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Traffic Emissions of Aerosols
Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM). (Luft)
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Non-exhaust traffic emissions are unregulated, yet the mass emission of non-exhaust particles may be larger than exhaust emissions. In addition, their emission factors (EF) are more uncertain than exhaust emissions. This thesis presents aerosol flux measurements in Stockholm using the eddy covariance method. Prior to this work, no direct flux measurements using micrometeorological techniques had been performed to study non-exhaust emissions. The challenges of measuring non-exhaust emissions include low particle number concentrations and the complex meteorological influences due to the dependence on road suspension processes. A novel approach applied here is the use of simultaneous flux measurements of CO2 and particles enabling improved parameterizations and understanding of source processes. It was shown that the EF for particles in the size range 0.25 to 2.5µm diameter are only 0.1% of the total particle number EF, indicating that ultrafine particles dominate the number fluxes. Sub- and super-micron particle emissions have been quantified, characterized, and parameterized. Dependence on wind speed, road surface condition and CO2 correlation indicates that the super-micron particles are primarily emitted through wind and vehicle induced turbulence. On the other hand, good correlation between sub-micron particles and CO2 flux indicates that these particles are primary emissions from traffic. The sub-micron particles (0.25 to 0.6 µm Dp) consisted of 60% semi-volatile components and 40% of non-volatile material. For the super-micron particle emissions, clear seasonal characteristics were found with the highest average values during spring. Heavy duty vehicles were found to emit 30 times more than light duty vehicles per km. This indicates that trucks and buses are more efficient at suspending super-micron particles than light duty vehicles. These results are consistent with the hypothesis that the particles originate from abrasion of road surfaces by studded tires.

Place, publisher, year, edition, pages
Stockholm: Department of Applied Environmental Science (ITM), Stockholm University , 2011. , 96 p.
Keyword [en]
Primary Aerosol Emissions, Carbon Dioxide Emissions, Traffic Aerosol, Urban Aerosol, Traffic Activity, Emission Factors, Eddy Covariance, Aerosol Flux
National Category
Meteorology and Atmospheric Sciences
Research subject
Applied Environmental Science
Identifiers
URN: urn:nbn:se:su:diva-61673ISBN: 978-91-7447-337-7 (print)OAI: oai:DiVA.org:su-61673DiVA: diva2:436966
Public defence
2011-09-28, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: In press. Paper 3: In press. Paper 4: Submitted.Available from: 2011-09-06 Created: 2011-08-25 Last updated: 2011-09-07Bibliographically approved
List of papers
1. Seasonal and diurnal cycles of 0.25–2.5 μm aerosol fluxes over urban Stockholm, Sweden
Open this publication in new window or tab >>Seasonal and diurnal cycles of 0.25–2.5 μm aerosol fluxes over urban Stockholm, Sweden
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2011 (English)In: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 63, no 5, 935-951 p.Article in journal (Refereed) Published
Abstract [en]

Size resolved aerosol and gas fluxes were measured in Stockholm from 1 April 2008 to 15 April 2009 over both urban and green sectors. CO2 and H2O fluxes peaked in daytime for all seasons. CO2 concentrations peaked in winter. Due to vegetation influence the CO2 fluxes had different diurnal cycles and magnitude in the two sectors. In the urban sector, CO2 fluxes indicated a net source. The sector dominated by residential areas and green spaces had its highest aerosol fluxes in winter. In spring, super micrometer concentrations for both sectors were significantly higher, as were the urban sector rush hour fluxes. The submicrometer aerosol fluxes had a similar diurnal pattern with daytime maxima for all seasons. This suggests that only the super micrometer aerosol emissions are dependent on season. During spring there was a clear difference in super micrometer fluxes between wet and dry streets. Our direct flux measurements have improved the understanding of the processes behind these aerosol emissions. They support the hypothesis that the spring peak in aerosol emissions are due to road dust, produced during the winter, but not released in large quantities until the roads dry up during spring, and explain why Stockholm has problems meeting the EU directive for aerosol mass (PM10).

