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
Titel:
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
Auteur:
E. I. Chang J. F. Pankow
Verschenen in:
Atmospheric chemistry and physics
Paginering:
Jaargang 10 (2010) nr. 12 pagina's 5475-5490
Jaar:
2010
Inhoud:
Secondary organic aerosol (SOA) formation in the atmosphere is currently often modeled using a multiple lumped "two-product" (<I>N</I>·2p) approach. The <I>N</I>·2p approach neglects: 1) variation of activity coefficient (ζ<I>i</I>) values and mean molecular weight <span style="border-top: 1px solid #000; color: #000;">MW</span> 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 (<I>N</I>·2p)<sup>ζp<span style="border-top: 1px solid #000; color: #000;">MW</span>,ζ 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 ζ<I>i</I> and <span style="border-top: 1px solid #000; color: #000;">MW</span> 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 <I>p</I>L,<I>i</I><sup>o (atm). To facilitate adoption of the (<I>N</I>·2p)<sup>ζ<I>p</I><span style="border-top: 1px solid #000; color: #000;">MW</span>,θ approach in large-scale models, this study also develops CP-Wilson.1 (Chang-Pankow-Wilson.1), a group-contribution ζ<I>i</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 ζ<I>i</I> values to predictions from UNIFAC. The (<I>N</I>·2p)<sup>ζ<I>p</I><span style="border-top: 1px solid #000; color: #000;">MW</span>,θ 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<sup>−3) when relative humidity (RH) ≈50%. Good agreement between the chamber and predicted results is obtained using both the (<I>N</I>·2p)<sup>ζ<I>p</I><span style="border-top: 1px solid #000; color: #000;">MW</span>,θ and <I>N</I>·2p approaches, indicating relatively small water effects under these conditions. However, for a hypothetical α-pinene/O3 case at ΔHC=30 μg m<sup>−3 and RH=50%, the (<I>N</I>·2p)<sup>ζ<I>p</I><span style="border-top: 1px solid #000; color: #000;">MW</span>,θ approach predicts that water uptake will lead to an organic PM level that is more double that predicted by the </I>N</I>·2p approach. Adoption of the (<I>N</I>·2p)<sup>ζ<I>p</I><span style="border-top: 1px solid #000; color: #000;">MW</span>,θ approach using reasonable lumped structures for SOA and POA compounds is recommended for ambient PM modeling.
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