Aarhus University Seal / Aarhus Universitets segl

No. 233: The Particle Project 2014-2016

Nøjgaard, J. K., Massling, A., Ellermann, T., 2017. The Particle Project 2014-2016. Aarhus University, DCE – Danish Centre for Environment and Energy, 40 pp. Scientific Report from DCE – Danish Centre for Environment and Energy No. 233. http://dce2.au.dk/pub/SR233.pdf

Summary

The Particle Project 2014-2016 covers data analysis 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 station HCAB using TEOM. Particle size distributions are furthermore analyzed at RISØ, HCØ, HCAB and the suburban site HVID. Elemental Carbon (EC) has been monitored at RISØ, HCØ and HVID and is a measure of airborne soot. 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 September 2014 to end of 2016.

PM2.5 was virtually unchanged from 2011-2013 (8.4 μg/m3) to 2014-2016 (8.2 μg/m3) at RISØ. At HCAB however, PM2.5 decreased from 12.9 μg/m3 to 11.9 μg/m3 during the same period. PM10 increased from 12.9 to 13.5 μg/m3 at HCØ and from 30.0 μg/m3 to 29.6 μg/m3 at HCAB during the same period. Since the first Particle Project in 2001 a slight decrease in ambient concentrations has been observed for PM10 (most evident at HCAB) and PM2.5 at all stations reflecting the general decreasing trend of PM2.5 in Europe and furthermore the importance of long-range transport.

Regional and long-range transported aerosols were expected to contribute to a large extent to the particle number in the sub micrometer size range, most evident at the rural site RISØ. Consequently, long-range transported aerosols also contributed to HVID, HCØ and HCAB, but the relative contribution of these particles is lower at HVID and HCØ and especially much lower at HCAB. The processed aerosol at the regional, rural and urban background sites resulted in a nearly mono modal particle number size distribution with mean diameters of about 40-60 nm at RISØ, HVID and HCØ. As a general difference the particle number size distributions at HCAB appears somewhat bimodal as an additional peak can be observed at around 20 nm from vehicle exhaust emissions. As a general finding it has been observed that the total sub micrometer number concentration has been constantly decreased throughout the whole measurement period from 2002 to 2016, most evident at HCAB but noticeable for all measurement stations. The particle number concentration has decreased from 20,320 cm-3 during 2002-2004 to 7,820 cm-3 during 2014-2016 for the smallest particle sizes (6-40 nm). At HCØ the corresponding decrease was from 4,220 cm-3 1,660 cm-3, that is to 39% and 38% of its initial values.

Measurements of EC in urban background was initiated by September 2014. EC averaged 0.22 μg/m3 at RISØ and 0.37 μg/m3 at HCØ in 2015, and 0.26 μg/m3 at RISØ, 0.33 μg/m3 at HCØ and 0.44 μg/m3 at HVID in 2016. Based on 2 years of data the urban increment was 0.11 μg/m3 equal to an ambient concentration of 0.33 μg/m3 at HCØ. The weekday average increment (0.12 μg/m3) was higher than the weekend average increment  (0.08 μg/m3). Furthermore, a repeating annual pattern with higher absolute concentrations during winter and lower concentrations during summer was observed at all three stations. However, no apparent correlations between EC increment and ambient temperature or degree days were observed for the 2-year dataset.

Several chemical species obtained from the Danish Air Quality Monitoring Programme, including EC, were tested as markers for residential wood combustion. Following subtraction of non-biomass combustion sources, including traffic as the largest source, residual EC was converted to PBBA (PM2.5) by a factor obtained from exposure studies on wood stove emissions. Traffic and other sources equaled 0.12 μg/m3 EC at RISØ using two independent methods. Following, the daily concentrations of PBBA were calculated for RISØ and HCØ and validated against an established and well documented marker species (levoglucosan) for PBBA. PBBA derived from EC and levoglucosan correlated well at the rural site (R2 = 0.67) as well as urban background (R2 = 0.68). In order to test the sensitivity of the method, back trajectory analysis were performed for days, where PBBA exceeded 3 μg/m3 in selected periods. Back trajectories with origin in Northern/Eastern Europe and the Baltic countries were overrepresented, which could lead to false high PBBA concentrations from long-range transport of EC. Furthermore, forest fires may contribute to part of the PBBA concentration in the summer months.

By use of this method daily PBBA concentrations were calculated for 2014 - 2016. Urban background PBBA averaged 1.1 μg/m3 equal to 48% of ambient EC, compared to the rural site RISØ, where PBBA averaged 0.9 μg/m3, which explained 54% of ambient EC. Implications are that the urban population could be exposed to 23% higher concentrations of primary particles from biomass burning than the rural population, excluding the populations living at hot spots close to sources of biomass burning. In this context, “primary” refers to particles directly emitted from wood stoves, unlike secondary particles formed in the atmosphere from volatile species or gasses. The latter is not evaluated in this report. During 2016, PBBA averaged 0.8 μg/m3 at RISØ, 0.9 at HCØ and 1.7 μg/m3 at HVID. Thus, the suburban population could be exposed to twice the PBBA concentration, the rural population is exposed to. The results are within the range of previous annual concentration estimates of 0.5 - 2 μg/m3 PBBA.

The daily time resolution of PBBA illustrates the large variation from season to season and day to day, but also that PBBA not only occurs during the coldest months, but also in the spring and autumn. Even during the summer PBBA prevails, however mostly in lower concentrations. By use of a second, yet more time consuming technique using Aerosol Mass Spectrometry (AMS), a time resolution as low as of 20 min was achieved online. This technique revealed transient peak concentrations of PBBA on all weekdays at all hours. This observation is of relevance for e.g. health aspects of Wood Stove emissions and is a potential topic in future research  projects.