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

Secondary organic aerosol (SOA) formation in the atmosphere is currently often modeled using a multiple lumped two-product (N center dot 2p) approach. The N center dot 2p approach neglects: 1) variation of activity coefficient (zeta 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 center dot 2p)(zeta p (MW) over bar, theta) approach (theta 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 zeta i and (MW) over bar 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 p(L,i)(o) (atm). To facilitate adoption of the (N center dot 2p)(zeta p (MW) over bar,theta) approach in large-scale models, this study also develops CP-Wilson. 1 (Chang-Pankow-Wilson. 1), a group-contribution zeta(i)-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 zeta(i) values to predictions from UNIFAC. The (N center dot 2p)(zeta p (MW) over bar,theta) approach is applied (using CP-Wilson. 1) to several real alpha-pinene/O-3 chamber cases for high reacted hydrocarbon levels (Delta HC approximate to 400 to 1000 mu gm(-3)) when relative humidity (RH) approximate to 50%. Good agreement between the chamber and predicted results is ob-tained using both the (N center dot 2p)(zeta p (MW) over bar,)theta and N center dot 2p approaches, indicating relatively small water effects under these conditions. However, for a hypothetical alpha-pinene/O-3 case at Delta HC= 30 mu g m(-3) and RH= 50%, the (N center dot 2p)(zeta p (MW) over bar,theta) approach predicts that water uptake will lead to an organic PM level that is more double that predicted by the N center dot 2p approach. Adoption of the (N center dot 2p)(zeta p (MW) over bar,theta) approach using reasonable lumped structures for SOA and POA compounds is recommended for ambient PM modeling.