Abstract
Abstract. The application of direct analysis in real-time mass spectrometry (DART-MS), which is finding increasing use in atmospheric chemistry, to two different laboratory model systems for airborne particles is investigated: (1) submicron C3–C7 dicarboxylic acid (diacid) particles reacted with gas-phase trimethylamine (TMA) or butylamine (BA) and (2) secondary organic aerosol (SOA) particles from the ozonolysis of α-cedrene. The diacid particles exhibit a clear odd–even pattern in their chemical reactivity toward TMA and BA, with the odd-carbon diacid particles being substantially more reactive than even ones. The ratio of base to diacid in reacted particles, determined using known diacid–base mixtures, was compared to that measured by high-resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS), which vaporizes the whole particle. Results show that DART-MS probes ∼ 30 nm of the surface layer, consistent with other studies on different systems. For α-cedrene SOA particles, it is shown that varying the temperature of the particle stream as it enters the DART-MS ionization region can distinguish between specific components with the same molecular mass but different vapor pressures. These results demonstrate the utility of DART-MS for (1) examining reactivity of heterogeneous model systems for atmospheric particles and (2) probing components of SOA particles based on volatility.
Highlights
Organic aerosol (OA) particles are responsible for ∼ 20– 90 % of atmospheric submicron particulate matter, with a substantial fraction being secondary organic aerosol (SOA) formed via oxidation of volatile organic compounds (Zhang et al, 2007; Jimenez et al, 2009; Hallquist et al, 2009; Ng et al, 2010; Finlayson-Pitts and Pitts, 2000)
We explore the online application of Direct analysis in real-time mass spectrometry (DARTMS) to particles from the reaction of submicron dicarboxylic acid particles with gas-phase trimethylamine (TMA) or butylamine (BA) and report more detailed studies on SOA particles from ozonolysis of the sesquiterpene αcedrene (C15H24)
The results show that measurements of diacid particles and α-cedrene SOA particles with direct analysis in real-time mass spectrometry (DART-MS) do not suffer from in-source oxidation and gas-phase-clustering artifacts
Summary
Organic aerosol (OA) particles are responsible for ∼ 20– 90 % of atmospheric submicron particulate matter, with a substantial fraction being secondary organic aerosol (SOA) formed via oxidation of volatile organic compounds (Zhang et al, 2007; Jimenez et al, 2009; Hallquist et al, 2009; Ng et al, 2010; Finlayson-Pitts and Pitts, 2000). Variants of ESI, such as extractive electrospray ionization mass spectrometry (Chen et al, 2006), have recently been applied to the real-time measurement of the molecular composition of OA particles (Doezema et al, 2012; Gallimore and Kalberer, 2013; Horan et al, 2012). In this technique, analyte ionization occurs when the charged solvent spray intersects the sample stream in front of the MS inlet. Of DART-MS to probe heterogeneous atmospheric reactions and provide additional insights into the nature of organic constituents in complex SOA particles
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