Temperature and Humidity Controlled Nephelometry: Improvements and Calibration

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Abstract
An aerosol sampling instrument that measures the light scattering extinction coefficient of submicrometer aerosol particles under controlled relative humidity and temperature conditions has been improved. The instrument has the capability to detect and quantify the sulfate ion mass concentration and the ammonium-to-sulfate molar ratio of aerosols by analysis of the aerosol's thermal decomposition profile, hygroscopic characteristics, and reactivity with gaseous NH3. As a result of the addition of a preheater-cooler assembly and improvements in the instrument's humidifier, heater, and cooler, sulfate ion mass concentration can be quantified at 0.5 μg/m3 with a 30-min averaging time. Also, the ammonium-to-sulfate molar ratio can be determined over a range from 0.0 to 2.0 with a precision of 0.2. The system was also calibrated to determine its response and sensitivity to aerosol particulate composition, relative humidity, thermal pretreatment, ammonia injection, and aerosol particle size.