COMPARISON OF APS AND BETA AS CONTINUOUS MONITORS

1. COMPARISON OF APS AND BETA AS CONTINUOUS MONITORS FOR MEASURING PM10 CONCENTRATIONS IN URBAN AIR Devraj Thimmaiah, Jan Hovorka Institute for Environmental Studies, Charles University in Prague, 128 01 Prague 2, Czech Republic devraj151078@yahoo.com, hovorka@cesnet.cz 1. Objective Fig. 2: Regression between PM10 mass concentrations by BETA and APS Diurnal variations of APS values had smoothened trend patterns than BETA. The BETA reflects faster than APS (Fig 5a). Urban atmosphere collocation measurements of 30 min integrates of PM10 by APS (3321) and BETA (FH 62 I-R) samplers are evaluated. Fig. 5 (a,b,c): Diurnal variations of PM10 by APS and BETA 2. Experimental Sampling site: School-yard building located in University Botanic garden in the Prague center (1.2 million population, 496 km2 urban territory). The site is under green cover with no direct exposure from vehicular traffic and street dust. No street canyon conditions. Sampling height: The sampling heads were placed 5m above the ground level within 1.5 meter distance. Apparatus: Aerodynamic Particle Sizer (APS, 3321, TSI Inc.). The particle density used for PM10 mass calculations was 1.5 g/cm3. Beta Attenuation Monitor (FH 62 I-R, Thermo Electron Corporation) equipped with PM10 selective inlet (Digitel) and tubing heated to 400C. Sampling period: April 20-30, 2005 and May 01-22, 2005. The APS and BETA recorded 5 min and 15 min averages respectively for the whole study period. The 30 min averages were found appropriate and chosen for evaluation. 21/04/05 Concerning diurnal variations, there were PM10 morning and late evening higher values recorded by BETA than by APS (Fig. 3). The APS PM10 was twice as much higher than BETA when RH dropped to 30 % between 12-17 hours on 24 April (Fig. 5b). Fig. 3: Diurnal variations of PM10 by APS and BETA for April and May 2005 24/04/05 Fig. 1: Location of sampling site But when RH increased to more than 70% BETA recorded twice as much higher than APS as recorded early morning on 21 May (Fig. 5c) The differences between APS and BETA values are associated with relative humidity (RH). The higher the RH the larger the differences. Fig. 4. 21/05/05 Fig. 4:Diurnal variations of PM10 by APS & BETA with Relative Humidity (RH) Sampling site 3. Results & Discussion Table 1: Summary of descriptive statistics for APS and BETA PM10 in ug/m3 4. Conclusion Paired t-test applied to the whole datasets showed no statistical significant difference (P(T<=t)two-tail=9.15E-7, tCrit=1.96). Also the slope of regression line with no intercept between the BETA and APS was close to unity (1.007) and high R2 (0.66), Fig. 2 There were no statistically significant difference between the two data sets of PM10 mass concentrations recorded by the APS and BETA during our measurements. Nevertheless, diurnal variations have shown that high RH (>70%) impose higher BETA than APS values while low RH (<35%) shows opposite effect. Though the lower values recorded by BETA than by APS should be expected due to occluded water evaporation in BETA heated inlet, our measurements showed opposite effect. This can be explained by partial aerosol drying in the jet of APS which also agrees with higher APS data than BETA when RH drops down to 30%. 5. Acknowledgement This work was supported by Grant No.:- 205/03/1560 of the Grant Agency of the Czech Republic. 7th IAC, September 10-15, 2006, St. Paul, Minnesota, USA