Context: Recently, a study by the Aryabhatta Research Institute of Observational Sciences (ARIES) has found that radiative forcing of aerosols i.e. effect of anthropogenic aerosols is much higher over the high altitudes of western trans-Himalayas.
Key Observations
Aerosol Optical Depth(AOD)

• The observations of the scientists show that the aerosol optical depth (AOD) exhibited a distinct seasonal variation with higher values (0.07) in May and lower (0.03) in winter months. 
• The lower values of Ångström exponent (AE) in spring indicated dominance of coarse-mode dust aerosols. 

Quality of Air

• Pure and polluted dust exhibited fractions between 16% and 23%, with a low frequency of less than 13% of absorbing aerosols, denoting weak influence of anthropogenic aerosols and Black Carbon over the trans-Himalayan sites.  

Aerosol Radiative Forcing

• Further, the aerosol radiative forcing ARF values at top of the atmosphere were mostly low (-1.3 Wm-2) over Hanle and Merak.
• ARF is the effect of anthropogenic aerosols on the radiative fluxes at the top of the atmosphere and at the surface and on the absorption of radiation within the atmosphere.
• Hanle and Merak, situated in Ladakh are the part of Indian Astronomical Observatory (IAO).

Importance of the Key Findings

• The atmospheric aerosols play a key role in the regional/global climate system through scattering and absorption of incoming solar radiation and by modifying the cloud microphysics. 
• Despite the large progress in quantifying the impact of different aerosols on radiative forcing, it still remains one of the major uncertainties in the climate change assessment. 
• Precise measurements of aerosol properties are required to reduce the uncertainties, especially over the oceans and high altitude remote location in the Himalayas where they are scarce.
• A deep scientific study of aerosol generation, transport, and its properties have important implications in our understanding and mitigation of climate change via atmospheric warming, which among many things, impacts the snow and glacier dynamics over the trans-Himalayan region.
• The results from the study can help better understanding the aerosol optical and microphysical properties and improving the modelling of aerosol effects in view of aerosol-climate implications via modifications in atmospheric warming and changes in the snow/glacier albedo over the trans-Himalayan region. 

Background

• Measurements of aerosol optical and microphysical properties started during the last decade at the Indian Astronomical Observatory (IAO) at the high altitude background sites of Hanle and Merak in the trans-Himalayas under the frameworks of Aerosol Radiative Forcing over India (ARFI) and Astronomical Site Survey program of Indian Institute of Astrophysics (IIA), Bangalore.
• The Indian Astronomical Observatory, located near Leh in Ladakh, has one of the world's highest located sites for optical, infrared and gamma-ray telescopes.
• In addition to this, few in-situ measurements of carbonaceous aerosols and ionic species have also been performed at Himansh Observatory (Spiti Valley) in the western Himalayas.

About Aerosol

• An aerosol is a colloid of fine solid particles or liquid droplets, in air or another gas.
• In simple terms, Aerosols are minute particles suspended in the atmosphere.
• Aerosols can be natural (fog, forest exudates and geyser steam) or artificial (haze, dust, particulate air pollutants and smoke). The bulk of aerosols—about 90% by mass—have natural origins.
• Aerosols also can act as sites for chemical reactions to take place (heterogeneous chemistry).
• Aerosols interact both directly and indirectly with the Earth’s radiation budget and climate.

Can we see Aerosols?

• When the suspended particles in aerosols are sufficiently large, we notice their presence as they scatter and absorb sunlight.
• Their scattering of sunlight can reduce visibility (haze) and redden sunrises and sunsets.

Direct effects of Aerosols

• As a direct effect most aerosols reflect sunlight and some also absorb it. Aerosols can, hence, alter the reflectivity, or albedo, of the planet. They are also likely to hasten the melting of ice in the Arctic.

Different Aerosols and their effects

• Pure sulfates and nitrates reflect nearly all radiation they encounter, cooling the atmosphere.
• Black carbon absorbs radiation readily, warming the atmosphere but also shading the surface.
• Brown carbon or organic matter has a warming influence on the atmosphere.
• Salt particles tend to reflect all the sunlight they encounter.

Indirect effects of Aerosols

• As an indirect effect, aerosols in the lower atmosphere can modify the size of cloud particles, changing how the clouds reflect and absorb sunlight.
• Aerosols also can act as sites for chemical reactions to take place: the most significant amongst them are the ones that lead to the destruction of stratospheric ozone.
• Indirectly aerosols typically work in opposition to greenhouse gases and cause cooling.

Road Ahead

• The transport of light-absorbing carbonaceous aerosols and dust from the polluted Indo-Gangetic Plain and desert areas over the Himalayas constitutes a major climatic issue due to severe impacts on atmospheric warming and glacier retreat. This heating over the Himalayas facilitates the “elevated-hat pump” that strengthens the temperature gradient between land and ocean and modifies the atmospheric circulation and the monsoon rainfall. 
• A better understanding of the aerosol optical and microphysical properties through the study can improving the modelling of aerosol effects in view of aerosol-climate implications via modifications in atmospheric warming and changes in the snow/glacier albedo over the trans-Himalayan region.