Measurements of carbon monoxide and nonmethane hydrocarbons during POPCORN

During the field campaign POPCORN (Photo oxidant formation by plant emitted compounds and OH radicals in North-eastern Germany) in Pennewitt (Mecklenb urg-Vorpommern, Germany) in August 1994, carbon monoxide and nonmethane hyd rocarbons were measured over a large maize held by in-situ gas chromatograp hy. Throughout the campaign CO and NMHC showed, even for a remote rural are a, unexpectedly low mixing ratios. Except a few episodes, CO mixing ratios were around 120 ppb. Ethane was the only hydrocarbon showing mixing ratios exceeding 1 ppb. The mixing ratios of all other NMHC ranged between several hundred ppt and the lower limit of detection which was between 20 and 5 pp t depending on the compound. During three frontal passages CO and NMHC mixi ng ratios increased significantly, while between August 13 and 16, 1994, po lar air masses were encountered with CO and NMHC mixing ratios dropping to values which are typical for North Atlantic background air. During this per iod average CO mixing ratios were 85 ppb and ethane as the most abundant hy drocarbon decreased to 650 ppt. The large-scale meteorological situation is reflected in an unusual frequency distribution of CO. The distribution sho ws three maxima which can be assigned to the periods of the frontal passage s, to the observation of polar air masses and the rest of the campaign. Two -day backward trajectories were calculated in order to obtain information a bout the origin of the air masses transported to the site. The observed NMH C and CO data can be attributed to the origin of the air masses and the air mass trajectories. NMHC and CO mixing ratios were well correlated indicati ng that these compounds originated from similar mostly anthropogenic source s. An exception was isoprene which showed no correlation with CO, With valu es below 100 ppt the mixing ratio of isoprene, which is emitted by terrestr ial vegetation, was also unexpectedly low during the first half of the camp aign although the maximum temperatures were around 35 degrees C.