Alpha-dicarbonyl compounds are believed to form brown
carbon in the atmosphere via reactions with ammonium sulfate (AS) in cloud
droplets and aqueous aerosol particles. In this work, brown carbon formation
in AS and other aerosol particles was quantified as a function of relative
humidity (RH) during exposure to gas-phase glyoxal (GX) in chamber
experiments. Under dry conditions (RH < 5 %), solid AS,
AS–glycine, and methylammonium sulfate (MeAS) aerosol particles brown within
minutes upon exposure to GX, while sodium sulfate particles do not. When GX
concentrations decline, browning goes away, demonstrating that this dry
browning process is reversible. Declines in aerosol albedo are found to be a
function of [GX]
Brown carbon is the name given to light-absorbing organic molecules present
in atmospheric aerosol. Estimates of the global direct radiative effect of
brown carbon aerosol range from
Glyoxal uptake to deliquesced ammonium sulfate particles is rapid (Kroll et al., 2005) but is difficult to detect on dry aerosol (Corrigan et al., 2008). Glyoxal reacts to form brown carbon imidazole derivatives in solutions containing ammonium ions (Shapiro et al., 2009; Noziere et al., 2009; Galloway et al., 2009; Yu et al., 2011; Kampf et al., 2012; Maxut et al., 2015) or primary amine species such as glycine or methylamine (De Haan et al., 2009a, b). While in bulk aqueous solution these reactions take hours to days (Shapiro et al., 2009; Noziere et al., 2009; Powelson et al., 2014), they can occur in minutes in aqueous aerosol particles, likely due to surface reactivity of glyoxal in its monohydrate form (De Haan et al., 2009a).
In this work, we report rapid and reversible browning of dry ammonium sulfate (AS), AS–glycine, and methylammonium sulfate (MeAS) aerosol particles upon exposure to gas-phase glyoxal. This browning process is not accompanied by appreciable particle growth and is reversed upon addition of water vapor.
CESAM is a 4.2 m
Additional experiments were conducted in a 300 L collapsible Tedlar chamber.
Aerosols were generated from 0.1 %
Reagents were used as received from Sigma-Aldrich unless otherwise
mentioned. Solutions for aerosol generation were generated by dilution of
glycine (>99 %) to 5 mM, AS (>99 %) to 1.2–10 mM, or sodium sulfate (
Summary of glyoxal gas addition experiments.
GX: glyoxal; AS: ammonium sulfate; gly: glycine; MeAS:
methylammonium sulfate.
Chamber experiments in which aerosol particles were exposed to gas-phase glyoxal are summarized in Table 1.
Experiment 1, in which dry AS aerosol was sequentially exposed to 0.05 and
then 0.50 ppm glyoxal at
The addition of 0.05 ppm glyoxal gas at
Pulse glyoxal addition Experiment 1 on dry AS aerosol in
CESAM chamber. Top: chamber RH. Middle panels: dilution- and water-corrected
PTR-MS traces for gas-phase glyoxal (
At
The 15 % aerosol mass loss upon humidification to 50 % RH is surprising
given that no corresponding mass gain was recorded during exposure to
glyoxal under dry conditions. However, the lack of mass gain under dry
conditions cannot be interpreted as a lack of glyoxal uptake or reactivity
given the large observed drop in albedo. Instead, the mass loss upon
humidification suggests that at least 15 % of the volume of AS seeds had
been replaced under dry conditions by glyoxal reaction products that could
break down into gas-phase species once water was added. Simultaneous
increases in gas-phase PTR-MS signals for
Dried seed aerosol particles atomized from AS–glycine mixtures were also
exposed to 0.25 ppm glyoxal under dry conditions in Experiment 2 (Fig. S1 in the Supplement). The response of these internally mixed seeds to glyoxal exposure was
comparable to that of pure AS seeds. No growth was observed by SMPS spectrometry, and
aerosol albedo at 450 nm was anticorrelated with PTR-MS glyoxal signals at
To better understand the reactive processes happening in the dry aerosol
particles, further experiments were conducted in a 300 L Tedlar chamber
probed by Q-AMS, SMPS, CAPS-ssa, CRD, and PAS spectrometry. Figure S2 shows an AMS ion
correlation plot comparing average signals before and after 40 ppb glyoxal
was added over a period of 25 min to the dry chamber containing AS
aerosol in Experiment 3. Unsurprisingly, the slow addition of this smaller
amount of glyoxal did not cause observable net particle growth or a decline
in aerosol albedo at 450 nm. A marginal (0.9
Proposed chemical mechanisms for brown carbon production at AS particle surfaces are summarized in Schemes 1 and S1 in the Supplement. Except for steps where new N-heterocyclic rings are formed, all processes are reversible (Kampf et al., 2012). Thus, a reduction in gas-phase glyoxal concentrations will shift reversible reactions away from brown carbon back towards simple N-heterocycle products, which do not absorb 450 nm light. Humidification to 50 % RH accelerates this shift by removing more glyoxal from the gas phase and perhaps also by hydrolysis of double bonds and dilution effects (Rincón et al., 2010; Phillips and Smith, 2014, 2015). Humidification also triggers the observed evaporation of formate as formic acid and perhaps the evaporation of other small N-containing products.
