Formation of Secondary Brown Carbon in Biomass Burning Aerosol Proxies through NO₃ Radical Reactions

Abstract

Atmospheric brown carbon (BrC) is an important contributor to the radiative forcing of climate by organic aerosols. Because of the molecular diversity of BrC compounds and their dynamic transformations, it is challenging to predictively understand BrC optical properties. OH radical and O3 reactions, together with photolysis, lead to diminished light absorption and lower warming effects of biomass burning BrC. The effects of night-time aging on the optical properties of BrC aerosols are less known. To address this knowledge gap, night-time NO3 radical chemistry with tar aerosols from wood pyrolysis was investigated in a flow reactor. This study shows that the optical properties of BrC change because of transformations driven by reactions with the NO3 radical that form new absorbing species and lead to significant absorption enhancement over the ultraviolet–visible (UV-vis) range. The overnight aging increases the mass absorption coefficients of the BrC by a factor of 1.3–3.2 between 380 nm and 650 nm. Nitrated organic compounds, particularly nitroaromatics, were identified as the main products that contribute to the enhanced light absorption in the secondary BrC. Night-time aging of BrC aerosols represents an important source of secondary BrC and can have a pronounced effect on atmospheric chemistry and air pollution.

Undersizing of Aged African Biomass Burning Aerosol by an Ultra High Sensitivity Aerosol Spectrometer

“…IR absorption by BrC has been measured in a few studies. While Li et al (2020) found negligible absorption for wood tar aerosol above 550 nm, Sumlin et al (2018b) did find a small amount of absorption, k = 0.002 ± 0.005, for 1047 nm light in smoke from smoldering peat fires in a laboratory. That absorption is only sufficient to heat 300 nm particles by about 25°C (Fig.…”

“…BC is a hypothetical material that absorbs more or less as a black body and includes graphitic soot nanospheres and amorphous C. BrC is organic material that absorbs light primarily at short wavelengths (Andreae and Gelencser, 2006) and to a small and very poorly known extent in the IR (e.g. Li et al, 2020;Sumlin et al, 2018b). Tarballs (Pósfai et al, 2003(Pósfai et al, , 2004 are a distinctively spherical variety of BrC prevalent in aged biomass burning plumes.…”

Laboratory Insights into the Diel Cycle of Optical and Chemical Transformations of Biomass Burning Brown Carbon Aerosols

“…However, the temporal continuity in atmospheric aging processes is generally overlooked, and most studies focus on a single reaction pathway in BB-BrC aging or ignore the modifications due to diel atmospheric transformations altogether in climate models. 15 , 16 , 18 , 19 …”

“… 26 − 28 Such wood tar aerosols have been applied in a number of recent studies to investigate the atmospheric aging of BB-BrC in the laboratory. 16 , 20 , 28 − 30 …”

Molecular Absorption and Evolution Mechanisms of PM2.5Brown Carbon Revealed by Electrospray Ionization Fourier Transform–Ion Cyclotron Resonance Mass Spectrometry During a Severe Winter Pollution Episode in Xi'an, China

“…Negative ESI‐FT‐ICR MS was applied to characterize molecules in ambient aerosols and smoke particles (Brege et al, 2018; Jiang et al, 2016; Mazzoleni et al, 2012; Song et al, 2018, Zhang et al, 2018). However, direct applications of ESI FT–ICR MS on studying BrC optical properties, such as exploring the relationship between absorption and molecules, are still very limited (Laskin et al, 2018; Li et al, 2019).…”