Abstract
Abstract. Vertical distributions (altitude profiles) of condensation nuclei (CN) and cloud condensation nuclei (CCN) and their spatial variations across the Indo-Gangetic Plain (IGP) have been investigated based on airborne measurements carried out during the SWAAMI field campaign (June to July 2016) capturing the contrasting phases of the Indian monsoon activity in 2016 just prior to its onset and during its active phase. Prior to the monsoon onset, high concentrations of CN and CCN prevailed across the IGP, and the profiles revealed frequent occurrence of elevated layers (in the altitude range 1–3 km). Highest concentrations and elevated peaks with high values occurred over the central IGP. The scenario changed dramatically during the active phase of the monsoon, when the CN and CCN concentrations dropped (CN by 20 % to 30 % and CCN by 6 % to 25 %) throughout the IGP with more pronounced changes at altitudes higher than 3 km where decreases as high as > 80 % were observed. These reductions have an east-to-west decreasing gradient, being most remarkable in the eastern IGP and very weak over the western IGP where the CN concentrations above 3 km increased during the monsoon. The activation ratios (ARs) showed contrasting features, increasing with increase in altitude, prior to the onset of monsoon, reversing the trend to decrease with increase in altitude during the active phase of the monsoon. The supersaturation spectrum became flatter during the active phase of the monsoon, indicating an increase in the hygroscopicity of aerosols following the mixing of surface-based emissions with the advected marine air mass.
Highlights
Spatio-temporal characteristics of aerosols and their interactions with clouds respectively are key parameters determining the direct and indirect climate forcing by aerosols (Twomey, 1974; Albrecht, 1989; Stocker et al, 2013)
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 cloud condensation nuclei (CCN) concentration and a weak increase in activation ratios (ARs), which otherwise remained featureless in the entire altitude region
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 subregions of the Indo-Gangetic Plain (IGP) and AMD during 2–7 July
Summary
Spatio-temporal characteristics of aerosols and their interactions with clouds respectively are key parameters determining the direct and indirect climate forcing by aerosols (Twomey, 1974; Albrecht, 1989; Stocker et al, 2013). Despite concerted efforts to understand the aerosol–cloud interactions and the associated feedback mechanisms in the atmosphere, large uncertainties still exist (McFiggans et al, 2006; Andreae and Rosenfeld, 2008; Stevens and Feingold, 2009) This mainly arises from the region-specific and heterogeneous nature of aerosols, their vertical mixing and advection to long distances in the real atmosphere, and sparseness of in situ measurements of the vital parameters of CCN, such as the vertical distribution of the CCN number concentration, CCN efficiency and its variation with supersaturation (SS) (Seinfeld et al, 2016). The campaign details along with the measurement protocols are briefly stated below (more details are available in Manoj et al, 2019), followed by the results and discussions
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