Formation of organosulfates from the photooxidation of isoprene and a-pinene

Abstract

Organosulfates of a no. of biogenic volatile org. compds. (BVOCs) have been obsd. both in lab.-generated and ambient secondary org. aerosol (SOA). While organosulfates are potentially an important component of SOA, relatively little is known about how they form and their aerosol mass concns. Organosulfates are most prevalent in acidic aerosols contg. high concns. of sulfate. A no. of mechanisms have been proposed for the aerosol-phase formation of organosulfates, including alc. esterification, aldehyde esterification, epoxide ring-opening, and sulfate-radical initiated reactions. In order to det. which mechanism produces organosulfates from a given VOC, the gas-phase precursor must first be detd. To elucidate the gas-phase precursor of organosulfates from isoprene and a-pinene, photooxidn. expts. were performed in the Caltech dual 28-m3 environmental chamber in the presence of a no. of different aerosol seeds varying in acidity and sulfate concn. The gas-phase compn. was followed by chem. ionization mass spectrometry and GC-FID and the aerosol-phase compn. was detd. by aerosol mass spectrometry and a no. of off-line mass spectrometric techniques from chamber filter samples. For isoprene, epoxydiols were detd. to be a main gas-phase precursor of the C5 trihydroxy sulfate ester found in many aerosol samples. The aq.-phase ring-opening reactions of epoxydiols was studied by NMR from solns. of H2SO4/Na2SO4 where trihydroxy sulfate esters were found to form in yields of up to 32% in solns. contg. 1 M SO42-. Similar studies were also performed to det. the gas-phase precursor of organosulfates from the photooxidn. of α-pinene.

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