Evolution of Brown Carbon Aerosols during Atmospheric Long-Range Transport in the South Asian Outflow and Himalayan Cryosphere

Abstract

The Himalayas sustain the water sources for ∼2 billion people in South and East Asia. The Himalayan cryosphere is particularly sensitive to climate change as a result of long-range transport of anthropogenic emissions from the Indo-Gangetic Plain. Atmospheric warming as a result of light-absorbing aerosols, i.e., black carbon (BC) and brown carbon (BrC), is responsible for accelerated loss of glaciers in the region. The BrC lifetime in the atmosphere over the Himalayan cryosphere has never been studied before, and observational determination of the lifetime of BrC is imperative in improving model performance. In this work, we collected bulk water-soluble BrC (WS-BrC) data from multiple campaigns conducted over the past 15 years in South Asia and the Himalayas, with an aim to provide constraints for two distinct receptor regions: South Asia outflow and Himalayan atmosphere. We observed decay in WS-BrC absorptivity for both receptor regions as air masses undergo long-range transport from the source region. However, the decay rate of the WS-BrC imaginary refractive index (k WS‑BrC‑365) in the Himalayan atmosphere (0.09 ± 0.02 day–1) was ∼2 times slower than the South Asian outflow (0.17 ± 0.04 day–1), implying their longer atmospheric half-life in the Himalayan atmosphere than the South Asian outflow (∼8 and 4 days, respectively). The comparison in the evolution of k WS‑BrC‑365 and absorption Ångström exponent revealed the less pronounced role of photochemical bleaching in the decay of BrC in the Himalayan atmosphere than in the South Asian outflow. Slower decay is consistent with highly viscous organic aerosol as a result of a low temperature and relative humidity in the cryospheric atmosphere.

Aqueous-phase photochemical oxidation of water-soluble brown carbon aerosols arising from solid biomass fuel burning

“…can transform the chemical composition and optical properties of BrC aerosols following their emissions or formation. 7,9,[31][32][33] Previous laboratory studies on aqueous-phase photochemical oxidation of secondary BrC showed rapid loss of chromophores also referred to as photobleaching. 29,30 However, photo-enhancement preceded photobleaching for biomass burning primary BrC aerosols.…”

Unexpected electrophiles in the atmosphere – anhydride nucleophile reactions and uptake to biomass burning emissions

“…Through reactions occurring inside particles, as well as interactions with gas-phase molecules, its composition and effects are additionally modified. 14–19…”

“…Through reactions occurring inside particles, as well as interactions with gas-phase molecules, its composition and effects are additionally modified. [14][15][16][17][18][19] Recent studies have demonstrated that photochemical processing of biomass burn plumes in the atmosphere leads to the formation of anhydrides through aromatic oxidation and furan chemistry. [20][21][22][23][24][25] Specifically, maleic and phthalic anhydride represent two significant primary emissions from the combustion of biomass.…”

Molecular signatures and formation mechanisms of particulate matter (PM) water-soluble chromophores from Karachi (Pakistan) over South Asia

“…Atmospheric WSOC originated from primary emissions, including biomass burning, coal burning, and other primary sources (Park and Yu, 2016;Li et al, 2018;Tang et al, 2020); as well as the secondary formations, such as the aqueous-phase reaction from anthropogenic or biogenic emission (Gilardoni et al, 2016;Lamkaddam et al, 2021;Updyke et al, 2012;Yu et al, 2021). Light-absorbing WSOC is an important component of brown carbon (BrC) and has a strong wavelength-dependent absorption that peaks in the ultraviolet (UV) spectral region and declines through the visible spectral region (Choudhary et al, 2022;Hecobian et al, 2010;Laskin et al, 2015;Sullivan et al, 2022). This fraction can contribute significantly to the global radiation balance and affects the photochemistry of the atmosphere (Feng et al, 2013;Kirchstetter and Thatcher, 2012).…”