<p>The hygroneph is a humidifier – dryer nephelometer system designed to measure the scattering coefficient σsp at three different wavelengths as a function of Relative Humidity (RH). </p>
<p>The system consists of a dryer stage followed by a humidifier stage connected to a TSI 3563 nephelometer’s inlet. In the first stage, particles experience low relative humidity in the dryer. Particles are directly conducted to the nephelometer when the humidifier is disabled (dehydration mode) and σsp at low RH will be measured. The dryer is made of a bundle of Nafion® polymer tubes surrounded by an outer tube. Dry purge gas flowing over the exterior surface of the Nafion tubing continuously extracts water vapor from the gas stream inside the tubing. The driving force is the difference in water concentration on the opposite sides of the tubing wall allowing the purge gas to carry the water vapor away. By controlling the flow of the purge gas it is possible to control the efficiency of the dryer. </p>
<p>Once the particles experience the minimum possible RH and the nephelometer measures the σsp at low RH the humidifier is enabled (hydration mode). The humidifier principle of operation involves a Nafion® membrane tubing surrounded by water flowing across the membrane to humidify gas streams. Water molecules are absorbed into walls of the Nafion® tube and transferred to dry gas stream. This transfer is driven by the difference in partial pressures of water vapor on opposing sides. The system allows measurements from RH ~30% to 85%. </p>
<p>Capacitive RH sensors will be used along the path of the aerosol particles throughout the system. RH will be read after the humidifier and dryer. Calibrations are carried out with filtered particle free air as the low span gas and CO2 as the high span gas. The absolute uncertainty of the measured σsp is 7% as estimated by Anderson et al., 1996. The scattering coefficient is corrected from angular truncation errors based on Anderson and Ogren (1998). </p>
<p>The data will be reported in a .txt file containing the time, scattering coefficient σsp at 440, 550 and 650nm, Relative Humidity (RH) and the enhancement factor, defined as:
Figure 1: UMBC Hygronephelometer. The TSI 3683 is on the far end standing vertically. The two values on the top of the rail are the wet and dry channel proportioning mixers. On the near end are the two Nafion humidity exchangers (one is the dry channel and one is the wet channel).