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Seasonal variation of ambient submicron particle size modes at a Boreal forest site
J Aerosol ScL Vol. 30, Suppl. 1, pp. S245-S246, 1999 O 1999 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0021-8502/99/$ - see front matter
Pergamon
SEASONAL VARIATION OF AMBIENT SUBMICRON PARTICLE SIZE MODES AT A BOREAL FOREST SITE Ismo K. Koponen, Jyrki M. M£kel~i, Pasi Aalto, and Markku Kulmala Department of Physics,B.O. Box 9, Siltavuorenpenger 20 D, FIN-00014 University of Helsinki, Finland
Keywords: aerosol size distribution, atmospheric aerosol, continental aerosol I n t r o d u c t i o n . Atmospheric aerosol particles influence the Earth's radiation balance both directly by scattering and absorbing solar radiation, and indirectly by acting as cloud condensation nuclei (CCN). Aerosol effects create uncertainties in the anthropogenic climate forcing. Lack of experimental data on diurnal, seasonal and annual size distibutions have made more detailed conclusions impossible. There is a need to characterize aerosol particle size distribution at different locations. Updating of the experimental data is also needed for generating more reliable input data for global models to estimate aerosol effects. Continental, lower tropospheric aerosols have been measured at Hyyti~l£ boreal forest site in Southern Finland since the beginning of 1996 (M~kel£ et al., 1997). Hyyti~il~ forest site may be considered as a remote boreal forest site in central southern Finland (61 51' N, 24 17' E). Often, the continental particle number size distribution is assumed to be three modally structured. Three size modes are observed to be present at slightly variable mean diameters at different locations. The data we present here have been measured at Hyyti£1£ site between the 1st of February 1996 and 31th of January, 1997. More than 50000 of ten minute's spectra measured by twin DMPS method have been analysed and, by using a standard lognormal fitting procedure, the three modal structure of submicron particle distribution has been verified. The occurence and evolution of different size modes are described, and the seasonal variation of the modes are discussed. Particle formation is observed to occur in the vicinity of the site mostly during spring time. Particle formation is often followed by intensive growth of the nucleation mode. This creates strong connection between the mean sizes and concentrations of nucleation and Aitken mode particles in spring. During winter time, more separate and stable size modes are obseved. Also the average concentration of nucleation mode particles (Dp < 20 nm) is usually less in winter time, although the presence of a clear ultrafine mode is frequently observed. I n s t r u m e n t a t i o n . The terrain of the boreal forest measurement site is relatively fiat and located within Scots pine forest (Pinus sylvestris) of 30 years of age. The nearest urban sources are the cities of Tampere (~ 50 km SW) and Jyv~kyl£ (~- 100 km NE). The sample flow for the aerosol number dize distribution measurement is presently taken through the wall of a cottage at the height of 2 m from the ground. Charge equilibrium in the sample flow is obtained by use of a bipolar SSKr aerosol neutralizer (TSI Model 3077). In our set-up we use two Vienna-type differential mobility analyzer (DMA) systems covering two subranges 3-25 nm and 20-500 nm (particle diameter) from where the number size distribution in the range of 3-500 nm is obtained. The actual measurement cycle for the whole size range is 10 minutes. Analysis. For data analysis conventional charging probabilities plus diffusion broadening of the DMA trensfer window are taken into account. Inversion to kernel matrix is done by using pinv(pseudoinversion) method. Several papers have discussed the applicability of lognormal
$245
S246
Abstracts of the 1999 European Aerosol Conference
Table 1: The annual values of modal mean diameter, number concentration and geometric standard deviation of three modes obtained using the fitting procedure. All Year 1996 Ultrafine Aitken Accumulation
Feb-May 1996
Dp
N~o~
ag
Dp
Ntot
a9
14.8 54.5 192
640 892 281
1.43 1.66 1.48
15.5 55.3 195
979 1348 370
1.41 1.74 1.47
