Long-term (2008-2018) aerosol properties and radiative effect at high-altitude sites over western trans-Himalayas

U C Dumka; Shantikumar S Ningombam; D G Kaskaoutis; B L Madhavan; H-J Song; Dorje Angchuk; Sonam Jorphail

doi:10.1016/j.scitotenv.2020.139354

Long-term (2008-2018) aerosol properties and radiative effect at high-altitude sites over western trans-Himalayas

Sci Total Environ. 2020 Sep 10:734:139354. doi: 10.1016/j.scitotenv.2020.139354. Epub 2020 May 16.

Authors

U C Dumka¹, Shantikumar S Ningombam², D G Kaskaoutis³, B L Madhavan⁴, H-J Song⁵, Dorje Angchuk⁶, Sonam Jorphail⁶

Affiliations

¹ Aryabhatta Research Institute of observational Sciences, Nainital 263001, India. Electronic address: dumka@aries.res.in.
² Indian Institute of Astrophysics, Bangalore 560034, India. Electronic address: shanti@iiap.res.in.
³ Institute for Environmental Research and Sustainable Development, National Observatory of Athens, Palaia Penteli, 15236 Athens, Greece. Electronic address: dkask@noa.gr.
⁴ National Atmospheric Research Laboratory (NARL), Department of Space, Gadanki 517112, India.
⁵ National Institute of Meteorological Sciences, Seogwipo, Jeju, South Korea.
⁶ Indian Astronomical Observatory, Indian Institute of Astrophysics, Skara, Leh-Ladakh, 194101, India.

PMID: 32470663
DOI: 10.1016/j.scitotenv.2020.139354

Abstract

Analysis of the climatology of aerosol properties is performed over Hanle (4500 m) and Merak (4310 m), two remote-background sites in the western trans-Himalayas, based on eleven years (2008-2018) of sun/sky radiometer (POM-01, Prede) measurements. The two sites present very similar atmospheric conditions and aerosol properties allowing us to examine them as continuous single-data series. The annual average aerosol optical depth at 500 nm (AOD₅₀₀) is 0.04 ± 0.03, associated with an Ångström exponent (AE₄₄₀_-₈₇₀) of 0.58 ± 0.35 and a single scattering albedo (SSA₅₀₀) of 0.95 ± 0.05. AOD₅₀₀ exhibits higher values in May (~0.07) and lower in winter (~0.03), while AE₄₀₀_-₈₇₀ minimizes in spring, indicating influence by coarse-mode dust aerosols, either emitted regionally or long-range transported. The de-convolution of AOD₅₀₀ into fine and coarse modes justifies the aerosol seasonality and sources, while the marginal diurnal variation in all aerosol properties reveals a weak influence from local sources, except for some few aerosol episodes. The aerosol-volume size distribution presents a mode value at ~10 μm with secondary peaks at accumulation (~ 2 μm) and fine modes (~0.03 μm) and low variability between the seasons. A classification of the aerosol types based on the fine-mode fraction (FMF) vs. SSA₅₀₀ relationship reveals the dominance of aerosols in the FMF range of 0.4-0.6, characterized as mixed (39%), followed by fine aerosols with high scattering efficiency (26%), while particles related to dust contribute ~21%, with low fractions of fine-absorbing aerosols (~13%). The aerosol radiative forcing (ARF) estimates reveal a small cooling effect at the top of the atmosphere (-1.3 Wm^-2), while at the surface, the ARF ranges from -2 Wm^-2 to -6 Wm^-2 on monthly basis. The monthly-mean atmospheric radiative forcing (~1 to 4 Wm^-2) leads to heating rates of 0.04 to 0.13 K day^-1. These ARF values are higher than the global averages and may cause climate implications over the trans-Himalayan region.

Keywords: Aerosol optical properties; Aerosol radiative forcing; Aerosol size distribution; Aerosol types; High-altitude site; Trans-Himalayas.