Peroxyacetyl nitrate (PAN) measurements during the POPCORN campaign

During the POPCORN campaign between 3 and 24 August 1994 we measured peroxy acetyl nitrate (PAW) in a rural area of Mecklenburg-Vorpommern (North-Easte rn Germany) above a corn field. A total of about 5000 PAN measurements were carried out within the three weeks of the campaign. Measured PAN mixing ra tios ranged from below the detection limit of 10 ppt up to an afternoon max imum of 1 ppb. The mean value of all data was 140 ppt. The daily mean PAN m ixing ratios were typically in the range of 50 to 250 ppt, but during a cle an air episode PAN mixing ratios of well below 40 ppt were observed. The ch aracteristic relative diurnal variation of the PAN mixing ratios with a lat e night/early morning minimum and an afternoon maximum persisted during the se episodes. The daily averages of the PAN mixing ratios showed clear episo dic variations which coincided with the duration of typical synoptic episod es of two to six days duration. Based on the measurements of the various pa rameters determining the PAN formation and destruction rates, the local bud get for PAN was calculated. During daytime the calculated net photochemical formation rate of PAN was nearly always significantly higher than the obse rved change of the PAN concentration. This demonstrates that substantial am ounts of PAN (often in the range of several hundred ppt/h) were exported fr om the corn field. The resulting removal of NOx to some extent effects the budget of nitrogen oxides (NOx), but the export of odd oxygen radicals in t he form of PAN during daytime often amounted up to 30-50% of the OH-radical formation by ozone photolysis. Thus the importance of PAN as reservoir and transport medium for odd oxygen radicals can be very substantial and may h ave a significant impact on the budget and distribution of odd oxygen radic als.