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Comment on “Atmospheric Particulate Matter Pollution during the 2008 Beijing Olympics” Wang et al. (1) report their findings on particulate matter (PM2.5 and PM10) pollution in Beijing prior to, during, and after the 2008 Olympics. The main findings include that (1) their observed PM10 concentrations were highly correlated with but 1.3 times higher than the PM10 concentrations measured at nearby sites by Beijing EnvironmentalProtectionBureau(BeijingEPB),(2)meteorological parameters (mainly air masses from the south and precipitation) accounted for 40% of the total variation in PM10 concentration, whereas the source control measures (to improve the air quality duringtheOlympics)onlyaccountedfor16% ofthetotalvariation, (3)PM10 concentrationsinBeijingduringtheOlympicperiodwere 2.9, 3.5, and 1.9 times higher than those in Atlanta, Sydney, and Athens,respectively,and(4)PM2.5 andPM10 concentrationsduring the Olympic period exceeded the World Health Organization (WHO) 24 h guidelines 100% and 81% of the time, respectively. We have serious concerns on the appropriateness of the PM sampling equipment and modeling method used by the authors to support their findings. Our main concerns are summarized below. (1) To measure mass concentrations of PM2.5 and PM10, the authors used a three-stage high volume cascade impactor (series 230, Tisch Environmental, Cleves, OH) and quartz fiber filters. It is well-known that it is not appropriate to use oven-baked quartz fiber filters to gravimetrically determine mass concentrations of PM2.5 and PM10 (2). The sampling system used by the authors is typically used to collect particulate matter for subsequent analyses of organic constituents rather than for gravimetric determination of PM mass concentrations. This particular sampler is not on the U.S.EnvironmentalProtectionAgency’srecentlyissued(December 28, 2008) List of Designed References and Equivalent Methods (www.epa.gov/amtic/criteria.html). The authors did not describe in detail how the high volume sampler was calibrated and how the QA/QC was performed. The authors should perform a thoroughtesttojustifytheuseofthehighvolumecascadeimpactor for determining mass concentrations of PM2.5 and PM10. (2) The authors corrected filter mass for humidity using linear regression between the change in filter mass after drying and relative humidity. However, the data set the authors used does not show good linear relationships. The correlation coefficients of the linear relationships between the change in filter mass after drying and relative humidity for >PM10, PM2.5-10, and PM2.5 are low (R 2 ) 0.40-0.73) (see Figure SI 2 of the Supporting Information in the paper by Wang et al. (1)). Therefore, this correction could lead to large errors in their reported mass concentrations of PM2.5 and PM10. (3) Perhaps to confirm that their observation is consistent with that of Beijing EPB, the authors state high correlations between PM10 concentrations measured by the authors and Beijing EPB (R 2 ) 0.92, slope ) 1.37, intercept ) 7.92 µg/m3) (Figure 3 in Wang et al. (1)). However, we found that this high correlation is questionable. Comparing PM10 concentrations reported by the authors in Figure 1 (1) and average PM10 concentrations for Beijing reported by Beijing EPB, we could only obtain a much weaker correlation (R 2 ) 0.399, slope ) 0.86, intercept ) 43.2 µg/ m3). We hope the authors could provide the data set to help clarify the discrepancy between the authors’ results and ours. This is critical for demonstrating the reliability of the PM measurement method used by the authors, given the concerns described above in point 1. (4) The PM monitoring site used by the authors was close to amainroadwaywithhightrafficdensity.Inactuality,itisequivalent 7588
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to a roadside monitoring station and should not be used to represent ambient levels of PM10/PM2.5 over the entire city of Beijing. The authors need to consider the potential bias of PM10/ PM2.5 mass concentration caused by local emission sources and should state what efforts they have made to reduce this bias. (5) The daily air quality index (API) reported by the Beijing EPB is calculated using the time frame starting at 12:00 a.m. the day before and ending at 12:00 a.m. of the reporting day. The authors did not report the time frame they used to determine daily (24 h average) concentrations. (6) Control measures designed and implemented to improve the air quality for the Beijing Olympics are rather complex and systematic. These measures include short-term (e.g., reduction of trafficdensity)andlong-term(e.g.,closureorrelocationofpolluting industries)controlactions.Differentcontrolmeasuresareeffective at different times, and there are no unified start and end dates for these control measures. Moreover, some of the control measures remain permanent. It is misleading to simply assign the period between July 20 and Sept 20, 2008 as the source control time and the period after Sept 20 as the nonsource control time. Therefore, theauthors’assessmentoftheeffectivenessofthecontrolmeasures based on the oversimplified source control versus noncontrol periods would generate misleading findings. For example, we found it very hard to support the authors’ statement that “meteorology accounted for more of the variation in PM concentration than source control measures”. (7) To assess the impact of meteorological conditions, the authors used a multivariable linear regression (MLR) model. This model oversimplifies complex physicochemical processes that determine the formation, transport, and transformation of air pollutants. It is well-known that meteorological conditions have a strong influence on vertical and horizontal diffusion, long-range transport, dry and wet deposition, and secondary aerosol formation. The relationship between meteorological conditions and secondary pollutants (such as certain constituents of PM2.5 and PM10) are nonlinear. Solely using an oversimplified multivariable linear regression (MLR) model would not be able to provide a creditable assessment on the impact of meteorological conditions.
Literature Cited (1) Wang, W. T.; Primbs, T.; Shu, T.; Simonich, S. L. M. Atmospheric prticulate matter pollution during the 2008 Beijing Olympics. Environ. Sci. Technol. 2009, 43 (14), 5314–5320. (2) Chow, J. C. Critical review: Measurement methods to determine compliance with ambient air quality standards for suspended particles. J. Air Waste Manage. Assoc. 1995, 45 (5), 320–382.
Xiaoyan Tang, Min Shao, and Min Hu College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China
Zifa Wang Institute of Atmospheric Physic, Chinese Academy of Science, Beijing 100029, China
Junfeng (Jim) Zhang Department of Environmental and Occupational Health, University of Medicine and Dentistry of New Jersey (UMDNJ), 683 Hoes Lane West, Piscataway, New Jersey 08854 ES902217X 10.1021/es902217x CCC: $40.75
2009 American Chemical Society
Published on Web 09/03/2009
