Measurements on Atmospheric Aerosols and of the Aerosol Optical Depth during 2006 at Uccle, Belgium Willy Maenhaut1, Wan Wang1, Xuguang Chi1, Nico Raes1, Jan Cafmeyer1, Anne Cheymol2, Andy Delcloo2, Veerle De Bock2, and Hugo De Backer2 1 Ghent University (UGent), Dept. Anal. Chem., Inst. Nucl. Sci., Proeftuinstraat 86, BE-9000 Gent, Belgium 2 Dept. Observations, Royal Meteor. Inst. (RMI) of Belgium, Ringlaan 3, BE-1180 Uccle, Belgium 1. Introduction Fig. 1 5. Aerosol chemical mass closure During 2006, a study undertaken at Uccle (50º48’N, 4º21’E, 100 m asl), near Brussels, to examine the relationship between the vertical column-integrated Aerosol Optical Depth (AOD), the boundary layer aerosol characteristics, and the meteorological parameters, including air mass origin and mixing height At 2006 meeting in Capetown preliminary results for the first 3 months of this study presented Here, we present the full data set and examine interrelations between the various data Mass closure calculations done for the separate PM2.5 and PM10 aerosol, and also for the coarse (PM10-2.5) size fraction, and this for each sampling As gravimetric PM, data from the PM2.5(N) and PM10(N) collections used For reconstituting PM: 8 aerosol types considered [Maenhaut et al., 2008] organic matter (OM), estimated as 1.4 OC elemental carbon (EC) ammonium nitrate non-sea-salt (nss) sulphate sea salt crustal matter other elements OC and EC data obtained from the PM2.5(Q) and PM10(Q) samplers data for the other 6 aerosol types obtained from PM2.5(N) and PM10(N) samplers Average concentrations of the 8 aerosol types (components) in the PM10 aerosol for each of the four seasons shown in Fig. 3 Percentage contributions of the different aerosol types to the seasonally averaged PM10 aerosol mass shown in Fig. 4 Secondary inorganic aerosols (SIA), which is the sum of ammonium, nitrate, and non-sea-salt sulphate, was the major component in each season for both PM2.5 and PM10, in each case followed by OM In PM2.5 SIA accounted for 59, 67, 46, and 42% of the mean aerosol mass in winter, spring, summer, and fall, respectively the corresponding percentages for OM were 34, 20, 41, and 32% 2. AOD measurements and aerosol collections Study site and study period instruments and samplers set up on roof at RMI at 17 meter above ground measurements from 4 Jan. to 2 May and from 13 July to 30 Nov. 2006 AOD obtained from Brewer ozone spectrophotometer derived from direct sun observations at 5 isolated wavelengths in the UV-B (306.3, 310.1, 313.5, 316.7 and 320.1 nm) [Cheymol and De Backer, 2003] 99 days with at least 1 valid AOD measurement up to 28 valid AOD data per day for these days; typically 13 Aerosol samplers (Fig. 1) PM2.5(N): PM2.5 filter holder with 0.4 µm Nuclepore polycarbonate filter PM10(N): PM10 filter holder with 0.4 µm Nuclepore polycarbonate filter PM2.5(Q): PM2.5 filter holder with two Whatman QM-A quartz fibre filters PM10(Q): PM10 filter holder, also with two Whatman QM-A filters all filters had a diameter of 47 mm the samplers operated at a flow rate of 17 L per min the quartz filters had been pre-fired at 550C to remove organic contaminants the purpose of the second quartz fibre filter in the PM2.5(Q) and PM10(Q) filter holders was to assess artifacts [Turpin et al., 2000] Aerosol samplings all samplers operated in parallel samples collected during the daytime only only on days with no or few clouds when 50% or more valid AOD data were to be expected average collection time per sampling: 7.5 hours 109 days with aerosol samplings Table 1 Overall and seasonal medians and ranges for the daily mean AOD (@320 nm), the sample-averaged mixing height (in m), and the PM2.5(N) mass (in µg.m–3). AOD Mixing height PM2.5 mass median (range) median (range) median (range) Overall 0.27 (0.052 – 1.08) 870 (161 – 1840) 13.8 (3.3 – 70) Winter 0.29 (0.122 – 0.60) 360 (161 – 1150) 35 (21 – 70) Spring 0.33 (0.084 – 0.90) 1200 (650 – 1840) 13.9 (4.8 – 56) Summer 0.42 (0.123 – 1.08) 1150 (900 – 1770) 10.2 (4.9 – 23) Fall 0.25 (0.052 – 1.04) 690 (300 – 1160) 10.2 (3.3 – 37) Fig. 3 Table 2 Overall median concentrations and interquartile ranges in the PM2.5 and PM10 aerosol. OC and EC were obtained from PM2.5(Q) and PM10(Q); ions and elements from PM2.5(N) and PM10(N). Data for the PM are in µg.m–3; all other data are in ng.m–3. PM2.5 PM10 median (interq. range) median (interq. range) PM (N) 13.8 (8.7 – 23) 24 (17.8 – 34) PM (Q) 18.4 (11.2 – 35) 29 (18.8 – 45) OC (Q) 2900 (690 – 4300) 4000 (2500 – 6200) EC (Q) 640 (350 – 1070) 890 (570 – 1490) Ammonium 1810 (690 – 4300) 1910 (740 – 4700) Nitrate 1620 (640 – 5700) 3900 (1780 – 8000) Sulphate 3000 (1670 – 5000) 3300 (2100 – 5500) Na (IC) 163 (94 – 380) 460 (250 – 1350) Mg (IC) 20 (9.6 – 49) 87 (57 – 192) Al DL (DL – 34) 157 (70 – 230) Si 80 (48 – 141) 490 (300 – 700) P DL (DL – 11.8) 22 (13.4 – 30) Cl (IC) 128 (29 – 500) 620 (185 – 1710) K 88 (53 – 162) 185 (122 – 250) K (IC) 52 (33 – 107) 94 (63 – 137) Ca 67 (48 – 112) 470 (300 – 740) Ca (IC) 89 (58 – 134) 570 (380 – 810) Ti 2.5 (DL – 5.5) 15.6 (9.9 – 20) V 1.65 (DL – 3.1) 2.5 (1.06 – 4.1) Mn 3.9 (2.1 – 8.7) 11.1 (5.9 – 17.7) Fe 116 (69 – 200) 390 (280 – 580) Ni 1.54 (0.80 – 2.5) 2.2 (1.28 – 3.6) Cu 4.2 (2.8 – 7.1) 11.8 (8.5 – 17.0) Zn 33 (15.6 – 78) 45 (23 – 109) Pb 9.6 (5.2 – 23) 11.6 (6.7 – 26) 3. Determination of particulate mass and chemical analyses All filters weighed before and after sampling with a microbalance to obtain the particulate mass (PM) weighings done at 20C and 50% relative humidity precision of net mass determination for the filter samples is estimated at 5 µg for the Nuclepore filters 30 µg for the Whatman QM-A quartz fibre filters Quartz fibre filters analysed for organic and elemental carbon (OC and EC) by a thermal-optical transmission (TOT) technique [Birch and Cary, 1996] OC on the 2nd quartz fibre filter of PM2.5(Q) or PM10(Q) assumed to be mainly due to adsorption of VOCs (positive artifact), and subtracted from the OC on the 1st filter in order to arrive at artifact-free OC data Nuclepore filters from the PM2.5(N) and PM10(N) samplers analysed for 29 elements (from Na to Pb) by particle-induced X-ray emission (PIXE) [Maenhaut and Cafmeyer, 1998] analysed for major anions and cations by ion chromatography (IC) [Maenhaut et al., 2002] Fig. 4 4. AOD data, mixing heights, and mass concentration data The boundary layer mixing height was estimated for every 3 hours from ECMWF data and the AOD data and mixing heights were averaged over the duration of each filter sampling The overall and seasonal medians and ranges for the daily mean AOD (@320 nm), the sample-averaged mixing height, and the PM2.5(N) mass are given in Table 1 The overall median of the daily mean AOD values (@320 nm) was 0.27 (range: 0.05 – 1.08) the AOD values were clearly higher in spring and summer than in winter and fall The overall median for the sample-averaged mixing height was 870 m (range: 160 – 1800 m) it was substantially higher in spring and summer than in fall and lowest in winter The overall median PM2.5 mass level was 14 μg/m3 (range: 3 – 70 μg/m3) median much larger in winter than in the other three seasons high winter levels were due to a strong air pollution episode with easterly winds, low wind speeds, and low mixing heights at the end of January – beginning of February The overall medians (and interquartile ranges) in PM2.5 and PM10 for the PM and several aerosol species and elements given in Table 2 PM2.5/PM10 mass ratios for the PM: on average, 0.75 ± 0.08, 0.60 ± 0.14, 0.55 ± 0.15, and 0.54 ± 0.12 during winter, spring, summer, and fall, resp. Time series for the PM10 PM and selected species shown in Fig. 2 Fig. 5 6. Relationship between AOD and boundary layer PM2.5 mass The relation between the daily mean AOD values and the PM2.5 and PM10 mass data was poor The linear regressions lines (when forced through the origin) were PM2.5 = 42 AOD(@320 nm) PM10 = 64 AOD(@320 nm) the coefficients are comparable to the data given by Schaap et al. [2009], who used AOD values at 550 nm It was estimated to which extent our daily mean AOD data could be explained by the boundary layer aerosol To this end, the PM2.5 aerosol mass data were multiplied by the sample-averaged boundary layer height and a mass extinction efficiency of 5 m2/g to obtain estimated AOD values The scatter plot of these estimated AOD versus the vertical column-integrated daily mean AOD data (@320 nm) is shown in Fig. 5 boundary layer aerosols contributed, on average, for 23% to the vertical column-integrated AOD the contribution for the individual days ranged from 6% to 63% there was a slight tendency for a larger contribution of boundary layer aerosols during winter than during the other three seasons Fig. 2 References Birch, M. E., & Cary, R. A. (1996). Aerosol Sci. Technol. 25, 221-241. Cheymol, A., & De Backer, H. (2003). J. Geophys. Res. 108 (D24), 4800, doi:10.1029/2003JD003758. Maenhaut, W., & Cafmeyer, J. (1998). X-Ray Spectrom. 27, 236-246. Maenhaut, W., et al. (2002). Nucl. Instr. and Meth. B189, 233-237. Maenhaut, W., et al. (2008). X‑Ray Spectrom. 37, 193-197. Schaap, M., et al. (2009). Atmos. Chem. Phys. 9, 909-925. Turpin, B. J., et al. (2000). Atmos. Environ. 34, 2983-3013. Acknowledgements We gratefully acknowledge the financial support from the Belgian Federal Science Policy Office (contract MO/34/014) and the Special Research Fund of Ghent University. We are also indebted to Sheila Dunphy for technical assistance.