Large contrast in the vertical distribution of aerosol optical properties and radiative effects across the Indo-Gangetic Plain during SWAAMI-RAWEX campaign

Abstract

<p><strong>Abstract.</strong> Measurements of the vertical profiles of the optical properties (namely the extinction / scattering and absorption coefficients; respectively σ<sub>ext</sub> / σ<sub>scat</sub> / σ<sub>abs</sub>) of aerosols have been made across the Indo-Gangetic Plain (IGP) using an instrumented aircraft operated from three base stations (Jodhpur (JDR) representing the semiarid western IGP; Varanasi (VNS) the central IGP characterized by significant anthropogenic activities; and the industrialised coastal location in the eastern end of the IGP (Bhubaneswar, BBR)), just prior to the onset of the Indian Summer monsoon. The vertical profiles depicted region-specific absorption characteristics, while the scattering characteristics remained fairly uniform across the region, leading to a west-east gradient in the vertical structure of single scattering albedo (SSA). Integrated from near ground to 3 km, the highest absorption coefficient and hence the lowest SSA occurred in the central IGP (Varanasi). Size distribution, inferred from the spectral variation of the scattering coefficient, showed a gradual shift from coarse particle dominance in the western IGP to strong accumulation dominance in the eastern coast with the central IGP coming in-between, arising from a change in the aerosol type from predominantly natural (dust and sea-salt) type in the western IGP to highly anthropogenic type (industrial emissions, fossil fuel and biomass combustion) in the eastern IGP; the central IGP exhibiting a mixture of both. Aerosol induced short-wave radiative forcing, estimated using altitude resolved SSA information, revealed significant atmospheric warming in the central IGP while a top-of-atmosphere cooling is seen, in general, in the IGP. Atmospheric heating rate profiles, estimated using altitude resolved SSA and column average SSA, revealed considerable underestimation in the latter case, emphasising the importance and necessity of having altitude resolved SSA information as against a single value for the entire column.</p>

Altitude profiles of CCN characteristics across the Indo-Gangetic Plain prior to the onset of the Indian summer monsoon

“…Hence it is imperative to examine altitudinal dependency of CCN on ground (identified by the colour) for each day of observation (identified by the shape of the point). Central IGP showed highest absorption coefficient (column averaged) of 26 ± 9 Mm -1 , followed 5 by west (16 ± 2 Mm -1 ) and east (15 ± 3 Mm -1 ) IGP (Vaishya et al, 2018). It is interesting to note that the linear relationship is maintained for low to moderate concentrations of BC (up to around 1000 ng m -3 , which occurs mostly above PBL), while significant scatter occurs for higher values of BC (exceeding 2000 ng m -3 ), which occurs mostly in the lower altitudes, supporting the hypothesis.…”

“…The efficacy of the inlet to sample aerosols below 4 μm, under such conditions, has been demonstrated during the DC-8 Inlet Characterization Experiment (McNaughton et al, 2007). Further details of the experimental setup are explained in Babu et al, (2016) and Vaishya et al, (2018). The air, aspirated through this inlet, is then fed to different instruments through a manifold.…”

“…During this period, being summer over the Indian region, the PBL would be quite deep as the thermal convections would be strong providing a thorough vertical mixing. Mean PBL heights at local noon time over the IGP regions, estimated from NCEP/NCAR global reanalysis product at 10 0.25° × 0.25° grid resolution data for the flight sortie days were found to be 1.4 ± 0.2 km, 2.3 ± 0.5 km, and 1.3 ± 0.5 km for BBR, VNS, and JDR, respectively (Vaishya et al, 2018). Due to unpressurised mode of operation of the aircraft, the ceiling altitude of airborne measurements was ~ 4 km amsl.…”

Reviewer comments

“…However, only six different profiles are presented, and only one pair of these consists of measurements made before and during the monsoon at the same location. Many other studies have explored monsoon-aerosol interactions over India using aircraft data (see Li et al, 2016 )-including an analysis of measurements from the same period as the present manuscript (Vaishya et al, 2018)-and it is not clear to me what makes the data presented in the present work particularly unique. Moreover, the supporting RF and heating rate calculations that are provided do not seem to generate novel insights.…”

Time-resolved black carbon aerosol vertical distribution measurements using a 356-m meteorological tower in Shenzhen

“…For example, Schwarz et al (2008) conducted aircraft-based BC measurements in Costa Rica. Vaishya et al (2018) reported vertically resolved light absorption profiles at Indo-Gangetic Plain (IGP) using aircraft measurements and found significant atmospheric warming induced by BC. Wang et al (2018a) found a decreasing trend of light absorption with height within 0-3 km, and the light absorption vertical profiles were influenced by the evolution of a boundary layer.…”