy in the seasonal evolution of the surface and column aerosol loading are not addressed in detail. The contribution of local and long-range transport of aerosols to the spring time enhancement in aerosol loading in the lower free troposphere is yet to be quantified. However, several studies have considered the gradients in warming due to these aerosols as potential forcing mechanism, which could influence large scale circulation pattern and Indian summer monsoon (Lau et al., 2006; Moorthy et al., 2009; Vinoj et al., 2014). 4.2. Absorption coefficient and validation experiments Spectral variation of absorption coefficient is estimated from the aethalometer measurements, which is corrected for scattering artifacts using simultaneous scattering coefficient measurements as described in Section 2. We have investigated the seasonal transformation of vertical distribution of absorption coefficient and its wavelength dependence to further understand the changes in the characteristics of BC aerosols over the Indian region and to delineate the possible contribution of different sources to it. In general, seasonal variation of vertical distribution of absorption coefficient (not shown here) resembled that of scattering coefficients, with high values above the boundary layer during spring and rapid decrease from the high near-surface values to very low value at 1 km during winter. Spectral variation of absorption coefficient is parameterized using a power law relationship (same as that of eqn. (1)) where α abs is the power law exponent, which is widely used to describe the source characteristics of absorbing aerosols qualitatively (Andreae and Gelencsér, 2006). Generally, α abs is close to 1 for fresh fine mode BC aerosols emitted from fossil fuel burning and α abs > 2 indicates either biomass burning or influence of dust particles (Kirchstetter et al., 2004; Liu et al., 2014). Fig. 5 shows the vertical distribution of α abs for all the base stations during RAWEX. It is found that α abs values are very close to 1.5 in the boundary layer during both the seasons. During winter, absorption coefficients above ∼ 1 km (the typical boundary layer height reported by Tripathi et al. (2006) and Nair et al. (2007)) were negative because of the low sensitivity of instrument to measure the extremely low concentration of absorbing aerosols. If any one of the seven wavelengths recorded negative values, we have screened the values from further analysis. However, during spring, as the aerosol concentration remains high above boundary layer, α abs could be estimated to higher altitudes, whereas σ abs was close to zero during winter. The high values of α abs over Jaipur and Jodhpur indicate the influence of dust aerosols (Fialho et al., 2005), as seen in the flat spectral dependence of scattering coefficients (α sca ) as shown in Fig. 6. In contrast to α abs , α sca represent the entire aerosol size distribution; values below one indicate the dominance of coarse mode aerosols and values above one indicate fine mode dominance. The low α sca during pre-monsoon over Jodhpur and Jaipur

Fig. 5. Vertical distribution of wavelength exponent of absorption coefficient during spring and winter over Dehradun, Jaipur, Jodhpur, Patna/Ranchi, Lucknow, Nagpur and Hyderabad. Notice the consistently higher value for α abs in spring at all the stations compared to winter.

Fig. 6. Vertical distribution of wavelength dependence of scattering coefficient (scattering angstrom exponent, α sca ) during winter and pre-monsoon over western Indian stations (Jaipur and Jodhpur).

