AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
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Semi-Continuous VOC Measurements in the SEARCH Network
KARSTEN BAUMANN, Eric Edgerton, Stephanie Shaw, John Jansen, Atmospheric Research & Analysis
Abstract Number: 706 Working Group: Effects of NOx and SO2 on BVOC Oxidation and Organic Aerosol Formation
Abstract In the fall of 2015, new auto-GC systems were implemented in four SouthEastern Aerosol Research and Characterization (SEARCH) network sites. The auto-GC consists of a thermal desorption air sampler (collection onto a sorbent at -30C, desorption at 100C/s holding at 280C for 3 minutes) coupled to a dual-column gas chromatograph (GC), allowing automated hourly collection of ambient air samples with subsequent GC analysis. Peaks were identified and quantitated using a multi-component calibration gas mix. In addition to the 54 target compounds from the Photochemical Assessment Monitoring Stations (PAMS) established by the US Environmental Protection Agency, iso-butene and 1,3-butadiene were measured routinely since January 2016 at two rural and two urban SEARCH sites. Compound specific lower detection limits (LDL) were assessed by periodic (once every 3-4 days) analysis of very small ambient sample volumes (20 ml), yielding LDL below 100 pptv for all VOCs except ethane and propane with ~200 pptv and ~150 pptv, respectively. Stability was tested at the same frequency by analysis of 200 ml volumes of the calibration gas, undergoing the same absorption/desorption process. Most compound recoveries were between 95% and 100% over a period of six months. Calibration via standard addition to the glass inlet under carefully controlled dilution, revealed a systematic reduction in recovery of some aromatics and alkenes but most significantly of isoprene. Removing ambient ozone from the sample drawn onto the cold trap by use of a quartz filter impregnated with 2 %v thiosulfate, helped improve the recoveries of these compounds, suggesting their removal from the cold adsorbent during sample collection is largely driven by reaction with ambient ozone. Diurnal behaviors of isoprene and other reactive VOC in urban and rural environments are compared for winter, spring and summer seasons, and discussed in light of their potential to forming secondary organic aerosol.