Abstract
Dicarboxylic acids have low volatilities and hence are present mostly in the particulate phase, including the surface of dust particles. Mineral dust, globally the most emitted aerosol, has photocatalytic properties that can initiate photo-induced heterogeneous chemistry of organic compounds, which is still poorly characterized. We investigated the photochemistry of five dicarboxylic acids (DCA) i.e., succinic (butanedioic) acid, glutaric (pentanedioic) acid, adipic (hexanedioic) acid, pimelic (heptanedioic) acid and suberic (octanedioic) acid on Arizona test dust (ATD) particles upon UV-A light irradiation (0–1.4 mW cm−2). Gas-phase products were monitored by a high-resolution proton-transfer-reaction mass spectrometer (PTR-ToF-MS), and surface sorbed products were extracted and analyzed by ultra-high-performance liquid chromatography coupled to a heated electrospray ionization high-resolution mass spectrometer (UHPLC-HESI-HRMS). Monoacids and aldehydes were the main observed and quantified gaseous products. In contrast, shorter chain DCA and highly oxygenated products were found at the surface of the dust particles. Interestingly, the photochemistry of these DCAs presented an even-odd alternation concerning their heterogeneous reactivity, with odd-numbered carbon diacids being more reactive than their even-numbered homologous ones. We present and discuss a reaction mechanism for the C4–C8 DCA heterogeneous photooxidation catalysed by TiO2/Fe2O3-rich dust particles. Our results suggest that photochemical processing on dust surfaces should be regarded as a possible efficient pathway for altering their surface properties impacting ice nucleation and cloud condensation properties.
Highlights
Dicarboxylic acids (DCA) are ubiquitous in the atmosphere and are especially abundant in aerosols (Kawamura and Bikkina, 2016)
For all dicarboxylic acids / Arizona test dust (ATD) films, illumination resulted in the immediate increase of various signals detected by the PTR-ToF-MS
We have shown that photo-induced reactions can take place on realistic dust surfaces coated with organic compounds
Summary
Dicarboxylic acids (DCA) are ubiquitous in the atmosphere and are especially abundant in aerosols (Kawamura and Bikkina, 2016). Direct emissions contribute to the diacids abundance, several studies have shown that low molecular weight (LMW) diacids are produced in the troposphere by photochemical chain reactions implying unsaturated hydrocarbons and fatty acids, as well as their oxidation products (Kawamura et al, 1996; Kawamura and Ikushima, 1993; Kawamura and Yasui, 2005) These results are supported by the observation of diurnal and seasonal variation where an increase of DCA concentration in the afternoon and summer times coincides with higher levels of atmospheric oxidants (Kawamura and Bikkina, 2016)
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