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 ratios, significantly increasing normalizedsensitivities for all organics and removing their humiditydependence due to reactions with H(HO). At an ratio of 80 Td, the formaldehyde normalizedsensitivity was 25 Hz/ppbv per MHz HO 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 an ratio of 100 Td or lower there was a significant ionsignal at =49, likely CHOOH. Sampling drying andoperation at low ratios enables sensitive measurementsof HCHO and potentially CHOOH, both important troposphericphotoproducts.