distribution to describe the aerosol size distribution (e.g. Whitby, 1978). We use lognormal distibution in our investigations. Since the real evolution of aerosol distribution is not known, manual analysis of each spectrum is needed. After visual analysis of the number of modes for each spectrum, a sum of one to three lognormal distributions have been fitted to the spectrum. The log normal fitting procedure for each spectrum results in gives 9 parameters of the distribution Dp,i, Ni, ai i.e. geometric mean diameter, standard deviation and number concentration in each mode. Since, mathematically, there is no unambiguous way to describe the size distribution as a sum of three modes, the fitting procedure must always contain some elements of experience and intelligent quess. The fitting procedure contains several MATLAB functions. For a rough fitting leastsq.m (Quasi-Newton method) was used and for final fitting fmins.m (the Nelder-Mead simplex, direct search method) was utilized. R e s u l t s . The selection of more than 50000 size distributions verifies a clear three modal structure. Most often those three modes are sufficient to explain the overall distribution. However, the modes cannot always be classified using constant size ranges, since the growth processes change size of particles during the day. Using our results of diurnal variation there axe several days when aerosol growth from nucleation mode to Aitken mode or even to accumulation mode can be observed. However, there are also several days when modal parameters are pretty constant. Since particles move from smaller modes to larger ones it is also important to be careful when considering different modes. The modal parameters obtained here can be compared with three similar data-sets, one obtained during the arctic expedition "IAOE-91" (Covert et al., 1996) one at high-alpine site Jungfraujoch (Weingartner et al., 1999) and one in Melpitz near Leipzig (Birmili, 1998). Our particle concentrations are one order of magnitude higher than the concentrations obtained in the arctic boundary layer. However, the modal mean sizes are surprizingly near to the values in the arctic. Also the GSD's are quite similar. When comparing with the data from the highalpine site, our GSD's are systematically smaller than in Jungfraujoch, also the mean sizes being different.
References Birmili W. (1998) Dissertation. University of Leipzig. l13p. Covert D.S., Wiedensohler A., Aalto, P., Heinzenberg J., McMurry, P.H. and Leck, C. (1996) TELLUS 48B, 197-212. Kulmala, M., Toivonen, A., M£kel£, J.M. and Laaksonen, A. (1998) TELLUS 50B449-462. M£kel£, J.M., Aalto, P., Jokinen, V., Pohja, T., Nissinen, A., Palmroth, S., Markkanen, T., Seitsonen, K., Lihavainen, H. and Kulmala, M. (1997a) Geophys. Res. Lett. 24, 1219-1222. Weingartner, E., Nyeki, S., and Baltensperger, U. (1999) J. Geophys. Res., Accepted. Whitby, K.T., (1978) Atm. Environment, 12, 135-159. Wiedensohler A., Covert D.S., Swietlicki E., Aalto, P., Heinzenberg J. and Leck, C. (1996) TELLUS 48B, 213-222.
Pergamon
SEASONAL VARIATION OF AMBIENT SUBMICRON PARTICLE SIZE MODES AT A BOREAL FOREST SITE Ismo K. Koponen, Jyrki M. M£kel~i, Pasi Aalto, and Markku Kulmala Department of Physics,B.O. Box 9, Siltavuorenpenger 20 D, FIN-00014 University of Helsinki, Finland
Keywords: aerosol size distribution, atmospheric aerosol, continental aerosol I n t r o d u c t i o n . Atmospheric aerosol particles influence the Earth's radiation balance both directly by scattering and absorbing solar radiation, and indirectly by acting as cloud condensation nuclei (CCN). Aerosol effects create uncertainties in the anthropogenic climate forcing. Lack of experimental data on diurnal, seasonal and annual size distibutions have made more detailed conclusions impossible. There is a need to characterize aerosol particle size distribution at different locations. Updating of the experimental data is also needed for generating more reliable input data for global models to estimate aerosol effects. Continental, lower tropospheric aerosols have been measured at Hyyti~l£ boreal forest site in Southern Finland since the beginning of 1996 (M~kel£ et al., 1997). Hyyti~il~ forest site may be considered as a remote boreal forest site in central southern Finland (61 51' N, 24 17' E). Often, the continental particle number size distribution is assumed to be three modally structured. Three size modes are observed to be present at slightly variable mean diameters at different locations. The data we present here have been measured at Hyyti£1£ site between the 1st of February 1996 and 31th of January, 1997. More than 50000 of ten minute's spectra measured by twin DMPS method have been analysed and, by using a standard lognormal fitting procedure, the three modal structure of submicron particle distribution has been verified. The occurence and evolution of different size modes are described, and the seasonal variation of the modes are discussed. Particle formation is observed to occur in the vicinity of the site mostly during spring time. Particle formation is often followed by intensive growth of the nucleation mode. This creates strong connection between the mean sizes and concentrations of nucleation and Aitken mode particles in spring. During winter