A new method for measuring the dependence of particle size distributions on relative humidity, with application to the Southeastern Aerosol and Visibility Study

A relative humidity-moderated differential mobility optical particle size s pectrometer (RH-DMOPSS) was developed to measure particle size distribution s at different humidity conditions in the 0.1 - to 1.1 -mum size range. The system has a high-flow differential mobility analyzer followed by an optic al counter and condensation particle counter in parallel. An aerosol dryer or humidity conditioner can be placed in line ahead of the mobility analyze r. A second dryer ahead of the optical counter permits measurements of the decrease in particle size when particles classified at high relative humidi ty were dried. The RH-DMOPSS data set includes (1) optical counter response to monodisperse ambient aerosols, (2) accumulation mode particle size dist ributions for dry and humid conditions, (3) selected hygroscopic growth fac tors for humid diameters of 0.2 to 1.1 mum, (4) inferred volatile particle water volume, and (5) inferred refractive index for dried particles. The RH -DMOPSS was operated in Smoky Mountains National Park as part of the Southe astern Aerosol and Visibility Study and compared with other collocated size and aerosol growth measurements. For dried ambient particles, size distrib ution parameters were correlated with those from impactor measurements, but volume median diameters were 12% smaller, on average. Growth factors for p articles below 0.5-mum diameter were in the same range as those measured by a tandem differential mobility analyzer and, for all sizes, were consisten t with the measured differences between dried and humidified particle size distributions. The mean ambient particle real refractive index inferred fro m the RH-DMOPSS dry-particle calibration data was 1.48, equal to that measu red by a multiangle light-scattering instrument.