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A significant improvement in the PTR-MS instrument sensitivity to formaldehyde was obtained by drying the air sample to a dew point of −30 °C using a cold trap to condense and freeze water vapour. At warmer trap temperatures there was significant uptake of formaldehyde and other water soluble organics, suggesting the presence of a quasi-liquid layer on the ice surface. By removing water vapour to a low constant dew point, the PTR-MS can be operated at low <i>E/N</i> ratios, significantly increasing normalized sensitivities for all organics and removing their humidity dependence due to reactions with H<sup>+</sup>(H<sub>2</sub>O)<sub>2</sub>. At an <i>E/N</i> ratio of 80 Td, the formaldehyde normalized sensitivity was 25 Hz/ppbv per MHz H<sub>3</sub>O<sup>+</sup> with an estimated detection limit of 78 pptv. Field testing demonstrated good agreement between HCHO measurements made at ambient humidity and corrected for water vapour effects compared to dehumidified sampling at −30 °C. Field testing also revealed that at an <i>E/N</i> ratio of 100 Td or lower there was a significant ion signal at <i>m/z</i>=49, likely CH<sub>3</sub>OOH. Sampling drying and operation at low <i>E/N</i> ratios enables sensitive measurements of HCHO and potentially CH<sub>3</sub>OOH, both important tropospheric photoproducts.