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Total aerosol flux measurements above a cityscape
,l A e r o s o l Sci.
Vol. 31, Suppl. 1, pp. $303 $304. 2000
Pergamon www.elsevier.com/locate/jaerosci
Poster Session I. Urban aerosols T O T A L A E R O S O L FLUX MEASUREMENTS ABOVE A CITYSCAPE J.R. DORSEY l, E. NEMITZ 2, P.I. WILLIAMS l, M.W. GALLAGHER l, K.N. BOWER l and D. FOWLER 2 1Atmospheric Physics Group, Physics Department, UMIST, PO Box 88, Manchester, M60 1QD, U K 2Centre for Ecology and Hydrology, Edinburgh Research Station, Bush Estate, Penicuick, Midlothian, EH26 0QB, UK.
Keywords: URBAN AEROSOLS, FLUX. INTRODUCTION Aerosols appear in high number concentration at all sizes in urban environments. They are thought to be linked to the high incidence of asthma in cities, and can have an effect upon rural areas within the urban airshed. For the first time, we have directly measured aerosol fluxes within a major UK city, and have tried to characterize emissions in terms of anthropogenic activity and meteorological conditions. Supporting measurements of aerosol size spectra, trace gas concentrations and urban micrometeorology were also made. This work was conducted as part of SASUA, an ongoing project aimed at quantifying the Sources and Sinks of Urban Aerosol. Total aerosol fluxes were measured using an eddy correlation system comprising a sonic anemometer and a condensation particle counter (CPC) adapted at UMIST for high frequency use. The measurement system is described and validating results presented. Size segregated fluxes were measured using standard optical particle counters for larger aerosol. Comparisons of the total (including fine mode) fluxes and size segregated accumulation mode fluxes show good agreement, and were used to infer fine mode vertical fluxes. These showed an emission of fine mode particles at most times of day, due to widespread and rapid particle production from combustion sources within the measurement footprint. METHODS Traditional aerosol eddy correlation measurements suffer from two major instrument related failings. The lower size cut off of many optical particle counters (above 100 nm) means that the majority of urban aerosols are not detected (figure 1). This, and low sample volume limitations gives rise to statistical problems in calculating fluxes by eddy covariance (EC), and means that the vertical transport of fine mode aerosol is not yet well understood. N/cc
Average Aerosol Size Spectrum (18:00 13110/99 to 00:00 14/10/99)
00000
I:oo':0 _So ,o..r
. . . .
...................................................................................................................................
.....................................\ - 1 ................. ", I
10
Dp (nm)
100
1 ,:°0°0',r ....
0.001
1000
Figure 1. A typical urban aerosol size spectrum
7 ..........7,: 0.01
. . . . . . . . . . . . . . .
0.1 R'equency (Hz)
I
Figure 2. A typical CPC power spectrum
$303
10
$304
Abstracts
of the 2000
European
Aerosol
Confcrcncc
In this work, a CPC (TSI model 3760A) was used for its higher flow rate and wider particle diameter measurement range to circumvent the problems outlined above, The detection of each individual particle was recorded using an event counter card (NI TIO series). The total counts over each report period (0.1 or -0.05 s) were integrated with data from a Gill Solent HS sonic anemometer. A similar approach has been used successfully before over a Scots Pine forest (Buzorius et. al.). Power spectral densities were calculated for the CPC time series in order to verify that the instrument response time was adequate for calculating fluxes by EC, and for quality control purposes (figure 2). RESULTS During the current work, the measurement system was deployed on top of a 35 m tower (the Nelson Monument), which itself is on a hill towards the eastern end of Edinburgh city centre. Depending on the wind direction, there were a variety of different surfaces within the measurement footprint. These included an urban park, with Leith Docks upwind, unmanaged rural grassland, and the city centre, including a busy station and bus route. Figure 3 shows particle flux data during a period where the footprint was in the city centre. P a r t i c l e Flux ( 2 6 / 1 0 / 9 9
N ~ m Z s -t
to 28110199)
N/cmZs 4
120000 100000 80000 60000 40000 20000 , , , , , 0 2~10/9906:00
, ' ' ' , ,
i , ' ~ ,,,,
26/10/99
18:00
J J J,
i , , , , , ' , , , , ,
27/10/9906:00
i,,
27/10/99
,,, 18:00
,,,
,,,
},,,,
,
28/10/9906:00
Date
Figure 3. Total aerosol number flux over a city centre Total aerosol fluxes and concentrations are related to trace gas and meteorological conditions and discussed in the context of the urban environment. It is found that the flux at the measurement site shows a linear proportionality to traffic activity in the city. Data from two different wind directions (footprint over the city centre and over a residential area) are compared, and calculations of the relative importance of different source types presented. ACKNOWLEDGEMENTS SASUA is supported by NERC under the URGENT thematic programme (Grant 022244). The help of Edinburgh City Council in providing additional data and infrastructure is gratefully acknowledged. REFERENCE Buzorius, G., Rannick, 0., M~hkel~, J. M., Vesala, T. and Kulmala, M. (1998). Vertical Aerosol Particle Fluxes Measured by Eddy Covariance Technique using Condensational Particle Counter, J. Aerosol Science 29, 1/2, 157-171.