National Category
Natural Sciences
Research subject
Applied Environmental Science
Identifiers
urn:nbn:se:su:diva-61682 (URN)10.1111/j.1600-0889.2011.00551.x (DOI)000296030800010 ()
Available from: 2011-08-26 Created: 2011-08-26 Last updated: 2017-12-08Bibliographically approved
2. The relationship between 0.25–2.5 μm aerosol and CO2 emissions over a city
Open this publication in new window or tab >>The relationship between 0.25–2.5 μm aerosol and CO2 emissions over a city
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2011 (English)In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 11, no 10, 4851-4859 p.Article in journal (Refereed) Published
Abstract [en]

Unlike exhaust emissions, non-exhaust traffic emissions are completely unregulated and in addition, there are large uncertainties in the non-exhaust emission factors required to estimate the emissions of these aerosols. This study provides the first published results of direct measurements of size resolved emission factors for particles in the size range 0.25–2.5 μm using a new approach to derive aerosol emission factors based on carbon dioxide (CO2) emission fluxes. Aerosol fluxes were measured over one year using the eddy covariance method at the top of a 105 m high communication tower in Stockholm, Sweden. Maximum CO2 and particle fluxes were found when the wind direction coincided with the area of densest traffic within the footprint area. Negative fluxes (uptake of CO2 and deposition of particles) coincided with periods of sampling from an urban forest area. The fluxes of CO2 were used to obtain emission factors for particles by assuming that the CO2 fluxes could be directly related to the amount of fuel burnt by vehicles in the footprint area. The estimated emission factor for the fleet mix in the measurement area was, in number 1.8 × 1011 particle veh−1 km−1 (for 0.25–2.5 μm size range). Assuming spherical particles of density 1600 kg m−3 this corresponds to 27.5 mg veh−1 km−1. For particles (0.8–2.5 μm) the emission factors were 5.1 × 109 veh−1 km−1 for number and 11.5 mg veh−1 km−1 for mass. But a wind speed dependence was noted for high wind speeds. Thus, for wind speeds larger than 9 m s−1, as measured in the tower at 105 m (U105), the emission factor for particle number and mass was parameterised as: Ef (Number, 0.8–2.5 μm) = (6.1 ± 1.7)109 U105 −50 ± 188 and Ef (Mass, 0.8–2.5 μm) = (20 ± 12) U105 − 171 ±122.

National Category
Natural Sciences
Research subject
Applied Environmental Science
Identifiers
urn:nbn:se:su:diva-61684 (URN)10.5194/acp-11-4851-2011 (DOI)000291094500019 ()
Available from: 2011-08-26 Created: 2011-08-26 Last updated: 2017-12-08Bibliographically approved
3. Traffic Aerosol emission velocity derived from direct flux measurements over urban Stockholm, Sweden
Open this publication in new window or tab >>Traffic Aerosol emission velocity derived from direct flux measurements over urban Stockholm, Sweden
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2011 (English)In: Atmospheric Environment, ISSN 1352-2310, E-ISSN 1873-2844, Vol. 45, no 32, 5725-5731 p.Article in journal (Refereed) Published
Abstract [en]

Size-resolved aerosol vertical number fluxes were measured using the eddy covariance method, 105 meters above the ground over the city of Stockholm, Sweden, between 1st April 2008 and 15th April 2009. The size range of the measurements cover particles from 0.25 to 2.5 μm diameter (Dp). Emission velocities (ve) were calculated for the same size range and were found to be well correlated with friction velocity (u*) and CO2 fluxes (FCO2). These variables were used to parameterize the emission velocity aswhere ve and u* are given in [m s−1], Dp in [μm], and FCO2 in [mmol m−2s−1].

The parameterization reproduces the average diurnal cycle from the observations well for particles sizes up to 0.6 μm Dp. For larger particles the parameterization tends to over predict the emission velocity. These larger particles are not believed to be produced by combustion and therefore not well represented by FCO2, which represents the traffic source through its fossil fuel consumption and the related CO2 emissions.

Keyword
Primary aerosol emissions, Carbon dioxide emissions, Traffic aerosol, Urban aerosol, Traffic activity, Emission factors, Eddy covariance, Aerosol flux
National Category
Natural Sciences
Research subject
Applied Environmental Science
Identifiers
urn:nbn:se:su:diva-61685 (URN)10.1016/j.atmosenv.2011.07.026 (DOI)000295607300009 ()
Available from: 2011-08-26 Created: 2011-08-26 Last updated: 2017-12-08Bibliographically approved
4. Sources and composition of urban aerosol particles
Open this publication in new window or tab >>Sources and composition of urban aerosol particles
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(English)In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324Article in journal (Refereed) Submitted
National Category
Natural Sciences
Research subject
Applied Environmental Science
Identifiers
urn:nbn:se:su:diva-61688 (URN)
Available from: 2011-08-26 Created: 2011-08-26 Last updated: 2017-12-08Bibliographically approved

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