Dynamic light absorption of biomass-burning organic carbon photochemically aged under natural sunlight

Abstract

Wood-burning aerosol produced under smoldering conditions was photochemically aged with different relative humidity (RH) and NO x conditions using a 104 m 3 dual outdoor chamber under natural sunlight. Light absorption of organic carbon (OC) was measured over the course of photooxidation using a UV-visible spectrometer connected to an integrating sphere. At high RH, the color decayed rapidly. NO x slightly prolonged the color of wood smoke, suggesting that NO x promotes the formation of chromophores via secondary processes. Overall, the mass absorption cross section (integrated between 280 and 600 nm) of OC increased by 11-54 % (except high RH) in the morning and then gradually decreased by 19-68 % in the afternoon. This dynamic change in light absorption of wood-burning OC can be explained by two mechanisms: chromophore formation and sunlight bleaching. To investigate the effect of chemical transformation on light absorption, wood smoke particles were characterized using various spectrometers. The intensity of fluorescence, which is mainly related to polycyclic aromatic hydrocarbons (PAHs), rapidly decreased with time, indicating the potential bleaching of PAHs. A decline of levoglucosan concentrations evinced the change of primary organic aerosol with time. The aerosol water content measured by Fourier transform infrared spectroscopy showed that woodburning aerosol became less hygroscopic as photooxidation proceeded. A similar trend in light absorption changes has been observed in ambient smoke aerosol originating from the 2012 County Line wildfire in Florida. We conclude that the biomass-burning OC becomes less light absorbing after 8-9 h sunlight exposure compared to fresh wood-burning OC.Published by Copernicus Publications on behalf of the European Geosciences Union.

Optical Properties of Laboratory and Ambient Biomass Burning Aerosols: Elucidating Black, Brown, and Organic Carbon Components and Mixing Regimes

“…Likewise are the combustion events that produce a much larger amount of rBC in addition to BrC, which would produce BrC products that are on the highly absorbing side of the spectrum, nearly approaching that of rBC. These results are consistent with many other studies, including Srinivas and Sarin (2013), Srinivas and Sarin (2014), Feng et al (2013), Sun et al (2007), Zhong and Jang (2014), and Kirchstetter and Thatcher (2012), who calculated AAE for the organic fraction (without rBC) of BB emissions. This implies that in our study BrC, in addition to rBC, is driving the optical properties observed in each laboratory burn, and what is observed in the right-to-left transition during a burn is the BrC optical properties are changing over time.…”

Source-diagnostic dual-isotope composition and optical properties of water-soluble organic carbon and elemental carbon in the South Asian outflow intercepted over the Indian Ocean

“…(c) Photochemical aging of WS‐BrC during long‐range transport leads to less absorptive chemical compounds. Both laboratory and field measurements have shown changes on aerosol absorption during photochemical aging, with aged OC being less absorptive than fresh OC [ Zhong and Jang , ; Adler et al ., ]. (d) Photochemical aging leading to the mixture of WS‐BrC with newly formed nonabsorbing WSOC during atmospheric transport, e.g., short‐chain dicarboxylic acids.…”

Agricultural fires in the southeastern U.S. during SEAC4RS: Emissions of trace gases and particles and evolution of ozone, reactive nitrogen, and organic aerosol

“…(BrC), a component of OA that absorbs light in the UV and visible spectral regions, is associated with incomplete combustion and SOA formation [Hecobian et al, 2011;Saleh et al, 2013;Forrister et al, 2015]. Recent studies indicate that BrC evolution in BB plumes is controlled by secondary processes such as chromophore formation and loss due to photobleaching, volatilization, and/or aerosol phase reactions, leading to different evolution of BrC and bulk OA Zhong and Jang, 2014;Forrister et al, 2015;Zhao et al, 2015]. Although agricultural field burning is ubiquitous in the U.S., no field study to date has characterized the emissions and smoke chemistry of these fires.…”