Vol. 31, Suppl. 1, pp. $303 $304. 2000
Pergamon www.elsevier.com/locate/jaerosci
Poster Session I. Urban aerosols T O T A L A E R O S O L FLUX MEASUREMENTS ABOVE A CITYSCAPE J.R. DORSEY l, E. NEMITZ 2, P.I. WILLIAMS l, M.W. GALLAGHER l, K.N. BOWER l and D. FOWLER 2 1Atmospheric Physics Group, Physics Department, UMIST, PO Box 88, Manchester, M60 1QD, U K 2Centre for Ecology and Hydrology, Edinburgh Research Station, Bush Estate, Penicuick, Midlothian, EH26 0QB, UK.
Keywords: URBAN AEROSOLS, FLUX. INTRODUCTION Aerosols appear in high number concentration at all sizes in urban environments. They are thought to be linked to the high incidence of asthma in cities, and can have an effect upon rural areas within the urban airshed. For the first time, we have directly measured aerosol fluxes within a major UK city, and have tried to characterize emissions in terms of anthropogenic activity and meteorological conditions. Supporting measurements of aerosol size spectra, trace gas concentrations and urban micrometeorology were also made. This work was conducted as part of SASUA, an ongoing project aimed at quantifying the Sources and Sinks of Urban Aerosol. Total aerosol fluxes were measured using an eddy correlation system comprising a sonic anemometer and a condensation particle counter (CPC) adapted at UMIST for high frequency use. The measurement system is described and validating results presented. Size segregated fluxes were measured using standard optical particle counters for larger aerosol. Comparisons of the total (including fine mode) fluxes and size segregated accumulation mode fluxes show good agreement, and were used to infer fine mode vertical fluxes. These showed an emission of fine mode particles at most times of day, due to widespread and rapid particle production from combustion sources within the measurement footprint. METHODS Traditional aerosol eddy correlation measurements suffer from two major instrument related failings. The lower size cut off of many optical particle counters (above 100 nm) means that the majority of urban aerosols are not detected (figure 1). This, and low sample volume limitations gives rise to statistical problems in calculating fluxes by eddy covariance (EC), and means that the vertical transport of fine mode aerosol is not yet well understood. N/cc
Average Aerosol Size Spectrum (18:00 13110/99 to 00:00 14/10/99)
00000
I:oo':0 _So ,o..r
. . . .
...................................................................................................................................
.....................................\ - 1 ................. ", I
10
Dp (nm)
100
1 ,:°0°0',r ....
0.001
1000
Figure 1. A typical urban aerosol size spectrum
7 ..........7,: 0.01
. . . . . . . . . . . . . . .
0.1 R'equency (Hz)
I
Figure 2. A typical CPC power spectrum
$303
10
$304
Abstracts
of the 2000
European
Aerosol
Confcrcncc
In this work, a CPC (TSI model 3760A) was used for its higher flow rate and wider particle diameter measurement range to circumvent the problems outlined above, The detection of each individual particle was recorded using an event counter card (NI TIO series). The total counts over each report period (0.1 or -0.05 s) were integrated with data from a Gill Solent HS sonic anemometer. A similar approach has been used successfully before over a Scots Pine forest (Buzorius et. al.). Power spectral densities were calculated for the CPC time series in order to verify that the instrument response time was adequate for calculating fluxes by EC, and for quality control purposes (figure 2). RESULTS During the current work, the measurement system was deployed on top of a 35 m tower (the Nelson Monument), which itself is on a hill towards the eastern end of Edinburgh city centre. Depending on the wind direction, there were a variety of different surfaces within the measurement footprint. These included an urban park, with Leith Docks upwind, unmanaged rural grassland, and the city centre, including a busy station and bus route. Figure 3 shows particle flux data during a period where the footprint was in the city centre. P a r t i c l e Flux ( 2 6 / 1 0 / 9 9
N ~ m Z s -t
to 28110199)
N/cmZs 4
120000 100000 80000 60000 40000 20000 , , , , , 0 2~10/9906:00
, ' ' ' , ,
i , ' ~ ,,,,
26/10/99
18:00
J J J,
i , , , , , ' , , , , ,
27/10/9906:00
i,,
27/10/99
,,, 18:00
,,,
,,,
},,,,
,
28/10/9906:00
Date
Figure 3. Total aerosol number flux over a city centre Total aerosol fluxes and concentrations are related to trace gas and meteorological conditions and discussed in the context of the urban environment. It is found that the flux at the measurement site shows a linear proportionality to traffic activity in the city. Data from two different wind directions (footprint over the city centre and over a residential area) are compared, and calculations of the relative importance of different source types presented. ACKNOWLEDGEMENTS SASUA is supported by NERC under the URGENT thematic programme (Grant 022244). The help of Edinburgh City Council in providing additional data and infrastructure is gratefully acknowledged. REFERENCE Buzorius, G., Rannick, 0., M~hkel~, J. M., Vesala, T. and Kulmala, M. (1998). Vertical Aerosol Particle Fluxes Measured by Eddy Covariance Technique using Condensational Particle Counter, J. Aerosol Science 29, 1/2, 157-171.