Cloud condensation nuclei characteristics during the Indian summer monsoon over a rain-shadow region

Abstract

Abstract. Continuous aerosol and cloud condensation nuclei (CCN) measurements carried out at the ground observational facility situated in the rain-shadow region of the Indian subcontinent are illustrated. These observations were part of the Cloud Aerosol Interaction Precipitation Enhancement Experiment (CAIPEEX) during the Indian summer monsoon season (June to September) of 2018. Observations are classified as dry–continental (monsoon break) and wet–marine (monsoon active) according to the air mass history. CCN concentrations measured for a range of supersaturations (0.2 %–1.2 %) are parameterized using Twomey's empirical relationship. CCN concentrations at low (0.2 %) supersaturation (SS) were high (>1000cm-3) during continental conditions and observed together with high black carbon (BC∼2000ngm-3) and columnar aerosol loading. During the marine air mass conditions, CCN concentrations diminished to ∼350cm-3 at 0.3 % SS and low aerosol loading persisted (BC∼800ngm-3). High CCN activation fraction (AF) of ≅0.55 (at 0.3 % SS) was observed before the monsoon rainfall, which reduced to ≅0.15 during the marine air mass and enhanced to ≅0.32 after that. There was mostly monomodal aerosol number size distribution (NSD) with a mean geometric mean diameter (GMD) of ≅85 nm, with least (≅9 %) contribution from nucleation mode (<30 nm) particles persisted before the monsoon, while multimode NSD with ≅19 % of nucleation mode particles was found during the marine air mass. Critical activation diameters (dcri) for 0.3 % SS were found to be about 72, 169, and 121 nm prior to, during, and after the marine conditions, respectively. The better association of CCN with aerosol absorption, and the concurrent accumulation mode particles during continental conditions, points to the possibility of aged (oxygenated) carbonaceous aerosols enhancing the CCN activity prior to the marine conditions. An enhancement in CCN concentrations and k values during the daytime along with absorption Ångström exponent was observed during the marine conditions. Best closure obtained using measured critical diameter and ammonium sulfate composition during continental conditions emphasizes the role of aged aerosols contributing to the accumulation mode, enhancing the CCN efficiency. The overestimation of CCN and less hygroscopicity of accumulation mode aerosols during the marine air mass indicate the role of size-dependent aerosol composition in CCN activity during the period.

Vertical profiles of cloud condensation nuclei number concentration and its empirical estimate from aerosol optical properties over the North China Plain

“…Previous studies have suggested that differences in air masses will lead to spatiotemporal differences in CCN activation ability and aerosol optical properties (e.g., Xu et al, 2020;Jayachandran et al, 2020b). To better understand air mass sources and aerosol transport pathways over the measurement area, 72-hour air mass back trajectories for all N CCN profiles at 0.5, 1.5, 2.5, and 3.5 km are analyzed using the NOAA Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model (Stein et al, 2015).…”

Measurement report: Altitudinal variation of cloud condensation nuclei activation across the Indo-Gangetic Plain prior to monsoon onset and during peak monsoon periods: results from the SWAAMI field campaign

“…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.…”

“…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).…”

Aerosol activation characteristics and prediction at the central European ACTRIS research station Melpitz, Germany

“…Besides the significant difference in spatial, also the temporal variations of aerosol activation characteristics are essential for NCCN prediction (Andreae and Rosenfeld, 2008). Most of the observations lasted 1-2 months or even less; they mainly focused on the effects of short-term weather processes or pollution events on aerosol particle activation, such as the effects of the summer monsoon (Jayachandran et al, 2017(Jayachandran et al, , 2020, wet removal (Croft et al, 2009), NPF events (Dusek et al, 2010;Wu et al, 2015), biomass burning (Rose et al, 2010), and aerosol particle aging as well as oxidation processes (Zhang et al, 2016(Zhang et al, , 2017. The long-term CCN concentration measurements (of at least one full year) are still rarely reported, resulting in insufficient knowledge concerning the seasonal and annual cycles of aerosol particle activation, which are also critical for model predictions and evaluations.…”