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in the western India clearly shows the advection of dust from the west. The low α sca and moderately high α abs indicate the dominance of dust aerosols, probably mixed with BC and other aerosols (Fialho et al., 2005). Using columnar spectral optical depths and Infrared Difference Dust Index (IDDI), Moorthy et al. (2007b) have shown that the dust over western India region is more absorbing than the purer dust over Africa, and attributed the mixing of it with BC as one of the possible reasons. Even though there are several observational studies on the black carbon mass concentration measurements from the surface, onboard aircraft, ship and balloon, the absorption coefficient and its spectral dependence remained rather less-explored over the Indian region due to the difficulties associated with filter based measurement techniques (CollaudCoen et al., 2010). The major limitation associated with the estimation of aethalometer measured optical attenuation to the absorption coefficient is the lack of simultaneous measurements of scattering coefficients at different wavelengths, which is essential for filter correction (Arnott et al., 2005; CollaudCoen et al., 2010). Due to this difficulty, most of the studies were restricted to the black carbon mass concentration measurements. Based on the different available algorithms for correcting aethalometer data, CollaudCoen et al. (2010) have reported that the estimation of α abs is highly sensitive to the scattering correction. Source characteristics of BC and organic carbon and their relative dominance are highly uncertain over the Indian region. There are few studies that have addressed the estimation of absorption cross section of black carbon, absorption due to brown carbon and OC/EC ratio over the region (Srinivas and Sarin, 2014). These authors compiled the measured OC/EC over the India and surrounding oceanic regions (Table 2 of Srinivas and Sarin (2014)), which clearly demonstrate the large variability in source characteristics as OC/EC ratios varied from 1.3 to 11. Most of the measurements over the subcontinent depicted values above 5 indicating the dominance of biomass burning sources over the fossil fuel (Kirchstetter et al., 2004; Srinivas and Sarin, 2014). Even though the magnitude of α abs may be underestimated due to the measurement uncertainties, the high values of α abs during winter and spring suggest the importance biomass burning. 4.3. Vertical distribution of SSA Aerosol single scattering albedo (SSA or ω) is defined as the ratio of scattering coefficient to extinction coefficient (σ ext = scattering + absorption) at a particular wavelength (ω = σ sca /σ ext ) and SSA subtracted from unity is generally known as co-albedo. Ideally SSA can vary from 0 to 1, former indicating completely absorbing and latter represent fully scattering aerosol system; however SSA of the atmospheric aerosols generally vary from 0.7 to 1 indicating the dominance of scattering aerosols. The aerosol SSA and surface albedo together (critical single scattering albedo) decides Table 2 Seasonal variation of near surface scattering coefficients and columnar AOD at different stations over the Indian sub-continent. Station

Scattering (Mm−1 ) Winter

Trivandrum Ananthapur Dibrugarh Delhi a b c d e

181 ± 66 140a , e

AOD

Spring a

565 ± 274b

ARFI database. Soni et al. (2010). Lodhi et al. (2013). Only for March 2011. Only for December 2011.

Winter

160 ± 46 105a , d

a

236 ± 96b

Spring

0.42 ± 0.04 0.40 ± 0.02a 0.41 ± 0.10a 0.77c

a

0.47 ± 0.07a 0.51 ± 0.02a 0.50 ± 0.07a 0.73c

319

whether aerosols cool or warm the top of the atmosphere. Even though there are several studies on the aerosol optical depth, total number and mass concentration and aerosol chemistry, there are only very few studies addressed the spatial and temporal variations of aerosol single scattering albedo (SSA) over the Indian subcontinent (Ganguly et al., 2006; Nair et al., 2008; Sreekanth et al., 2011). This is mostly attributed to the uncertainties in characterizing the aerosol absorption accurately. The mean vertical distributions of SSA at 520 nm over different base stations, derived from the aircraft measurements, are shown in Fig. 7. The SSA values are very low within the boundary layer (<1.5 km) during winter and spring at all the stations indicating the dominance of absorbing aerosols at near-surface level throughout the year over this region. The lower free tropospheric SSA values indicate high aerosol absorption (low SSA values) during spring compared to winter. Spring time SSA at all stations showed steady values throughout the free troposphere, whereas large fluctuations are seen in winter values because of very low scattering and absorption coefficients. During winter season, the high SSA values (relatively low contribution of absorbing aerosols to the total extinction) above the boundary layer do not have significant implications on the radiative forcing since winter time aerosols are mostly confined within the boundary layer. It should be noted that, SSA values were as low as 0.85 to 0.9 over central India and Indo Gangetic Plain (IGP) during spring, which would contribute significantly to the diabatic heating due to aerosols (Moorthy et al., 2009). Extinction weighted columnar SSA (ratio of scattering AOD to extinction AOD) estimated from the RAWEX measurements is given in Table 3.

