Abstract
Particles in the upper troposphere and lower stratosphere (UT/LS) consist mostly of concentrated sulfuric acid (40–80 wt %) in water. However, airborne measurements have shown that these particles also contain a significant fraction of organic compounds of unknown chemical composition. Acid-catalyzed reactions of carbonyl species are believed to be responsible for significant transfer of gas phase organic species into tropospheric aerosols and are potentially more important at the high acidities characteristic of UT/LS particles. In this study, experiments combining sulfuric acid (H2SO4) with propanal and with mixtures of propanal with glyoxal and/or methylglyoxal at acidities typical of UT/LS aerosols produced highly colored surface films (and solutions) that may have implications for aerosol properties. In order to identify the chemical processes responsible for the formation of the surface films, attenuated total reflectance–Fourier transform infrared (ATR-FTIR) and 1H nuclear magnetic resonance (NMR) spectroscopies were used to analyze the chemical composition of the films. Films formed from propanal were a complex mixture of aldol condensation products, acetals and propanal itself. The major aldol condensation products were the dimer (2-methyl-2-pentenal) and 1,3,5-trimethylbenzene that was formed by cyclization of the linear aldol condensation trimer. Additionally, the strong visible absorption of the films indicates that higher-order aldol condensation products must also be present as minor species. The major acetal species were 2,4,6-triethyl-1,3,5-trioxane and longer-chain linear polyacetals which are likely to separate from the aqueous phase. Films formed on mixtures of propanal with glyoxal and/or methylglyoxal also showed evidence of products of cross-reactions. Since cross-reactions would be more likely than self-reactions under atmospheric conditions, similar reactions of aldehydes like propanal with common aerosol organic species like glyoxal and methylglyoxal have the potential to produce significant organic aerosol mass and therefore could potentially impact chemical, optical and/or cloud-forming properties of aerosols, especially if the products partition to the aerosol surface.
Highlights
Aerosols in the upper troposphere and lower stratosphere (UT/LS) are composed primarily of sulfuric acid (40– 80 wt %) (Clegg et al, 1998; Finlayson-Pitts and Pitts, 2000; Tabazadeh et al, 1997) and water, but they contain significant fractions of organic compounds (Froyd et al, 2009; Murphy et al, 2007, 2014, 1998)
Since the solid material in the laboratory samples rises to the surface of the solution due, at least in part, to its low density relative to sulfuric acid, it is unclear whether similar insoluble acetals potentially formed from reactions of aldehydes in liquid UT/LS aerosols would exist as solid inclusions or as surface coatings; the latter of which would be more likely to alter aerosol optical, chemical and/or cloudnucleating properties
Neither the solubility nor the reactive uptake coefficient of propanal in sulfuric acid has been measured, but, based on the low concentration of propanal vapor in the UT/LS (∼ 15 ppt at 11 km and presumably much lower in the stratosphere; Singh et al, 2004) and on the short lifetimes of gas phase aldehydes with respect to photolysis, uptake and reaction of propanal alone to form polyacetals is not expected to be a significant source of organic material in UT/LS aerosols
Summary
Aerosols in the upper troposphere and lower stratosphere (UT/LS) are composed primarily of sulfuric acid (40– 80 wt %) (Clegg et al, 1998; Finlayson-Pitts and Pitts, 2000; Tabazadeh et al, 1997) and water, but they contain significant fractions of organic compounds (Froyd et al, 2009; Murphy et al, 2007, 2014, 1998). In the case of UT aerosols, the amount of organic material can even exceed the amount of sulfate present (Murphy et al, 1998). The potential impacts of this organic material on chemical, optical and cloudforming properties of UT/LS aerosols are highly uncertain since relatively little is known about the chemical composition of the organic fraction because available sampling techniques and frequencies are limited by the high-altitude airborne missions required. Van Wyngarden et al.: Composition of colored surface films formed on propanal/H2SO4
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