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Title:
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JOAQUIN - Work Package 1 Action 1 and 3 - Monitoring of ultrafine particles and black carbon
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Author(s):
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Staelens, J.; Hofman, J.; Kos, G.P.A.; Weijers, E.P.; Hama, S.M.L.; Helmink, H.; Delaunay, T.; Smallbone, K.L.; Matheeussen, C.; Cordell, R.; Dijkema, M.; Monks, P.; Roekens, E.
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Published by:
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Publication date:
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ECN
Environment & Energy Engineering
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16-3-2016
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ECN report number:
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Document type:
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ECN-E--15-080
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ECN publication
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Number of pages:
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Full text:
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124
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Download PDF
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Abstract:
This report describes 2-year long measurements of ultrafine particles (UFP) and black carbon (BC) at four monitoring stations and a mobile trailer in the North-West European region. The study was carried out as part of the Joint Air Quality Initiative (Joaquin project, Work Package 1 Action 1 and 3).
Background
Epidemiological studies attribute the most important health impacts of air pollution to particulate matter (PM), although it is still unclear which specific particle properties (such as size and chemical composition) or sources are most relevant to health effects. Current air quality legislation on PM is focused on the mass concentration of airborne particles (expressed in µg/m3). However, there are indications that other metrics are also relevant to human health. There is for example considerable interest in particles with an aerodynamic diameter smaller than 0.1 µm (ultrafine particles, UFP). Ultrafine particles contribute little to the PM mass concentration in ambient air, but have high number concentrations (expressed in particles/cm3). The negative health impacts of UFP have been shown by toxicological studies, but epidemiological evidence is still scarce due to the limited number of UFP monitoring sites and long-term studies. As UFP measurements are not included in policy-oriented air quality monitoring programmes, one of the aims of the Joaquin project was to set up a long-term UFP
monitoring network in different cities in NW Europe.
Aims
The main objectives were:
- To evaluate the feasibility of long-term UFP measurements in air quality monitoring networks.
- To gain a better understanding of the spatiotemporal variation in UFP number concentration
and size distribution in urban environments.
- To assess the added value of UFP data compared to more commonly measured parameters
such as BC and nitrogen oxides (NOx).
To do so, it was important to assess the comparative usefulness and reliability of different instruments, to harmonize the instrumental operation within the network, to assure the quality and comparability of the data gathered, to investigate the temporal and intra-urban spatial variation of UFP and to investigate the relationships between UFP, traffic and other traffic-related air pollutants.
Methods
From April 2013 to March 2015, UFP were continuously measured at an urban background site in four
cities in NW-Europe (Amsterdam, Antwerp, Leicester and London). Results are available for 1-2 years, depending on the site. At all sites the total particle number concentration (TNC) was measured with a water-based condensation particle counter (TSI 3783, particles in the range of 7-1000 nm) and BC with a MAAP (Thermo 5012). Information on the particle size distribution was obtained by a scanning mobility particle sizer (SMPS, Grimm 5420/L-DMA, 10-1000 nm) in Amsterdam and Antwerp and by a differential mobility analyser with corona discharger and electrometer (TSI 3031, 20-200 nm) in Leicester and London. Instrument comparability was assessed by an initial measurement campaign at one site and follow-up comparisons at each of the four sites using a mobile trailer equipped with the same instruments as the monitoring stations. The trailer was also used for short-term campaigns (2-4 weeks) at a second urban background site in Amsterdam, Antwerp and Leicester.
Results.
Over the span of the reported period, data coverage of the UFP instruments was reasonable (81-84%
at the 30-min level) but below that of more commonly used NOx and PM monitoring equipment. The
comparability of the instruments was good for each type of instrument, but TNC was underestimated
by the size-resolved devices (SMPS, TSI 3031) compared with the particle counters.
Results showed a traffic-related diurnal variation of UFP, BC and NO2 with distinct morning and
evening peaks on weekdays, coinciding with traffic rush hours. In the weekends only an evening peak could be observed. For all monitoring sites, the highest monthly-averaged concentrations were found in the cold season (September to March), likely due to meteorological conditions. The site in Antwerp showed the highest UFP, BC and NO2 concentrations, which can be explained by its proximity (30 m) to a traffic-intensive road.
