Halogen-induced organic aerosol (XOA): a study on ultra-fine particle formation and time-resolved chemical characterization

Abstract

The concurrent presence of high values of org. SOA precursors and reactive halogen species (RHS) at very low ozone concns. allows the formation of halogen-induced org. aerosol, so-called XOA, in maritime areas where high concns. of RHS are present, esp. at sunrise. The present study combines aerosol smog-chamber and aerosol flow-reactor expts. for the characterization of XOA. XOA formation yields from α-pinene at low and high concns. of chlorine as reactive halogen species (RHS) were detd. using a 700 L aerosol smog-chamber with a solar simulator. The chem. transformation of the org. precursor during the aerosol formation process and chem. aging was studied using an aerosol flow-reactor coupled to an FTIR spectrometer. The FTIR dataset was analyzed using 2D correlation spectroscopy. Chlorine induced homogeneous XOA formation takes place at even 2.5 ppb of mol. chlorine, which was photolysed by the solar simulator. The chem. pathway of XOA formation is characterized by the addn. of chlorine and abstraction of hydrogen atoms, causing simultaneous carbon-chlorine bond formation. During further steps of the formation process, carboxylic acids are formed, which cause a SOA-like appearance of XOA. During the ozone-free formation of secondary org. aerosol with RHS a special kind of particulate matter (XOA) is formed, which is afterwards transformed to SOA by atm. aging or degrdn. pathways.