Effect of Relative Humidity on Secondary Brown Carbon Formation in Aqueous Droplets

Abstract

Atmospheric brown carbon (BrC) is a significant contributor to particulate light absorption. Reactions between small aldehydes and reduced nitrogen species have been shown to produce secondary BrC in atmospheric droplets. These reactions can be substantially accelerated upon droplet evaporation. Despite aqueous droplets undergoing continuous water evaporation and uptake in response to the surrounding relative humidity (RH), secondary BrC formation in these droplets under various RH conditions remains poorly understood. In this work, we investigate BrC formation from reactions of two aqueous-phase precursors, glyoxal and methylglyoxal, with ammonium sulfate or glycine in aqueous droplets after drying at a range of RH (30−90%). Our results illustrate, for the first time, that BrC production varies as a function of RH. For all four chemical reaction systems being investigated, mass absorption efficiencies (MAE, m 2 /g C) of aqueous aerosol products (from 270 to 512 nm wavelength range) generally increase with reducing RH to reach a maximum at ∼55−65% RH and subsequently decrease, caused by further drying. Chemical characterization using high-resolution aerosol mass spectrometry shows that the formation of nitrogen-containing organic species also follows a similar variation with RH. Our observations reveal that the acceleration of BrC production from evaporation of water may be diminished by other factors, such as limited particle-phase water content, phase transition, and volatility of reactants and products. Overall, our results highlight that intermediate RH conditions in the atmosphere may be more efficient in secondary BrC formation, indicating that the effect of RH needs to be included in atmospheric models for a more accurate representation of light-absorbing aerosol formation in aqueous droplets.

Photoreaction of biomass burning brown carbon aerosol particles

“…There is also potential for concentration effects, given that low RH leads to more concentrated solutes. 33,46 We note that previous studies on the effects of RH on radical production from photosensitized reactions have observed complex behavior, which may be driven by a variety of factors, including concentration and viscosity. 33…”

Measurement Report: Optical properties and sources of water-soluble brown carbon in Tianjin, North China: insights from organic molecular compositions

“…Moreover, Abs365 and the estimated SOC showed similar variations with strong correlations in both summer (R = 0.693, p < 0.01) and winter (R = 0.881, p < 0.01) (Figure 6d and S1d), suggesting the significant impacts of secondary formation processes on BrC, even in the winter season. This is reasonable because SOA are also major contributors to BrC through photochemical reactions and/or aqueous/heterogeneous chemical processes (Laskin et al, 2014;Lin et al, 2015;Braman et al, 2020;Kasthuriarachchi et al, 2020). The correlation between Abs365 and SOC in winter was even stronger than that between Abs365 and EC, indicating the greater impacts of secondary formation on winter BrC compared with the primary emissions from combustion-related activities.…”

Aging of Atmospheric Brown Carbon Aerosol

“…200 A recent study demonstrated that the BrC formation process from evaporating aqueous aerosol particles occurs most efficiently at intermediate relative humidity values of roughly 60%. 208 Recent work has expanded the investigation of BrC formation by evaporation processes of aqueous mixtures of SOA in the presence of sulfuric acid. 209 For SOA made from high concentrations of α-pinene, β-pinene, limonene, and isoprene oxidation that was collected and extracted into water and adjusted to pH 2 with sulfuric acid, evaporation led to increases in absorption at ultra-violet and visible wavelengths, with the largest increases observed for limonene SOA (Figure 8).…”