Abstract
Abstract. The heterogeneous reaction between gas phase glyoxal and ammonium sulfate (AS) aerosols, a proxy for inorganic atmospheric aerosol, was studied in terms of the dependence of the optical, physical and chemical properties of the product aerosols on initial particle size and ambient relative humidity (RH). Our experiments imitate an atmospheric scenario of a dry particle hydration at ambient RH conditions in the presence of glyoxal gas followed by efflorescence due to decrease of the ambient RH. The reactions were studied under different RH conditions, starting from dry conditions (~20% RH) and up to 90% RH, covering conditions prevalent in many atmospheric environments, and followed by consequent drying of the reacted particles before their analysis by the aerosol mass spectrometer (AMS), cavity ring down (CRD) and scanning mobility particle sizer (SMPS) systems. At λ = 355 nm, the reacted aerosols demonstrate a substantial growth in optical extinction cross section, as well as in mobility diameter under a broad range of RH values (35–90%). The ratio of the product aerosol to seed aerosol geometric cross section reached up to ~3.5, and the optical extinction cross-section up to ~250. The reactions show a trend of increasing physical and optical growth with decreasing seed aerosol size, from 100 nm to 300 nm, as well as with decreasing RH values from 90% to ~40%. Optically inactive aerosols, at the limit of the Mie range (100 nm diameter) become optically active as they grow due to the reaction. AMS analyses of the reaction of 300 nm AS at RH values of 50%, 75% and 90% show that the main products of the reaction are glyoxal oligomers, formed by acetal formation in the presence of AS. In addition, imidazole formation, which is a minor channel, is observed for all reactions, yielding a product which absorbs at λ = 290 nm, with possible implications on the radiative properties of the product aerosols. The ratio of absorbing substances (C-N compounds, including imidazoles) increases with increasing RH value. A core/shell model used for the investigation of the optical properties of the reaction products of AS with gas phase glyoxal, shows that the refractive index (RI) of the reaction products are n= 1.68(±0.10)+0.01(±0.02) at 50% RH and n = 1.65(±0.06)+0.02(±0.01) at 75% RH at 355 nm. The observed increase in the ratio of the absorbing substances is not indicated in the imaginary part of the products at RH 50% and 75%. A further increase in the ratio of absorbing substances and a resulting increase in the imaginary part of the RI at higher RH values is expected, and may become even more substantial after longer reaction times, possibly in cloud or fog droplets. This study shows that the reaction of abundant substances present in atmospheric aerosols, such as AS, and gas phase glyoxal alters the aerosols' optical, physical and chemical properties and may have implications on the radiative effect of these aerosols.
Highlights
Aging of aerosol particles in the atmosphere is an important process, leading to products with new chemical and physical properties, higher molecular weight (MW), higher oxidation state, and hygroscopicity, as well as altered optical properties
This study shows that the reaction of abundant substances present in atmospheric aerosols, such as ammonium sulfate (AS), and gas phase glyoxal alters the aerosols’ optical, physical and chemical properties and may have implications on the radiative effect of these aerosols
An 80 cm3 min−1 flow is directed into the Aerodyne High Resolution Time of Flight Aerosol Mass Spectrometer (HR-ToF-aerosol mass spectrometer (AMS), Aerodyne) system for determination of the chemical properties, and 800 cm3 min−1 are introduced into a pulsed cavity ring down (CRD) system for determination of their optical properties
Summary
Aging of aerosol particles in the atmosphere is an important process, leading to products with new chemical and physical properties, higher molecular weight (MW), higher oxidation state, and hygroscopicity, as well as altered optical properties. Glyoxal is found in urban environments, such as mega-cities, over the oceans and in forests, including tropical rain forests, and in biomass burning regions, it is found in marine and coastal sites, and rural locations (Fu et al, 2008, 2009; Huisman et al, 2011; Myriokefalitakis et al, 2008; Sinreich et al, 2007; Volkamer et al, 2007) Due to their high solubility and reactivity in the aqueous phase, glyoxal and other α-dicarbonyls contribute substantially to SOA formation, mainly via cloud processing, while the photolysis of glyoxal significantly contributes to HOx (OH + HO2) chemistry in the gas phase (Carlton et al, 2007; Ervens et al, 2008; Fu et al, 2008; Hallquist et al, 2009). We determine the effect of the heterogeneous reaction on the optical properties of the aerosol particles using cavity ring down aerosol spectrometer (CRD-AS)
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