Cryogenic trapping of atmospheric organic acids under laboratory and field conditions

A new and easy to handle method for the sampling of atmospheric organic acids is presented. The acids are cryogenically cotrapped with atmospheric water, using a specially designed siliconised glass trap, constantly cooled to − 70°C. A constant air flow is chosen ranging between 2 and 8 min−1 Samples are analysed by ion chromatography. With this method several acids may be detected, such as formic, acetic, lactic, oxalic, propionic and pyruvic acids. Sampling efficiency was tested for formic and acetic acid for different sample volumes, sampling times and flow rates, all performed under typical ambient conditions (relative humidity 67 ± 3°; temperature 25.5 ± 0.5°C; mixing ratios: 0.87 ± 0.015 and 1.07 ± 0.023 ppbv for acetic acid and formic acid, respectively). For both acids sampling efficiency was found to be 100 (± 3)%. Errors for parallel samples were 3% maximum. ZEFLUOR filters, used to separate particles from the gas stream, showed no impact on the composition and concentration of formic, acetic and propionic acids, whereas oxalic acid showed severe losses on the filter. Field measurements confirmed the reliability of the method. Under varying field conditions the relative accuracy seemed to be lower than in the laboratory tests but still lied in a satisfying range. Typical concentrations (averages of field measurements) yield relative errors of 6.4% for formic acid, 6.1% for acetic acid and 7.1% for propionic acid.