Monitoring of Water Content of Ambient Aerosol During the Pittsburgh Air Quality Study

The absorption of water by atmospheric particles affects air quality, visibility, climate, particle lifetime, and interactions with semi-volatile compounds. Often, the water content of aerosols is calculated by a thermodynamic equilibrium approach. Due to a lack of knowledge about hysteresis effects, aerosol homogeneity, and chemical interactions, there is significant uncertainty in estimates of water concentrations for continental tropospheric aerosols. To address this, aerosol size distributions measured at both dried and ambient relative humidities are being monitored at a sampling site in the Northeastern United States dominated by regional transport of sulfate- and organic-containing aerosols. Four ambient and four dry size distributions are measured each hour, along with a suite of gas-phase, particle-phase, and meteorological parameters. Here we report hourly observations of aerosol water content for over 3500 different hours spanning a wide range of relative humidities, temperatures, and aerosol compositions during a period of several months. The observations are compared with thermodynamic equilibrium calculations based on the simultaneously measured chemical composition data. The effects of organic compounds are evaluated by analyzing changes in water uptake with the fraction of organic compounds. The aerosol size distributions were measured by an automated in-situ system which conditioned aerosols at ambient and dry conditions using computer controlled valves and Nafion dryers. The system included two Scanning Mobility Particle Sizers (SMPS) and an Aerodynamic Particle Sizer (APS) which measured the aerosol size distribution between 5 nm and 10 μm in diameter.