time, more separate and stable size modes are obseved. Also the average concentration of nucleation mode particles (Dp < 20 nm) is usually less in winter time, although the presence of a clear ultrafine mode is frequently observed. I n s t r u m e n t a t i o n . The terrain of the boreal forest measurement site is relatively fiat and located within Scots pine forest (Pinus sylvestris) of 30 years of age. The nearest urban sources are the cities of Tampere (~ 50 km SW) and Jyv~kyl£ (~- 100 km NE). The sample flow for the aerosol number dize distribution measurement is presently taken through the wall of a cottage at the height of 2 m from the ground. Charge equilibrium in the sample flow is obtained by use of a bipolar SSKr aerosol neutralizer (TSI Model 3077). In our set-up we use two Vienna-type differential mobility analyzer (DMA) systems covering two subranges 3-25 nm and 20-500 nm (particle diameter) from where the number size distribution in the range of 3-500 nm is obtained. The actual measurement cycle for the whole size range is 10 minutes. Analysis. For data analysis conventional charging probabilities plus diffusion broadening of the DMA trensfer window are taken into account. Inversion to kernel matrix is done by using pinv(pseudoinversion) method. Several papers have discussed the applicability of lognormal
$245
S246
Abstracts of the 1999 European Aerosol Conference
Table 1: The annual values of modal mean diameter, number concentration and geometric standard deviation of three modes obtained using the fitting procedure. All Year 1996 Ultrafine Aitken Accumulation
Feb-May 1996
Dp
N~o~
ag
Dp
Ntot
a9
14.8 54.5 192
640 892 281
1.43 1.66 1.48
15.5 55.3 195
979 1348 370
1.41 1.74 1.47
distribution to describe the aerosol size distribution (e.g. Whitby, 1978). We use lognormal distibution in our investigations. Since the real evolution of aerosol distribution is not known, manual analysis of each spectrum is needed. After visual analysis of the number of modes for each spectrum, a sum of one to three lognormal distributions have been fitted to the spectrum. The log normal fitting procedure for each spectrum results in gives 9 parameters of the distribution Dp,i, Ni, ai i.e. geometric mean diameter, standard deviation and number concentration in each mode. Since, mathematically, there is no unambiguous way to describe the size distribution as a sum of three modes, the fitting procedure must always contain some elements of experience and intelligent quess. The fitting procedure contains several MATLAB functions. For a rough fitting leastsq.m (Quasi-Newton method) was used and for final fitting fmins.m (the Nelder-Mead simplex, direct search method) was utilized. R e s u l t s . The selection of more than 50000 size distributions verifies a clear three modal structure. Most often those three modes are sufficient to explain the overall distribution. However, the modes cannot always be classified using constant size ranges, since the growth processes change size of particles during the day. Using our results of diurnal variation there axe several days when aerosol growth from nucleation mode to Aitken mode or even to accumulation mode can be observed. However, there are also several days when modal parameters are pretty constant. Since particles move from smaller modes to larger ones it is also important to be careful when considering different modes. The modal parameters obtained here can be compared with three similar data-sets, one obtained during the arctic expedition "IAOE-91" (Covert et al., 1996) one at high-alpine site Jungfraujoch (Weingartner et al., 1999) and one in Melpitz near Leipzig (Birmili, 1998). Our particle concentrations are one order of magnitude higher than the concentrations obtained in the arctic boundary layer. However, the modal mean sizes are surprizingly near to the values in the arctic. Also the GSD's are quite similar. When comparing with the data from the highalpine site, our GSD's are systematically smaller than in Jungfraujoch, also the mean sizes being different.
References Birmili W. (1998) Dissertation. University of Leipzig. l13p. Covert D.S., Wiedensohler A., Aalto, P., Heinzenberg J., McMurry, P.H. and Leck, C. (1996) TELLUS 48B, 197-212. Kulmala, M., Toivonen, A., M£kel£, J.M. and Laaksonen, A. (1998) TELLUS 50B449-462. M£kel£, J.M., Aalto, P., Jokinen, V., Pohja, T., Nissinen, A., Palmroth, S., Markkanen, T., Seitsonen, K., Lihavainen, H. and Kulmala, M. (1997a) Geophys. Res. Lett. 24, 1219-1222. Weingartner, E., Nyeki, S., and Baltensperger, U. (1999) J. Geophys. Res., Accepted. Whitby, K.T., (1978) Atm. Environment, 12, 135-159. Wiedensohler A., Covert D.S., Swietlicki E., Aalto, P., Heinzenberg J. and Leck, C. (1996) TELLUS 48B, 213-222.