Experimental and modeling studies of secondary organic aerosol formation and some applications to the marine boundary layer

A series of controlled experiments were carried out in the Calspan Corporat ion's 600 m(3) environmental chamber to study some secondary organic aeroso l formation processes. Three precursor-ozone systems were studied: cyclopen tene-ozone, cyclohexene-ozone, and alpha -pinene-ozone. Additionally, SO2 w as added to the initial gas mixture in several instances and was likely pre sent at trace levels in the ostensibly organic-only experiments. It was fou nd that all three systems readily formed new submicron aerosols at very low reactant levels. The chemical composition of formed aerosols was consisten t with some previous studies, but the yields of organic products were found to be lower in the Calspan experiments. A three-step procedure is proposed to explain the observed particle nucleation behavior: HO. production --> H 2SO4 formation --> H2SO4-H2O (perhaps together with NH3) homogeneous nuclea tion. It is also proposed that some soluble organic products would partitio n into the newly formed H2SO4-H2O nuclei, enhance water condensation, and q uickly grow these nuclei into a larger size range. While the observations i n the two cycloolefin-ozone systems could be well explained by these propos ed mechanisms, the exact nature of the nucleation process in the alpha -pin ene-ozone system remains rather opaque and could be the result of nucleatio n involving certain organics. The results from three simple modeling studie s further support these proposals. Their applicability to the marine bounda ry layer (MBL) is also discussed in some detail. Particularly, such a parti cle nucleation and growth process could play an important role in secondary aerosol formation and, quite likely, CCN formation as well in certain MBL regions.