Sample drying to improve HCHO measurements by PTR-MS instruments: laboratory and field measurements

摘要

A significant improvement in the PTR-MS instrument sensitivity toformaldehyde was obtained by drying the air sample to a dew point of−30 °C using a cold trap to condense and freeze watervapour. At warmer trap temperatures there was significant uptake offormaldehyde and other water soluble organics, suggesting thepresence of a quasi-liquid layer on the ice surface. By removingwater vapour to a low constant dew point, the PTR-MS can be operatedat low E/N ratios, significantly increasing normalizedsensitivities for all organics and removing their humiditydependence due to reactions with H+(H₂O)₂. At anE/N ratio of 80 Td, the formaldehyde normalizedsensitivity was 25 Hz/ppbv per MHz H₃O+ with an estimateddetection limit of 78 pptv. Field testing demonstrated goodagreement between HCHO measurements made at ambient humidity andcorrected for water vapour effects compared to dehumidified samplingat −30 °C. Field testing also revealed that at anE/N ratio of 100 Td or lower there was a significant ionsignal at m/z=49, likely CH₃OOH. Sampling drying andoperation at low E/N ratios enables sensitive measurementsof HCHO and potentially CH₃OOH, both important troposphericphotoproducts.