Proposed brown carbon formation pathways of glyoxal reacting at solid AS aerosol particle surfaces. Products detected in this study are shown in blue. IC: 1H-imidazole-2-carboxaldehyde. BI: 2,2'-biimidazole (Kampf et al., 2012). AS: ammonium sulfate. GX: glyoxal. We assume, following Kampf et al., 2012, that all reactions are reversible except for formation of N-heterocycle rings. See Scheme S1 for corresponding diagram of pathways under conditions of humidification.
The anticorrelation of albedo with glyoxal concentrations in Experiments 1–4
is summarized in Fig. 2. Although AS–glycine–glyoxal bulk aqueous mixtures
have been shown to brown more than mixtures without glycine (Trainic et
al., 2012; Powelson et al., 2014), here we see that dry AS and AS–glycine
aerosol particles brown similarly for a given concentration of gas-phase
glyoxal. This may indicate that glycine is not at the aerosol surface or
that glycine surfaces, when present, are less able to retain adsorbed water
in the dry chamber. We therefore fit the combined dataset from all four
experiments. Albedo shows a clear downward curvature at high glyoxal
concentrations such that the relationship is best fit by a second-order
polynomial. This suggests that the formation of the compounds absorbing at
450 nm is proportional to [glyoxal]
Top: anticorrelation of particle single-scattering albedo
at 450 nm (with a 3–7 min delay) with gas-phase concentrations of glyoxal
(Experiments 1, 3, and 4: dry AS, red
Reversible surface browning of AS aerosol under dry conditions was also recently observed during exposures to methylglyoxal gas (De Haan et al., 2017). The albedo values observed before and after two methylglyoxal additions are shown for comparison in Fig. 2. Although the data show a slight negative offset due to particle size effects, judging by the slope it is clear that methylglyoxal's effect on the albedo of dry AS aerosol is significantly less than glyoxal. This is the opposite of the trend in brown carbon production in bulk aqueous solutions at pH 5, where methylglyoxal is much more effective in generating light-absorbing products (Powelson et al., 2014), perhaps due to the fact that its ketone functional group is far less likely to be inactivated by hydration than the aldehyde groups on both molecules. However, in these dry aerosol experiments in which water is scarce, glyoxal's greater attraction to water (seen in its much higher Henry's law coefficient; Betterton and Hoffmann, 1988; Ip et al., 2009; Kampf et al., 2013) may allow it to interact with small amounts of adsorbed water at the AS aerosol surface far more effectively than methylglyoxal.
Gas-phase glyoxal was added to a few other types of seed particles in the
small chamber. In experiments on MeAS seeds (Experiment 5, Fig. 3, top panel),
the slow addition of 140 ppb of glyoxal caused a matching drop of
Gradual glyoxal addition experiments in a small Tedlar
chamber on
Albedo at 405 nm was calculated from PAS and CRD signals in Experiment 5 (Fig. S3), showing that albedo had dropped to 0.30 by 13:11 and remained at this
level for 45 min. These albedo values indicate that maximum light
absorbance at 405 nm was 4.7 times greater than at 450 nm and persisted
for a longer period of time after glyoxal gas concentrations declined. At
even longer wavelengths (530 nm), PAS aerosol absorbance reached only 0.9 Mm
In an experiment on dry sodium sulfate seeds at
Finally, three experiments exploring browning on wet rather than dry AS aerosol were conducted at RH ranging from 38 % to 81 %. The highest-humidity experiment (Experiment 7) is summarized in Fig. S8. In Experiments 7–9, albedo declines of 0.013 or less were observed following addition of 1.1, 1.2, and 0.12 ppm of glyoxal gas, respectively. If graphed in Fig. 2, the resulting slopes for Experiments 7–8 would be more than 1000 times flatter than the methylglyoxal data shown for comparison. While some of the glyoxal gas added may have been quickly lost to the walls of the humid chambers as an equilibrium is established (Kroll et al., 2005), especially in Experiments 7 and 9, it is clear that wet AS aerosol particles brown much less than dry AS, AS–glycine, or MeAS aerosol upon exposure to glyoxal.