At all sites BC and NO2 were correlated with the TNC and size-specific particle number concentration (PNC), but the relationships depended on the site and season, likely reflecting differences in local site and traffic characteristics and meteorological effects. The relationships between UFP and BC/NOx confirm that vehicle engines are an important source of UFP in urban environments. Nevertheless, the relationship was weakest during summer, which may be due to non-traffic-emitted UFP, for example originating from new particle formation. For the monitoring site in Amsterdam, relations between the typical traffic-related pollutants and UFP were weaker. Therefore, road traffic may not be the dominant source of UFP at this site, for example due to the presence of a low emission zone in Amsterdam and/or other sources that contribute significantly to the measured UFP concentrations. The relative UFP size distribution was quite similar for all sites, with the highest particle numbers in the 30-50 nm size class. The 10-20 nm particle size class (only measured in Antwerp and Amsterdam) showed a higher relative contribution in Amsterdam and persisted through the day and during the weekend,
suggesting a non-traffic related UFP source for this site. The spatial variation in TNC between the sites was evaluated using coefficients of divergence (COD) and Spearman rank correlation coefficients. This suggested that TNC is not covarying well at the regional level and that much of the variation in UFP is due to local factors. An increased association (smaller COD and larger correlation) was obtained for increasing particle sizes. Therefore larger
particles tend to be more uniform, which may indicate the regional nature of these aerosols. The
spatial variation within a city was investigated by simultaneous mobile trailer measurements at a
second site in Amsterdam, Antwerp and Leicester and by two intra-urban campaigns in Antwerp. The
UFP concentrations within a city generally covaried over time, although meaningful intra-urban
differences between the sites were observed, depending on the considered particle size class. This can be explained by an overall urban contribution mostly originating from traffic emissions that follow a similar behaviour in time but differ in quantity, depending on the distance to and intensity of these emissions source. In addition, specific local sources may affect a single site. On average, the largest variation in TNC between the monitoring station and the trailer site was observed in Antwerp (38%), followed by Amsterdam (24%) and Leicester (20%). While the spatial variation in particle mass concentration is relatively low over an urban region, this is not the case for particle numbers. Polar plots of the PNC as a function of wind direction and wind speed indicated site-dependent UFP sources. For Antwerp the highest UFP concentrations were obtained for low wind speeds coming from the south, pointing at the main road near the site. For Amsterdam, a clear increase in TNC due to increases in the PNC of 10-20 and 20-30 nm particles was observed during strong SW winds. In combination with the high and continuous 10-20 nm contribution through the day and the weekends and the weaker relationships between UFP and BC/NOx, this suggests an influence of Schiphol airport on UFP measured at a distance of 8 km in Amsterdam. For the site in Leicester, the polar plots and diurnal patterns indicate that the main road is contributing significantly to the local UFP concentrations. For the site in London, the PNC seemed rather independent from the wind direction.
Conclusions
While UFP sizing instruments represent feasible additions to air quality monitoring networks, to obtain the best data coverage more maintenance and expertise may be required than for traditional monitors. Care should be taken to minimize particle losses due to air sampling. An SMPS provides the most comprehensive data coverage over the largest range of particle sizes. The TSI 3031 monitor appears to provide reliable data mostly in the mid (30-200 nm) size range. Total particle counters can offer a cheaper, simpler yet still reliable solution if particle size fractionation is not required. Size-resolved measurements, however, offer more information on the type, origin and transformation processes of atmospheric aerosols.
The obtained time series provide important insights into the spatiotemporal variation of total and sizeresolved UFP in urban environments. The degree of correlation between UFP and other traffic-related pollutants shows that traffic is a significant, but not exclusive, UFP source at all the sites investigated. Due to the short atmospheric lifetime of UFP and their strong dependence on local sources, total and size-specific PNC can vary meaningfully on short spatial and temporal scales. Therefore, UFP monitoring at a single site may not be indicative of the actual exposure in the communities surrounding the site. This pleads for thoughtful consideration when selecting urban background stations for UFP measurements in heterogeneous urban environments. To more accurately estimate human exposure and subsequent health impacts of UFP, measurements and/or modelling on finer spatial scales is valuable.
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