Spectroscopic and Photochemical Properties of Secondary Brown Carbon from Aqueous Reactions of Methylglyoxal

Abstract

Chromophoric organic compounds in the atmosphere termed brown carbon (BrC) play an important role in tropospheric photochemistry and radiative forcing due to their absorption in the near-UV region of the solar spectrum (300−400 nm). BrC chromophores can form in secondary reactions of carbonyls with ammonium and amine precursors commonly found in aqueous aerosols and cloud water, although there is still uncertainty regarding the contribution of these reactions to atmospheric BrC. Herein, experimental results are presented characterizing the spectroscopic properties of BrC generated from aqueous aerosol/cloud water mimics containing methylglyoxal and ammonium sulfate (MG + AS) or methylamine chloride (MG + MA). Spectroscopic results include mass absorption coefficients, fluorescence quantum yields, and excitation− emission matrix (EEM) spectra, including parallel factor (PARAFAC) analysis. This work also provides a detailed comparison to previous laboratory and field measurements on the classification of aerosols and environmental water with EEM spectroscopy. In addition to spectroscopic characterization, the chemical composition of these samples is investigated using electrospray ionization mass spectrometry and shows the formation of oligomers of methylglyoxal-and nitrogen-containing heterocycles as contributors to absorption in the near-UV consistent with the previous work. The BrC samples were also irradiated using a solar simulator to study the changes in the optical properties and estimate the atmospheric photolytic lifetimes, which have not been previously reported for MG + MA BrC. These results reveal that BrC derived from MG + MA is more stable under solar irradiation than MG + AS BrC with measured atmospheric photolytic half-lives of 39 ± 8 and 12 ± 3 min, respectively.

Aging of Atmospheric Brown Carbon Aerosol

“…184 Numerous combinations of atmospherically relevant aldehydes and ammonia or amines have been observed to participate in this chemistry, including aldehydes such as methylglyoxal, glyoxal, glycolaldehyde and amines such as glycine and alkylamines. 180,182,[185][186][187][188][189][190][191][192][193][194][195] More recently, pyrazine compounds have also been observed as products from the reaction of methylglyoxal and ammonium (e.g., dimethylpyrazine, Figure 1). 185,196 While these small aldehydes have high volatilities, they are water-soluble and partition to the aqueous phase.…”

“…Loss of absorption from imine BrC has been investigated in laboratory studies using a variety of precursors to form imine BrC. 159,160,192,194,[202][203][204][205] The lifetime of methylglyoxal and ammonium derived imine BrC with respect to UV light exposure occurred over minutes, primarily due to the loss of imidazole-2-carboxaldehyde + methylglyoxal oligomers and other nitrogen containing compounds (Figure 7). 160,192,206 OH oxidation in the aqueous phase also proceeds rapidly, with similar lifetimes on the order of minutes to hours.…”

Competing Photochemical Effects in Aqueous Carbonyl/Ammonium Brown Carbon Systems

“…Similar to other approaches for comparable photochemical reaction systems, − all reactions modeled were assumed to have constant, first-order kinetics independent of solution concentrations. This assumption allows for the representation of rate constants for each feature by a Gaussian curve in terms of fitted time constants: …”

“…Studies exploring the MG/AS reaction system in the presence of OH or O 3 suggest the photolysis of aqueous-phase oligomerization products within the order of minutes . Direct photolysis studies on bulk MG/AS mimic solutions demonstrate a decrease in MAC and the extent of oligomerization after hours of UV irradiance; those studies fit the decrease in absorbance to first-order kinetics. − Zhao et al studied both the direct photolysis and OH oxidation of bulk MG/AS and Gly/AS solutions and attributed the observed nonfirst-order decrease in absorbance in both systems to the presence of multiple chromophores with differing photolabilities . Vo et al studied the light-absorbing properties of the methyl-substituted imidazole carboxaldehyde (1-(4-methyl-1 H -imidazol-2-yl)-ethanone, dimethyl-IC) and postulated that there may be generation and photolysis of carbonyl species beyond this molecule that are affecting the light absorption at 280 nm .…”

Photodegradation of atmospheric chromophores: changes in oxidation state and photochemical reactivity

“…11582 Z. Mu et al: Photodegradation of atmospheric chromophores: changes in oxidation state Altieri et al, 2008;Haynes et al, 2019;Holmes and Petrucci, 2006;Perri et al, 2009). SOA may have a more significant light absorption than primary organic aerosol (POA) in the short-wavelength visible and near-UV range (Zhong and Jang, 2014;Saleh et al, 2013;Harrison et al, 2020).…”