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Abstract:
In EGU General Assembly 2024, Vienna, Austria & Online, 14-19 April
Aerosols, with their direct and indirect effects impacting the climate, have been established to
significantly perturb Earth's radiative budget and hydrological cycle. The climate impact of aerosols
is complex and multifaceted, with various factors influencing the combined net effect. The
intricacies of aerosol effects, mainly through aerosol-cloud interactions, necessitate precise
measurements to reduce the uncertainty in forecasting future climate fluctuations1. Studying their
characteristics in pristine settings can provide an enhanced scientific understanding of aerosol
impact in background conditions, as opposed to polluted ones2. With this motivation, we
conducted a comprehensive field measurement campaign during the second phase of the COVID-
induced lockdown in Munnar, a relatively clean high-altitude site in the Western Ghats of India.
Munnar is surrounded by lush tea plantations and extensive forest reserves, and tea production
and tourism are the major human activities in the area. However, suspended tourist activities due
to the pandemic and frequent precipitation during monsoon enabled us to study the ambient
aerosol characteristics in near-natural conditions3. This study presents results from the size-
resolved Cloud Condensation Nuclei (SR-CCN) measurements conducted along with aerosol size
distribution and chemical composition at the Natural Aerosol and Bioaerosol High Altitude
Laboratory (NABHA; 10.09 N, 77.06 E; 1605m asl) during the Southwest Monsoon season between
June-October 2021. The median number concentration for 10–450nm particles was observed to be
533cm-3, with 357cm-3and 908cm-3 as first and third quartiles, respectively, similar to other pristine
locations, such as Amazonia during the wet season4. The average non-refractory particulate
matter (NR-PM1) concentration was 2.28±1.81 μg/m3 (mean ± one standard deviation). The SR-CCN
measurements were carried out for set supersaturations between 0.1% and 0.85% for particles
ranging between 20-350 nm in diameter. The critical dry diameter varied from 60 to 150nm for
highest to lowest supersaturation, similar to previously reported studies elsewhere4,5. During the
campaign, the efficiency spectra of CCN often reached unity despite organic aerosols dominating
the submicron aerosol composition.
Further, hygroscopicity, a particle size and composition function, was investigated using the kappa-
Köhler theory. The hygroscopicity parameter, kappa, derived from SR-CCN measurements(kCCN)
varied between 0.26 and 0.57. kCCN did not exhibit much variation in the Aitken mode regime
(60-80nm) but increased in the accumulation mode (100-160nm), suggesting higher hygroscopic
fraction in larger (aged) particles. Assuming a linear mixing of organic and inorganic aerosols,
chemically derived hygroscopicity (kchem) was comparable to kCCN, following similar diurnal
variation. Further details will be presented.
References:
1.Lohmann, U. & Ferrachat, S. Impact of parametric uncertainties on the present-day climate and
on the anthropogenic aerosol effect. AtmosChemPhys (2010).
2.Andreae, M. O. Aerosols Before Pollution. Science (2007).
3.Navasakthi, S., Pandey, A., Bhari, J. S. & Sharma, A. Significant variation in air quality in South
Indian cities during COVID-19 lockdown and unlock phases. EnvironMonitAssess (2023).
4.Gunthe, S. S. et al. Cloud condensation nuclei in pristine tropical rainforest air of Amazonia: size-
resolved measurements and modeling of atmospheric aerosol composition and CCN activity.
AtmosChemPhys (2009).
5.Singh, A. et al. Rapid growth and high cloud-forming potential of anthropogenic sulfate aerosol
in a thermal power plant plume during COVID lockdown in India. NPJClimAtmosSci (2023).
