Environ. Sci. Technol. 2009, 43, 7589
Comment on “Atmospheric Particulate Matter Pollution during the 2008 Beijing Olympics” Wang et al. (1) used a quartz filter-based gravimetric method to measure the particulate matter (PM) mass concentration during the 2008 summer Olympics in Beijing. They found their filter data were 1.3 times higher than the Beijing Environmental Protection Bureau’s PM10 concentrations measured by a tapered element oscillating microbalance (TEOM RP1400a). The authors attributed the observed difference to a negative artifact in the TEOM. They arrived at the conclusion that the PM10 mass concentrations grossly exceeded a number of PM air quality standards. Then, using a mixed regression model, the authors concluded that meteorological conditions accounted for more of the variation (40%) in PM concentrations than source control measures (16%) during the 2008 summer Olympics in Beijing. We have reservations about the conclusions reported by Wang et al. (1) because (1) their filter-based measurements are subjected to large artifacts, and (2) the interpretation of the modeling results may be inappropriate. (1a) In our opinion, the method (U.S. Environmental Protection Agency Method 5 40 CFR Part 60) the authors used to weigh the filters is inappropriate. Instead, Appendix J to Part 50, Reference Method for the Determination of Particulate Matter as PM10 in the Atmosphere, should have been used. The omission of the relative humidity (RH) control required in Appendix J resulted in mass concentrations with large positive artifacts. Figure SI.2 of the Supporting Information in Wang et al. (1) shows the relationship between the change in filter masses (after drying) and RH (at the time when the samples were collected). There is a 1-4% error for the RH range of 70-85%, suggesting the samples collected at this RH would experience an overcorrection of the filter mass by 1-4%. Let us suppose the PM2.5 concentration is 100 µg m-3, and the 24 h sampling volume is 1500 m3. The mass of PM2.5 collected would then be 0.15 g. According to Appendix J to Part 50, only filters with masses of 3.7-4.7 g should be used. Therefore, a 1-4% underestimation in the dried filter mass would lead to a 20-120% overestimation of the PM2.5 concentration. As reported, the mass concentration of PM2.5 accounted for 75-76% of the total mass of PM10, and all high concentration episodes occurred when the RH was 70-85%. This positive artifact is sufficiently large that the conclusions in the study about the frequency of air quality standard violations would not be valid. (1b) Turpin et al. (2) reported that the organic carbon (OC) loading on the backup quartz filter was approximately 10-55% of that of the front filter in four cities in the United States. In Beijing, OC carbon accounts for 30-40% of the total mass of PM2.5 (3). Thus, the potential positive artifact of OC, inherent to quartz filter-based sampling, could be very important and should account for part of their observed difference. (1c) In about 20% of the days (Figure SI.3 of the Supporting Information in Wang et al. (1)), filter PM10 mass concentrations were reported to be higher than the TEOM 1400a data when the RH was