Enhanced AS aerosol browning under dry conditions is surprising given that glyoxal Maillard chemistry is normally considered to be an aqueous-phase process. One clue to the nature of the dry browning process is seen in the slight depletion of water signals observed in all dry experiments probed by Q-AMS spectrometry (nos. 3, 4, 5, Figs. S2 and S9) after browning caused by glyoxal exposure (most water is removed from aerosol particles in the AMS inlet). The extra water depletion associated with glyoxal exposure of dry aerosol, which was not observed in deliquesced aerosol experiments probed by Q-AMS spectrometry (nos. 7–8), suggests that even under dry conditions, glyoxal is able to access and deplete trace amounts of aerosol-phase surface water. Any adsorbed water would be saturated with ammonium (or methylammonium) sulfate, and the presence of dissolved AS is known to greatly increase glyoxal uptake via a “salting-in” effect (Kampf et al., 2013; Waxman et al., 2015), while methylglyoxal solubility is reduced by salting out (Waxman et al., 2015). Thus, both glyoxal and AS are expected to be concentrated in any surface-adsorbed water present. In a previous study, similar reasoning was used to explain glyoxal uptake on solid seed particles at RH levels as low as 10 % (Corrigan et al., 2008). Furthermore, the scarcity of water will favor dehydration of products, helping to form light-absorbing conjugated double bonds.
Since methylglyoxal is generally less abundant in the atmosphere than glyoxal (Igawa et al., 1989; Munger et al., 1995; Matsumoto et al., 2005) and since the browning of dry AS by methylglyoxal is much less than that of glyoxal, likely due to salting effects (Kampf et al., 2013; Waxman et al., 2015), we focus on the effects of instantaneous browning of atmospheric aerosol particles due to interaction with glyoxal. We assume that all tropospheric sulfate particles contain ammonium and, as an upper limit, that solid-phase tropospheric sulfate particles would brown as much as the pure, fully dry AS particles used in this study regardless of the presence of additional aerosol species. The first assumption is generally reasonable (Jimenez et al., 2009) since acidic sulfate aerosol takes up ammonia in the atmosphere, while the second assumption will clearly result in the estimation of an upper limit since the presence of other materials at aerosol particle surfaces has been shown to limit the extent of the interactions between glyoxal and ammonium ions (Drozd and McNeill, 2014), and few locations in the troposphere are as dry as in this study. Tropospheric aerosol particles are typically semisolid- or solid-phase except low over the Amazon and Arctic (Shiraiwa et al., 2017).
Using the function for albedo of
The underlying data are publicly available at
Supporting information is available: proposed reaction scheme including
humidification; data summaries of experiments 2, 4, 7, 8, and 9;
Ångstrom coefficient plots for experiments 4 and 8; Q-AMS plots
summarizing the effects of glyoxal addition in experiments 3 and 8; and
estimated spectrum of absorbance of sulfate-scattered solar radiation due to
glyoxal uptake. The supplement related to this article is available online at:
DODH guided the project and wrote the manuscript. LNH and J-FD guided large chamber experiments. MAT guided small chamber experiments. KJ conducted small chamber experiments. HGW, RP, AdL, NGJ, MC, and EP conducted large chamber experiments. AG and AB quantified glyoxal by FTIR in the large chamber. PF provided assistance in interpreting optical measurements.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Simulation chambers as tools in atmospheric research (AMT/ACP/GMD inter-journal SI)”. It is not associated with a conference.
Lelia N. Hawkins was supported by the Barbara Stokes Dewey Foundation. The authors thank Mila Ródenas García (CEAM) for access to the Main Polwin MATLAB program and for glyoxal FTIR reference spectra. CNRS-INSU is gratefully acknowledged for supporting CESAM as an open facility through the National Instrument label.
This research has been supported by the National Science Foundation, Division of Atmospheric and Geospace Sciences (grant nos. AGS-1523178 and AGS-1826593) and the Research Corporation for Science Advancement (grant no. CCSA 22473). The CESAM chamber has received funding from the European Union's Horizon 2020 research and innovation program through the EUROCHAMP-2020 Infrastructure Activity (grant no. 730997).
This paper was edited by Christian George and reviewed by three anonymous referees.