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Evaluation of Continuous and Filter-Based Methods for Measuring PM2.5 Mass Concentration

Real-time continuous PM2.5 mass measurements were made by a continuous ambient mass monitor (CAMM), a real-time total ambient mass sampler (RAMS), and a tapered element oscillating microbalance (TEOM) in Houston during summer and in Seattle during winter. Hourly PM2.5 mass concentrations measured by the three samplers were compared with one another to evaluate the sampling performance for total ambient PM2.5 mass measurements. Reasonably good agreement was observed between pairs of the continuous mass samplers. The regression parameters were calculated between PM2.5 mass concentrations of the instruments. The mass difference of PM2.5 between the continuous adjusted RAMS and 30°C TEOM in Houston with relative humidity and temperature suggests that the loss of semivolatile ammonium nitrate contributed to the mass difference. The results also suggest that setting the TEOM at a lower temperature than the standard configuration still could lose semivolatile materials and thus would not be enough to measure total ambient PM2.5 mass. The continuous PM2.5 mass samplers showed consistently higher PM mass concentrations than the integrated PM2.5 mass sampler measurements in Houston. The majority of the mass difference between the continuous and integrated PM2.5 mass resulted from the loss of semivolatile ammonium nitrate and organic compounds from the integrated PM2.5 samples. The results in Seattle showed more comparable values between the continuous and integrated PM2.5 masses, which was probably due to decreased volatilization loss of semivolatile materials from the integrated sample filters at the colder winter temperature. Some of the 30°C TEOM samples showed lower mass concentrations than the integrated mass monitor because of loss of semivolatile materials at 30°C. There was a suggestion that some of the RAMS mass measurements underestimated the total ambient PM2.5 mass concentration in Seattle.