Organic particulate matter formation at varying relative humidity using surrogate secondary and primary organic compounds with activity corrections in the condensed phase obtained using a method based on the Wilson equation E. I. Chang1 and J. F. Pankow2 1Bonneville Power Administration, P.O. Box 3621, Portland, OR 97208-3621, USA 2Department of Chemistry and Department of Civil and Environmental Engineering, Portland State University, P.O. Box 751, Portland, OR 97207-0751, USA
Abstract. Secondary organic aerosol (SOA) formation in the atmosphere is currently
often modeled using a multiple lumped "two-product"
approach neglects: 1) variation of activity coefficient (ζi)
values and mean molecular weight MW in the particulate
matter (PM) phase; 2) water uptake into the PM; and 3) the possibility of
phase separation in the PM. This study considers these effects by adopting
approach (θ is a phase index). Specific chemical structures are
assigned to 25 lumped SOA compounds and to 15 representative primary organic
aerosol (POA) compounds to allow calculation of ζi and
MW values. The SOA structure assignments are based on
chamber-derived 2p gas/particle partition coefficient values coupled with
known effects of structure on vapor pressure pL,io
(atm). To facilitate adoption of the
approach in large-scale models, this study also develops CP-Wilson.1
(Chang-Pankow-Wilson.1), a group-contribution ζi-prediction
method that is more computationally economical than the UNIFAC model of
Fredenslund et al. (1975). Group parameter values required by CP-Wilson.1
are obtained by fitting ζi values to predictions from UNIFAC.
approach is applied (using CP-Wilson.1) to several real α-pinene/O3
chamber cases for high reacted hydrocarbon levels
(ΔHC≈400 to 1000 μg m−3) when relative humidity
(RH) ≈50%. Good agreement between the chamber and predicted
results is obtained using both the
approaches, indicating relatively small water effects under these
conditions. However, for a hypothetical α-pinene/O3 case at
ΔHC=30 μg m−3 and RH=50%, the
approach predicts that water uptake will lead to an organic PM level that is
more double that predicted by the
approach. Adoption of the
approach using reasonable lumped structures for SOA and POA compounds is
recommended for ambient PM modeling.
Citation: Chang, E. I. and Pankow, J. F.: Organic particulate matter formation at varying relative humidity using surrogate secondary and primary organic compounds with activity corrections in the condensed phase obtained using a method based on the Wilson equation, Atmos. Chem. Phys., 10, 5475-5490, doi:10.5194/acp-10-5475-2010, 2010.