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
Atmospheric Chemistry & Physics
Organic compounds, Aerosols -- Measurement, Aerosols -- Environmental aspects, Atmospheric chemistry, Environmental chemistry
Secondary organic aerosol (SOA) formation in the atmosphere is currently often modeled using a multiple lumped "two-product" (N · 2p) approach. The N · 2p approach neglects: 1) variation of activity coefficient (i) 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 an (N ·2p)pMW, approach (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 i 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 (N · 2p)pMW, 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 i values to predictions from UNIFAC. The (N · 2p)pMW,approach is applied (using CP-Wilson.1) to several real α-pinene/O3 chamber cases for high reacted hydrocarbon levels (HC400 to 1000 g m-3) when relative humidity (RH)50%. Good agreement between the chamber and predicted results is obtained using both the (N ·2p)pMW, and N ·2p approaches, indicating relatively small water effects under these conditions. However, for a hypothetical α-pinene/O3 case at HC=30 g m-3 and RH=50%, the (N·2p)pMW, approach predicts that water uptake will lead to an organic PM level that is more double that predicted by the N · 2p approach. Adoption of the (N · 2p)pMW, approach using reasonable lumped structures for SOA and POA compounds is recommended for ambient PM modeling.
Chang, E. I., & Pankow, J. F. (2010). 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. Atmospheric Chemistry & Physics, 10(12), 5475-5490.