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