 4km

ωcolumn =

0

ω (h )σext (h )dh σext (h )dh 0

 4km

(3)

In general columnar SSA values are high during winter and low during spring. The mean SSA values decrease from 0.93 over Ranchi to 0.87 over Patna (both stations are at the eastern part of IGP) and 0.88 to 0.86 over Lucknow as season progressed from winter to spring. The winter time measurements of SSA could be biased due to strong confinement of aerosols close to the surface and technical difficulties in sampling the near surface aerosols prior to the aircraft sortie. To further understand the seasonal difference in the absorption potential of aerosols over India, the monthly mean variation of columnar SSA at 441 nm retrieved from the sun-sky radiometer (AERONET, NASA, Dubovik et al., 2000) measurements made at Kanpur, Gandhi College, Jaipur and Pune are shown in Fig. 8. The lowest value of monthly mean columnar SSA (high aerosol absorption) is observed during the spring season over the Indo gangetic plain and central India. AERONET measurements are consistent with the RAWEX findings. It is seen that columnar SSA values estimated from the RAWEX measurements are close to that of AERONET retrieved SSA values over Indo Gangetic plain. Further, the RAWEX measurements revealed that spring time aerosols are more absorptive than winter time haze over the Indian region, which is independently verified with AERONET measurements. The high SSA values observed during the summer monsoon season are attributed to the significant wet deposition of aerosol due to large rainfall associated with Indian monsoon. The major highlight of this study was the high absorption potential of aerosols over the northern parts of India prior to the onset of Indian summer monsoon. The decrease in columnar SSA from winter to summer is mostly attributed to the relative dominance of absorbing aerosols in the free tropospheric altitudes rather than due to the changes in the

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Fig. 7. Vertical distribution of single scattering albedo at 520 nm during winter and spring over (a) central India (Hyderabad, Nagpur) (b) Indo Gangetic Plain (Lucknow, Patna/Ranchi) (c) western India (Jaipur, Jodhpur) and (d) Himalayan foothill (Dehradun).

Table 3 Columnar SSA estimated from the RAWEX measurements during spring and winter seasons. Station

Spring

Winter

Dehradun Jodhpur Jaipur Hyderabad Nagpur Lucknow Ranchi/Patna

0.904 0.935 0.910 0.880 0.790 0.859 0.870

0.931 0.891 0.904 – 0.889 0.878 0.927

aerosol source type in the boundary layer. Interestingly, in-situ measurements of boundary layer aerosols depict an increase in SSA from winter to spring (Ganguly et al., 2006). Hence, it should be noted that the boundary layer aerosol contribution to the columnar SSA is higher during winter, whereas free tropospheric aerosols contribute more to the column SSA during pre-monsoon season. This indicates the importance of high resolution aerosol measurements well within the boundary layer during winter (using tethersonde Nair et al., 2007) along with the aircraft measurements. Even though the scattering coefficients over Dehradun did not show significant seasonal variability, SSA during pre-monsoon was lower than in winter.

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Fig. 8. Monthly mean variation of columnar single scattering albedo retrieved from the sky radiance measurements at AERONET stations at Kanpur, Gandhi College and Jaipur. The corresponding column integrated (up to 4 km) SSA values measured during RAWEX aircraft experiment is also shown.

The uncertainty in the columnar SSA values estimated from the RAWEX measurements originates mainly from (i) instrumental uncertainties (ii) sampling errors onboard aircraft and (iii) large spatial averaging and limited vertical coverage. In the case of instrument uncertainties, in general, the aethalometer overestimates the absorption coefficient and the nephelometer underestimates the scattering coefficient, which results in about 15% uncertainty in the SSA values estimated using the combination of these two instruments [following Moorthy et al., 2009; Nair et al., 2008]. Uncertainties associated with the iso-kinetic sampling onboard aircraft and its size segregated efficiencies are difficult to estimate. Nevertheless, McNaughton et al. (2007), who used a similar inlet system, have reported that the measured scattering coefficient values are close (10–30%) to that of the tower measurements. Large spatial averaging (100–150 km) and limited vertical coverage are the intrinsic limitations of the aircraft measurements, standard deviation of the data represent the spatial averaging. Several earlier studies, as well as the profiles in the current measurements, reveal that >70% of the aerosol burden is confined within the first three km of the atmosphere especially if no elevated aerosol layers are present. In the light of the above, it is reasonable that the columnar aerosol loading estimated from the RAWEX aircraft measurements up to 4 km altitude represents winter time aerosols more accurately, while there would be an underestimate of the spring aerosol loading. It should be keep in mind that, since SSA is an intrinsic property of the aerosol system, which depends on the fractional contribution of chemical species rather than absolute magnitude, AERONET and RAWEX measurements encountered the same aerosol system qualitatively. During INDOEX and ICARB experiments, there are spectral measurements of SSA over the ocean around India (Nair et al., 2008), whereas, the vertical distribution of SSA is least understood over the Indian region (Sreekanth et al., 2011). Synthesizing the ICARB measurements onboard ship and aircraft into regional radiative forcing, Moorthy et al. (2009) have emphasized the importance of realistic profiles of SSA for the estimation of diabatic heating structure of aerosols. Using these observation based estimates of meridional gradients in aerosol heating structure, Nair et al. (2013b) have hypothesized the role of absorbing aerosols on circulation patterns prior to the onset of Indian summer monsoon.

