Heated stainless steel tube for ozone removal in the ambient air measurements of mono- and sesquiterpenes

摘要

Heated stainless steel inlets were optimized for the ozone removal and for the measurements of mono-and sesquiterpenes in ambient air. Five different inlets were used with different flows, temperatures and ozone and biogenic volatile organic compound (BVOC) concentrations. Both ozone removal capacities and recoveries of BVOCs were determined. Ozone and BVOCs were flushed through the inlet and recoveries were measured by an ozone monitor and adsorbent tube sampling of BVOCs with subsequent analysis with thermal desorption - gas chromatograph (GC) - mass spectrometer (MS). Recovery tests of BVOCs were conducted both with zero air and with ozone rich air. Inlets were optimized especially for online-GC and adsorbent tube measurements of mono- and sesquiterpenes. The results of this study show that it was possible to remove ozone without removing most VOCs with this set-up. Setting the temperature, stainless steel grade and flow correctly for different inlet lengths was found to have a crucial role. The results show that the ozone removal capacity increases with increasing temperature and inlet length. Stainless steel grade 316 was found to be more efficient than grade 304 with respect to ozone removal. Based only on the ozone removal capacity, the longest possible stainless steel inlet with heating would be the optimum solution. However, the recoveries of studied compounds had to be considered too. Of the tested set-ups, a 3 m inlet (1/4 in. grade 304) heated to 120 ℃ with a flow of 1 or 2 1 min~(-1) was found to give the best results with respect to the ozone removal efficiency and compound recovery. This inlet was removing ozone efficiently for at least 4 months when used for ambient air sampling at a rural forested site with a flow of 1 1 min~(-1) (～ 170 m~3 of air flushed through the tube). A heated (140 ℃) 1 m inlet (1/4 in. grade 304 or Vs in. grade 316) was able to remove ozone with a constant flow of 0.8-1 1 min~(-1) for about two weeks (～18 m~3 of air) and had acceptable recoveries for all other studied compounds except for linalool. This inlet was found to be suitable also for ozone removal in adsorbent tube sampling when the flow is low (0.1 1 min~(-1)).