Brown Carbon Formation from Nighttime Chemistry of Unsaturated Heterocyclic Volatile Organic Compounds

Abstract

Nighttime atmospheric processing enhances the formation of brown carbon aerosol (BrC) in biomass burning plumes. Heterocyclic compounds, a group of volatile organic compounds (VOCs) abundant in biomass burning smoke, are possible BrC sources. Here, we investigated the nitrate radical (NO 3 )-initiated oxidation of three unsaturated heterocyclic compounds (pyrrole, furan, and thiophene) as a source of BrC. The imaginary component of the refractive index at 375 nm (k 375 ), the single scattering albedo at 375 nm (SSA 375 ), and average mass absorption coefficients (⟨MAC⟩ 290−700 nm ) of the resulting secondary organic aerosol (SOA) are reported. Compared to furan and thiophene, NO 3 oxidation of pyrrole has the highest SOA yield. Pyrrole SOA (k 375 = 0.015 ± 0.003, SSA = 0.86 ± 0.01, ⟨MAC⟩ 290−700 nm = 3400 ± 700 cm 2 g −1 ) is also more absorbing than furan SOA (⟨MAC⟩ 290−700 nm = 1100 ± 200 cm 2 g −1 ) and thiophene SOA (k 375 = 0.003 ± 0.002, SSA 375 = 0.98 ± 0.01, ⟨MAC⟩ 290−700 nm = 3000 ± 500 cm 2 g −1 ). Compared to other SOA systems, MACs reported in this study are higher than those from biogenic precursors and similar to high-NO x anthropogenic SOA. Characterization of SOA molecular composition using high-resolution mass spectrometric measurements revealed unsaturated heterocyclic nitro products or organonitrates as possible chromophores in BrC from all three precursors. These findings reveal nighttime oxidation of fire-sourced heterocyclic compounds, particularly pyrrole, as a plausible source of BrC.

Organic coating on sulfate and soot particles during late summer in the Svalbard Archipelago

“…Although increased humaninduced emissions of long-lived greenhouse gases are certainly one of the driving factors, air pollutants, such as aerosols and ozone, are also important contributors to climate change in the Arctic (Law and Stohl, 2007;Shindell, 2007). Spatial and temporal variations of aerosol composition, size distribution, and sources of Arctic aerosols have been studied extensively in numerous ground-based, ship, and airborne observations and various atmospheric models (Brock et al, 2011;Burkart et al, 2017;Chang et al, 2011;Dall'Osto et al, 2017;Fu et al, 2008;Hara et al, 2003;Hegg et al, 2010;Iziomon et al, 2006;Karl et al, 2013;Lathem et al, 2013;Leck and Bigg, 2008;Leck and Svensson, 2015;Raatikainen et al, 2015;Wöhrnschimmel et al, 2013;Winiger et al, 2017;Yang et al, 2018;Zangrando et H. Yu et al: Organic coatings on secondary sulfate particles al., 2013). These studies show that regional pollutants and local natural aerosol production affect sea ice albedo and the heat balance of the atmosphere, especially in the summer when mid-latitude transport is not as frequent relative to that during the winter-spring Arctic Haze season (Hansen and Nazarenko, 2004;Jacob et al, 2010;Shindell, 2007).…”

Photochemical aging of atmospherically reactive organic compounds involving brown carbon at the air–aqueous interface

“…Volatility is generally inversely correlated with the O : C ratio (Aiken et al, 2008). The photochemical reaction at the air-aqueous interface under the condition of BrC is an efficient and common pathway to oxidize organic films toward low volatile organic compounds (O : C ratio of 0.25 to 1) (Jimenez et al, 2009). Moreover, the photosensitized reaction of organic aqueous aerosol likely depends on the filmforming species, given the different reactivities of saturated and unsaturated phospholipids.…”

Laboratory studies of fresh and aged biomass burning aerosol emitted from east African biomass fuels – Part 2: Chemical properties and characterization

“…Aerosols obtained from a 500 • C burn classified as BrC aerosol are spherical in morphology as seen in transmission electron microscopy (TEM) images and are yellowbrown in color due to values of the imaginary portion of the refractive index (κ) that increase sharply toward shorter visible and ultraviolet wavelengths (Bond and Bergstrom, 2006). BrC is comprised of a wide range of poorly characterized compounds exhibiting highly variable light absorption properties, with reported κ values spanning 2 orders of magnitude (Saleh et al, 2013;Chen and Bond, 2010;Kirchstetter et al, 2004;McMeeking et al, 2009). Its optical properties have been shown to change through atmospheric processing, such as oxidation and the absorption of solar radiation, leading to particle-phase reactions and aqueous-phase processing within aerosol particles (Lambe et al, 2011(Lambe et al, , 2013Sareen et al, 2013;Lee et al, 2014;Zhao et al, 2015;Nguyen et al, 2012;Laskin et al, 2015).…”