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Narasimhan and Satheesh (2013) have reported the influence of realistic vertical profiles of aerosol extinction in deriving the SSA using satellites MODIS and OMI. In this context, RAWEX experiment provided the first and most exhaustive measurements of the SSA of aerosols over the Indian region representing south, north, west and Himalayan regions. It has been reported by several investigators using observational and modelling techniques that, aerosol absorption due to BC and dust over the northern India during spring season has a positive feedback on Indian summer monsoon through the change in synoptic circulation associated with direct/semi direct effect of aerosols (Lau et al., 2006; Bollasina et al., 2008; Vinoj et al., 2014). However, the plausible pathways or mechanisms involved in the aerosol–hydroclimate interactions due to aerosol induced radiative perturbations are beyond the scope of this study. Even though the Lau et al. (2006) and Bollasina et al. (2008) proposed an increase in summer monsoon precipitation over the Indian region, the pathways are distinctly different. The RAWEX measurements of high aerosol loading and low SSA during pre-monsoon support the hypothesis by Lau et al. (2006) and Bollasina et al. (2008) but its contribution to the diabatic heating to the land surface, lower and upper free troposphere needs to be further quantified. It is confirmed from this study that pre-monsoon aerosols over the Indian region are more absorbing than winter haze and absorbing aerosol loading in the lower free troposphere is significantly higher in spring compared to the winter season. This would have significant implications on the aerosol heating structure over the Indian region. 5. Conclusions Seasonal transformation of vertical distribution of aerosol properties over the Indian region is characterized using extensive measurements of multi-instrumented aircraft experiments carried out as a part of Regional Aerosol Warming Experiment (RAWEX). The simultaneous measurements of scattering and absorption properties of aerosols were carried out regionally, seasonally, vertically and spectrally to understand its implications on regional climate. The major findings of this study are. 1. Free tropospheric enhancement in scattering coefficient during pre-monsoon is almost 10 to 20 times as that of winter values. 2. Extremely high aerosol loading during the winter season is mainly confined to the boundary layer and free tropospheric values of scattering and absorption coefficients are close to zero. 3. Boundary layer aerosols are more absorbing than free tropospheric aerosols during both winter and spring. 4. During both the seasons, the wavelength exponent of the absorption coefficient is higher than one (α abs ∼1.5) over all the stations, indicating the contribution from biomass burning and dust aerosols up to lower free troposphere. 5. Low values of columnar aerosol single scattering albedo confirms that pre-monsoon aerosols are more absorptive than winter time aerosols and aerosol absorption is significantly higher at lower free troposphere in pre-monsoon season. Acknowledgements This work formed a part of the RAWEX aircraft campaign of ISRO-GBP. We thank Director, National Remote Sensing Centre (NRSC), Hyderabad and the Aerial Services and Digital Mapping Area (AS & DMA) for providing the aircraft support for this experiment. The authors wish to thank the crew of the aircraft for their help throughout the field campaign and the wholehearted support of the NRSC aircraft team.We thank the AERONET (data available at http://aeronet/gsfc. nasa.gov) PIs and their staff for establishing and maintaining the sites used in this investigation.

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