Analysis off Atmospheric Sesquiterpenes: Sampling Losses and Mitigation of Ozone Interferences

摘要

Atmospheric standards containing parts-per-billion levels of 14 semivolatile hydrocarbon compounds,including eight sesquiterpenes (SQTs) (longipinene,alpha-copaene,isolongi-folene,alpha-cedrene,trans-caryophyllene,aromadendrene,alpha-humulene,delta-cadinene),two oxidized sesquiterpenoids (cis-nerolidol,trans-nerolidol),one biogenic ketone (geranyl-acetone) and three aromatic compounds (1,3,5-tri-isopropylbenzene,diphenylmethane,nonylbenzene),were collected onto four solid adsorbent materials at increasing ozone mixing ratios (0-100 ppbv O_3) for analysis by thermodesorption-gas chromatography. Substantial sampling losses of up to >90% were found for the most reactive SQT,even at the lowest ozone level investigated of 20 ppbv. Loss rates from the ozone-SQT reaction were used to derive estimates of gas-phase ozone reaction rate constants for longipinene,alpha-copaene,isolongifolene,geranylacetone,aromadendrene,delta-cadinene,cis-nerolidol,and trans-nerolidol. Three different ozone mitigation techniques were investigated to prevent these sampling losses. These strategies included (a) placing glass fiberfilters impregnated with sodium thiosulfate (Na)2S)2O_3) into the sampling line,(b) titration of ozone in the sampling stream with nitric oxide (NO),and (c) catalytically removing ozone with a commercially available manganese dioxide (MnO_2) catalyst. All three techniques reduced ozone-mixing ratios from 100 ppbv to <0.6 ppbv at sampling flow rates of 1 L min~(-1). When the Na_2S_2O_3 filters and the NO-titration techniques were applied,SQT loss rates decreased from 25-60% to 0-5% for most SQT compounds and from >90% to approx 10-50% for the two most reactive compounds at ozone mixing ratios of up to 100 ppbv. The commercial manganese dioxide scrubber,however,caused complete analyte losses (>98%) even at 0 ppbv ozone. These results underline the need and present applicable techniques for removal of ozone in air samples for SQT analysis by solid adsorption techniques.