Large‐scale enhancement in aerosol absorption in the lower free troposphere over continental India during spring

Abstract

Aerosol absorption in the lower troposphere over continental India was assessed using extensive measurements of the vertical distribution of absorption coefficients aboard an instrumented aircraft. Measurements were made from seven base stations during winter (November–December 2012) and spring (April–May 2013), supplemented by the data from the networks of surface observatories. A definite enhancement in aerosol absorption has been observed in the lower free troposphere over the Indo‐Gangetic Plain (IGP) during spring, along with a reduction near the surface. The regional mean aerosol absorption optical depth (AAOD) over IGP, which was derived from aircraft observations (integrated from the ground to 3 km), increased from 0.020 ± 0.009 in winter to 0.048 ± 0.01 in spring. The columnar AAOD depicted weak and distinctly different seasonal variations than that of surface level black carbon mass concentrations. This contrasting difference in the seasonality indicates the presence of elevated layers of absorbing aerosols during spring in association with the long‐range transport and vertical convective lofting of aerosols.

Air-borne in-situ measurements of aerosol size distributions and BC across the IGP during SWAAMI

“…A constant volumetric flow of 70 LPM (liters per minute) was maintained using an external pump connected to the main inlet assembly, which provided iso-kinetic flow for the average speed of 300 km/hr maintained by the aircraft during the entire campaign. The efficiency of this inlet system has been already proven in several previous campaigns (Gogoi et al, 2019;Nair et al, 2016;Babu et al, 2016).…”

“…The measurements were carried out aboard the instrumented aircraft (Beechcraft-200) fitted with an iso-kinetic inlet, mounted (front facing) at the bottom of the fuselage for aspirating ambient aerosols and detailed in a few earlier papers (Babu et al, 2016;Vaishya et al, 2018;Gogoi et al, 2019). A constant volumetric flow of 70 LPM (liters per minute) was maintained using an external pump connected to the main inlet assembly, which provided iso-kinetic flow for the average speed of 300 km/hr maintained by the aircraft during the entire campaign.…”

“…In recent years, a few campaign-mode airborne measurements have been made over this region to estimate the altitude-resolved properties of aerosols that are important in aerosol-radiation interactions (Gogoi et al, 2019;Vaishya et al, 2018;Babu et al, 2016;Nair et al, 2016;Padmakumari et al, 2013). These include the measurements of aerosol scattering and absorption coefficients conducted as part of the Regional Aerosol Warming Experiment (RAWEX; Babu et al, 2016) to delineate the spatio-temporal variability in the altitude distribution of aerosol single scattering albedo (SSA) across the IGP during winter and pre-monsoon seasons and aerosol and cloud parameter measurements conducted as part of the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX; Konwar et al, 2015). Some studies have also reported significant contribution of dust and BC as the elevated aerosol load (Pandey et al, 2016;Li et al, 2016;Praveen et al, 2012;Kedia et al, 2014) and their potential role to act as ice nuclei (Padmakumari et al, 2013).…”

Possible climatic implications of high-altitude black carbon emissions

“…Examining the vertical profile of aerosols over Bangalore (an urban station in the southern part of India) using a groundbased micro-pulse lidar, Satheesh et al (2006) have reported morning-time (nighttime) vertical lifting (confinement) of aerosols during summer days. Carrying out airborne measurements of BC and absorption coefficients over several stations across India, Babu et al (2008Babu et al ( , 2010 and Nair et al (2016) have reported a near-steady vertical distribution up to an altitude of 3 km above the ground with isolated peaks of higher BC concentration and absorption coefficients during the pre-monsoon period above most of the landmass. Synthesising multi-platform measurements, Satheesh et al (2008) have revealed the existence of elevated aerosol layers in the middle troposphere (4-5 km).…”

“…With this backdrop, a few studies have examined the vertical distribution of aerosols (extinction coefficient values and/or BC mass concentration values) over the Indian region using multiple observational platforms, such as ground-based (Satheesh et al, 2006), airborne or space-based lidar Prijith et al, 2016), instrumented aircraft (Moorthy et al, 2004;Babu et al, 2008Babu et al, , 2010Nair et al, 2016) and tethered and high-altitude balloons (Babu et al, 2011). Carrying out such observations for the first time over India, Moorthy et al (2004) noticed an exponential decrease in BC concentration within the boundary layer and a steady value above it over an urban station (Hyderabad) in south-central India during February 2004.…”