Temperature-induced volatility of molecular markers in ambient airborne particulate matter Department of Civil and Environmental Engineering, University of California, Davis, USA
04 Jan 2011
Received: 30 Jul 2010 – Published in Atmos. Chem. Phys. Discuss.: 27 Aug 2010 Abstract. Molecular markers are organic compounds used to represent known sources of
particulate matter (PM) in statistical source apportionment studies. The
utility of molecular markers depends on, among other things, their ability
to represent PM volatility under realistic atmospheric conditions. We
measured the particle-phase concentrations and temperature-induced
volatility of commonly-used molecular markers in California's heavily
polluted San Joaqin Valley. Concentrations of elemental carbon, organic
carbon, levoglucosan, and polycyclic aromatic hydrocarbons were not reduced
by mild (~10 K) heating. In contrast, both hopane/sterane and
n-alkane concentrations were reduced, especially during the summer sampling
events at the urban site. These results suggest that hopanes and steranes
have effective saturation concentrations ~1 μg m−3, and
therefore can be considered semi-volatile. The volatility of an individual
compound depends both on its inherent properties (primarily vapour pressure)
and the interactions between itself and any potential absorbing phase. The
volatility behavior of n-alkanes during the urban summer is consistent with
that predicted for absorption by suberic acid (a C8 diacid) using a
group contribution modelling method. Observations can also be matched by an
absorbent whose composition is based on recently-obtained high-resolution
aerosol mass spectrometer factors (approximately 33% "hydrocarbon-like"
and 67% oxygenated organic aerosol). The reduced evaporation of the
n-alkanes, hopanes, and steranes with mild heating during rural and/or winter
experiments could be explained by a more oxygenated absorbing phase along
with a non-absorptive partitioning mechanism, such as adsorption to soot.
This suggests that the temperature-induced volatility of large hydrocarbons
in PM is most important if a relatively non-polar absorbing organic phase
exists. While the activity coefficients of most organic aerosol compounds
may be close to unity, the assumption of ideality for large hydrocarbons
(e.g., hopanes) may result in large errors in partitioning calculations.
Revised: 16 Nov 2010 – Accepted: 07 Dec 2010 – Published: 04 Jan 2011
Citation: Ruehl, C. R., Ham, W. A., and Kleeman, M. J.: Temperature-induced volatility of molecular markers in ambient airborne particulate matter, Atmos. Chem. Phys., 11, 67-76, doi:10.5194/acp-11-67-2011, 2011.