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Abstract Spatial and temporal shifts in rainfall patterns over South Asia and the adjoining Seas during the pre-monsoon season have been observed over the past 2 decades from 2000 to 2019. Aerosol particles suspended above the boundary layer are a contributing factor to these changes. These particles not only alter cloud characteristics, but also diminish the lapse rate, thereby suppressing convective activity, leading to precipitation anomalies. Over the past 2 decades, high-rainfall regions have experienced declining rainfall, while low-rainfall regions have received increased rainfall. Coinciding with notable anomalies in precipitation, contrasting trends in aerosol optical depth, particularly due to absorbing aerosols in the elevated regions of the atmosphere, are seen. Apart from aerosols, several factors are considered that are critical in modifying precipitation patterns over the study region, such as water vapor content, convective processes, and lower-level relative humidity. We observed a potential transport of excess water vapor by ambient circulation from the oceanic regions having reduced rain, such as Bay of Bengal and the Arabian Sea, to higher latitudes enabling precipitation anomaly at distant locations.

Abstract The dust plume tracks from the Middle East and Eastern Africa to the Indian subcontinent have an impact on the atmospheric and ocean biogeochemistry of the Arabian Sea (AS). Here, we present the impact of dust on net primary productivity (NPP) over the AS using satellite-based observation and model simulation. Seasonal episodes and long-term trends in dust optical depth (DOD), dust mass flux (DMF) and dust deposition flux (DDF) from 2007 to 2020 are quantified. Nearly 32% of the total dust is advected to the AS during transport (maximum in JJA; DMF ~ 33.1 Tg year−1 ~ 56% of annual and DDF ~ 5.5 Tg year−1 ~ 63% of annual). Over the last one and half decades, there has been a statistically significant decreasing trend in DOD, associated with precipitation, enhanced vegetation index and surface soil moisture over the landmasses in the proximity of the AS. Similarly, the depletion in DDF suppresses the NPP over different regions of the AS, especially over the central AS, where the oceanic supply of nutrients is limited.

Continuous ground-based observations of aerosols over the Himalayan region are highly beneficial in studying aerosol trends over the complex geographic terrain. This study presents the results from continuous long-term observations (2004–2016) of aerosols over a high-altitude site in the central Himalayas – Nainital, quantifying multihued aspects of black carbon (BC) and columnar aerosol optical depth (AOD). The results reveal that BC and AOD have a positive trend of 23.04 ± 3.00 ng m−3 year−1 and 0.0076 ± 0.0003 year−1, respectively. Simultaneously, a positive trend of 0.0066 ± 0.0059 year−1 is observed in the Angstrom exponent, thus indicating an increase in anthropogenic aerosols. The satellite observations covering a broader region around Nainital also depicted increasing trends in AOD (0.0042–0.0074 year−1), absorbing aerosol optical depth (0.0001 ± 0.0001 year−1) and aerosol index (0.0131 ± 0.0019 year−1), along with a slight decline in single scattering albedo (−0.0004 year−1). These results suggest that aerosols of absorbing nature might have increased over this region in recent times. Investigation of the AOD ratio, obtained from aerosol vertical profiles, revealed an increasing trend in aerosol load below 1 km. The observed trends in aerosols are found to be associated with enhanced biomass burning activity, reflected in terms of the positive trend in fire count ∼38.65 ± 5.88 year−1 over the region.

Abstract Global distribution of aerosol direct radiative forcing (DRF) is estimated using Clouds and Earth's Radiant Energy System (CERES) synoptic (SYN) 1° datasets. During 2001–2017, a statistically significant change of global DRFs is revealed with a general decreasing trend (i.e., a reduced cooling effect) at the top of the atmosphere (DRFTOA ~ 0.017 W⋅m−2⋅year−1) and at the surface (DRFSFC ~ 0.033 W⋅m−2⋅year−1) with rapid change over the land compared to the global ocean. South Asia and Africa/Middle East regions depict significant increasing trend of atmospheric warming by 0.025 and 0.002 W·m−2⋅year−1 whereas, the rest of the regions show a decline. These regional variations significantly modulate the global mean DRF (−5.36 ± 0.04 W·m−2 at the TOA and − 9.64 ± 0.07 W·m−2 at the surface during the study period). The observed DRF trends are coincident with the change in the underlying aerosol properties, for example, aerosol optical depth, Ångström exponent and partly due to the increasing columnar burden of SO2 over some of the regions. This indicates that increasing industrialization and urbanization have caused prominent change in the DRF during recent decades.

Formation of ultrafine particles and their subsequent growth have been examined during new particle formation (NPF) events in two contrasting environments under varying ambient conditions, one for a tropical semi-urban coastal station, Trivandrum, and the other for a high-altitude free-tropospheric Himalayan location, Hanle. At Trivandrum, NPF bursts took place in the late evening/night hours, whereas at Hanle the burst was a daytime event. During the nucleation period, the total number concentration reached levels as high as ~ 15 900 cm−3 at Trivandrum, whereas at Hanle, the total number concentration was ~ 2700 cm−3, indicating the abundant availability of precursors at Trivandrum and the pristine nature of Hanle. A sharp decrease was associated with NPF for the geometric mean diameter of the size distribution, and a large increase in the concentration of the particles in the nucleation regime (Dp < 25 nm). Once formed, these (secondary) aerosols grew from nucleation (diameter Dp < 25 nm) to Aitken (25 ≤ Dp ≤ 100 nm) regime and beyond, to the accumulation size regimes (100 ≤ Dp ≤ 1000 nm), with varying growth rates (GR) for the different size regimes at both the locations. A more rapid growth ~ 50 nm h−1 was observed at Trivandrum, in contrast to Hanle where the growth rate ranged from 0.1 to 20 nm h−1 for the transformation from the nucleation to accumulation – a size regime that can potentially act as cloud condensation nuclei (CCN). The faster coagulation led to lifetimes of < 1 h for nucleation mode particles.