New insights into atmospherically relevant reaction systems using direct analysis in real time-mass spectrometry (DART-MS)
Fairhurst, Michelle C.
Wingen, Lisa M.
Ezell, Michael J.
Finlayson-Pitts, Barbara J.
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 C<sub>3</sub>-C<sub>7</sub> dicarboxylic acid (diacid) particles reacted with gas phase trimethylamine (TMA) or butylamine (BA); (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 acids in reacted particles, determined using known acid-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 mainly surface layers, 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.