Evolving mass spectra of the oxidized component of organic aerosol: results from aerosol mass spectrometer analyses of aged diesel emissions A. M. Sage, E. A. Weitkamp, A. L. Robinson, and N. M. Donahue Center for Atmospheric Particle Studies, Carnegie Mellon University; Pittsburgh, PA 15213, USA
Abstract. The species and chemistry responsible for secondary organic aerosol
(SOA) formation remain highly uncertain.
Laboratory studies of the oxidation of individual, high-flux SOA precursors
do not lead to particles with mass spectra (MS) matching those of ambient
aged organic material.
Additionally, the complexity of real organic particles challenges efforts
to identify their chemical origins.
We have previously hypothesized that SOA can form from the atmospheric oxidation
of a large suite of precursors with varying vapor pressures.
Here, we support this hypothesis by using an aerosol mass spectrometer to track
the chemical evolution of diesel exhaust as it is photochemically oxidized in
an environmental chamber.
With explicit knowledge of the condensed-phase MS of the primary emissions from
our engine, we are able to decompose each recorded MS into contributing primary
and secondary spectra throughout the experiment.
We find that the SOA becomes increasingly oxidized as a function of time, quickly
approaching a final MS that closely resembles that of ambient aged organic particulate matter.
This observation is consistent with our hypothesis of an evolving suite of SOA precursors.
Low vapor pressure, semi-volatile organic emissions can form condensable products
with even a single generation of oxidation, resulting in an early-arising,
relatively less-oxidized SOA.
Continued gas-phase oxidation can form highly oxidized SOA in surprisingly young
air masses via reaction mechanisms that can add multiple oxygen atoms per generation
and result in products with sustained or increased reactivity toward OH.
Citation: Sage, A. M., Weitkamp, E. A., Robinson, A. L., and Donahue, N. M.: Evolving mass spectra of the oxidized component of organic aerosol: results from aerosol mass spectrometer analyses of aged diesel emissions, Atmos. Chem. Phys., 8, 1139-1152, doi:10.5194/acp-8-1139-2008, 2008.