Semicontinuous Automated Measurement of Organic Carbon in Atmospheric Aerosol Samples

A fully automated measurement system for ambient aerosol organic carbon, capable of unattended operation over extended periods, is described. Particles are collected in a cyclone with water as the collection medium. The collected sample is periodically aspirated by a syringe pump into a holding loop and then delivered to a wet oxidation reactor (WOR). Acid is added, and the WOR is purged to measure dissolved CO2 or inorganic carbonates (IC) as evolved CO2. The IC background can often be small and sufficiently constant to be corrected for, without separate measurement, by a blank subtraction. The organic material is now oxidized stepwise or in one step to CO2. The one-step oxidation involves UV-persulfate treatment in the presence of ozone. This treatment converts organic carbon (OC) to CO2, but elemental carbon is not oxidized. The CO2 is continuously purged from solution and collected by two sequential miniature diffusion scrubbers (DSs), a short DS preceding a longer one. Each DS consists of a LiOH-filled porous hydrophobic membrane tube with terminal stainless steel tubes that function as conductance-sensing electrodes. As CO2 is collected by the LiOH-filled DSs, hydroxide is converted into carbonate and the resulting decrease in conductivity is monitored. The simultaneous use of the dual short and long DS units bearing different concentrations of LiOH permits both good sensitivity and a large dynamic range. The limit of detection (LOD, S/N = 3) is ∼140 ng of C. With a typical sampling period of 30 min at a sampling rate of 30 L/min, this corresponds to an LOD of 160 ng/m3. The approach also provides information on the ease of oxidation of the carbonaceous aerosol and hence the nature of the carbon contained therein. Ambient aerosol organic carbon data are presented.