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No. 285: The Particle Project 2017-2018

Nøjgaard, J. K., Massling, A., Ellermann, T., 2018. The Particle Project 2017-2018. Aarhus University, DCE – Danish Centre for Environment and Energy, 30 pp. Scientific Report from DCE – Danish Centre for Environment and Energy No. 285. http://dce2.au.dk/pub/SR285.pdf

Summary

The Particle Project 2017 presents time series of particle mass concentrations smaller than 2.5 µm (PM2.5) and particles smaller than 10 µm (PM10) at the rural site RISØ and curbside stations in Aarhus (AARHG) and Copenhagen (HCAB) using TEOM. Particle size distributions are furthermore analyzed at the rural site (RISØ), in urban background (HCØ), street station (HCAB) and at the suburban site (HVID). Also presented are analysis of Elemental Carbon (EC) in particles at RISØ, HVID and HCØ, and at the latter monitoring station also benzene, which is a Volatile Organic Compound (VOC). As special issue, long time series of Primary Biomass Burning Aerosols (PBBA) mainly originating from residential wood combustion has been calculated for RISØ, HCØ and HVID from late 2009 to end of 2017. 


Regional and long-range transported aerosols are expected to contribute to a large extent to the particle number in the submicrometer size range. On a relative base the highest contribution of such aerosols are found at rural background sites as e.g. RISØ in Denmark. As a consequence, long-range trans-ported aerosols also contribute to other sites which are located with smaller distances to local sources. For Denmark, this means that the relative contribu-tion of such aerosols decreases when considering the suburban site at HVID, the urban background site HCØ and finally the urban curbside site HCAB. The processed aerosol at the rural, suburban and urban background sites results in a nearly mono modal particle number size distribution with mean diameters of about 40 nm and larger at RISØ, HVID and HCØ. As a general difference the particle number size distributions at HCAB appears somewhat bimodal as an additional peak was observed at around 20 nm from vehicle exhaust emissions during previous projects. This finding cannot be concluded for the year 2017 as no data is available for the smallest size range at HCAB for this specific year. As a general finding it has been observed that the total submicrometer number concentration as discussed in this report in the size range between 41 and 550 (old instruments) and 41 and 478 nm (new instruments) has been nearly constant between the years 2016 and 2017. Smaller differences are most likely due to meteorological variations. A trend of de-creasing concentrations can be found at all stations when considering time periods of the order of a decade. Obviously higher particle number concen-trations were observed at the suburban background station HVID compared to the urban background station HCØ. A reason might be the location of HVID surrounded within a residential area with potentially high wood stove emissions during wintertime and surrounded by a number of high-trafficked highways.

PM2.5 measured by TEOM at HCAB continued to decrease in 2017 (10 µg/m3) in agreement with the overall trend since 2002. PM10 as measured by TEOM, however, increased at RISØ (12 µg/m3) and HCAB (26 µg/m3) in 2017. A new time series for PM10 was initiated at AARHG in 2017 (15 µg/m3).

Measurements of EC in urban background was initiated in September 2014. In 2017, EC averaged 0.27 µg/m3 at RISØ and 0.33 µg/m3 at HCØ. Higher concentrations were measured at the suburban station HVID (0.39 µg/m3) and even higher at HCAB (1.16 µg/m3). EC concentrations are thus showing an overall decreasing trend at HCAB since 2010, and at HVID since measurements were initiated in 2015, whereas little changes are observed at rural and urban background. 

Measurements of benzene in urban background was initiated in end 2009. Benzene has decreased by 52% from 2010 (1.33 µg/m3) to 2017 (0.64 µg/m3) at HCAB. In urban background, benzene has decreased by 35% from 0.75 µg/m3 to 0.49 µg/m3 in the same period. 

Benzene and EC was utilized to calculate Primary Biomass Burning Aerosols (PBBA) from residential wood combustion (RWC) for the period 2010 – 2017. The applied technique was introduced in the previous Particle Project 2014-2016. Daily and annual concentrations of PBBA were calculated for the rural site RISØ, urban background HCØ and suburban site HVID. The longest time series from 2010-2017 are found for RISØ based on the marker EC, and urban background based on benzene. Since the availability of the marker is a prerequisite of calculating PBBA, the shortest time series is at HVID, where meas-urements of EC were not initiated until late 2015. At HCØ, both benzene (from 2010) and EC (from late 2014) measurements are available, which provided two independent measures of PBBA. Generally, PBBA concentrations decrease from 2010-2017 at all sites. The highest concentrations were calculated at the suburban station. 

Annual averages of PBBA at the rural site and urban background are gener-ally showing comparable concentrations and trends. PBBA concentrations in urban background exceeded those at the rural site during 2015 – 2017, where the same marker was measured at both sites. However, this increment has decreased from 40% in 2015 to 5% in 2017. 


Annual averages of PBBA were highest in 2010 and 2011. That is, as an annual average at these sites, ambient air contained 1.6 - 2.5 µg/m3 primary particles in the PM2.5 fraction originating from biomass burning of which residential wood combustion is believed to be the major contributor. In the winter season, these concentrations were higher and conversely lower during summer.

PBBA concentrations were also calculated season wise during winter (Decem-ber, January, and February) in two calendar years. Urban background PBBA decreased from 4.0 µg/m3 in 2010/2011 to 2.7 µg/m3 in 2016/2017. A comparable trend was calculated for the rural site, i.e. 3.9 µg/m3 in 2010/2011 decreasing to 2.5 µg/m3. At the suburban site 3.3 µg/m3 was calculated for 2016/2017. These results indicate that exposure to primary particles from residential wood combustion may be higher in the suburbs than rural and urban background. Furthermore, the exposure may be higher in urban background than at rural locations. Time series of annual average PBBA concentrations shows an initially decreasing trend, which has increased slightly in 2017. By comparison with a product of RWC modelled from emission inventories, measured concentrations decreased more than modelled ones. Both techniques showed an initial decrease in pollutants from RWC from 2010.