A robust, portable H-TDMA for field use.

The complexity of a typical hygroscopicity tandem differential mobility analyser (H-TDMA) can make it an awkward instrument for fieldwork. An improved design is presented here which is both light weight and portable and permits very accurate, reliable hygroscopic growth, deliquescence and efflorescence measurements for aerosols over a continuous relative humidity range from 10 % to 98 % without reconfiguration. The method achieves this through three significant advances in H-TDMA design: Firstly, the approach regulates the humidity by rapidly alternating the air flow between a dryer and a saturator, with the time spent in each path controlled by a feedback loop connected to the humidity sensor. Secondly the temperature of the aerosol humidifier is directly coupled to that of the wet DMA by enveloping the humidifier in the wet DMA’s excess air flow. Thirdly, a novel arrangement of two NafionTM tubes in the humidifier allows for high humidities without needing to pre-wet the aerosol flow