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No. 450: The Particle Project 2020

Ellermann, T., Massling, A., Bossi, R., & Nøjgaard, J.K., T., 2021. The Particle Project 2020. Aarhus University, DCE – Danish Centre for Environment and Energy, xx pp. Scientific Report from DCE – Danish Centre for Environment and Energy No. 450. http://dce2.au.dk/pub/SR450.pdf

Summary

The Particle Project 2020 reports time series from measurements of particle number concentrations of submicrometer particles and Elemental Carbon (EC) in fine particles with diameter smaller than 2.5 µm (PM2.5) in urban background in Copenhagen. These measurements are carried out as a supplement to the Danish air quality monitoring program under NOVANA. Trends in urban background (HCØ) are compared to rural location (RISØ), suburban location (HVID) and urban curbside (HCAB).

Regional and long-range transported aerosols and precursors hereof contribute to the particle number concentration in the submicrometer size range. The highest relative contribution from long-range transported particles is found at rural background locations while the other locations are more influenced by local sources. For these reasons, long-range transported particles make up a relative smaller fraction at the suburban site (HVID) and the urban background site (HCØ) and an even smaller fraction at the urban curbside station (HCAB), where the highest particle number concentrations are measured due to the significant contributions from local traffic.

A trend of decreasing particle number concentrations is observed at all stations when considering time periods of the order of a decade. At the suburban site (HVID) only measurements covering about half a decade are available. A general decreasing trend is also observed for the rural background (RISØ), the urban background (HCØ) and the urban curbside (HCAB) when considering the measurements of particles in the range 41 - 110 nm and 110 – 478/550 nm of the last 15 – 20 years. However, the measurements show for these fractions very stable numbers for the rural location (RISØ) and the urban background station (HCØ) in the last five years with an increased value in 2018. This might be due to an unusual dry year in 2018 where summer precipitation was about 40% lower than summer precipitation in 2017 and 2019, thus leading to reduced wet deposition. In contrast the decreasing trend is still observed at the urban curbside station (HCAB) for these two fractions in the last five years where also local contributions from traffic are expected.

Data for particle numbers in the size range 11 – 41 nm do show a gap due to instrumental problems. The general tendency shows stable particle numbers for this small size fraction in the last ten to fifteen years at the rural background (RISØ) and the urban background (HCØ).  A decreasing trend can be observed at the urban curbside (HCAB) which more and more levels out in the last years.

The total number of particles for the full size range (11 – 478/550nm) shows a small decrease at the rural background (RISØ) and the urban background (HCØ) while the decrease is much more pronounced at the curbside station (HCAB) especially between 2002 and the following ten years. It is still not possible to make a firm conclusion for the suburban site (HVID) due to the limited length of the data series.

The annual mean concentration of EC at the urban background station in Copenhagen was 0.27 µg/m3 in 2020, which is about 17% lower than measured in 2019. Similar changes were observed at the rural (16%) and suburban (15%) stations while the decrease was higher at the urban curbside station HCAB (29%).

Since 2015, EC has decreased with about 29% (0.1 µg/m3) at the background station HCØ. The main reason for the decrease in the concentrations at HCØ is the general reductions in the emissions both at national and international level.

EC has decreased by 50% (0.23 µg/m3) at RISØ over the time period 2010 to 2019. At the suburban site (HVID) there has been a reduction since 2016 of about 25% (0.1 µg/m3). Since 2016 the decreasing trends have been quite similar at urban background, suburban and rural sites indicating that the general reductions in emissions on national and international level can explain the major part of the decreasing trends.

At HCAB there has been a pronounced reduction of about 67% (1.7 µg/m3) in the annual average concentrations over the time period 2010 to 2020, predominantly due to a reduction in local road traffic emissions.

Analysis of the temporal variation of EC in 2020 showed that the Covid-19 restrictions in 2020 only had minor impact on the concentration change from 2019 to 2020 at the urban background station (HCØ). The same was concluded for the rural station (RISØ) and suburban station (HVID). At these three stations the main reasons for the change between 2019 and 2020 is the continuing decreases in the emissions nationally and internationally and the natural variations in the meteorological conditions from year to year.

At the urban curbside station (HCAB) a nearly 50% reduction in the concentrations of EC were measured for the first period with widespread Covid-19 restrictions in 2020 when compared to 2019. This large decrease shows that the Covid-19 restrictions had impact on the concentrations of EC as expected from the large reductions in traffic in connection to the widespread Covid-19 restrictions in early spring. However, it is also evident that the reductions in emissions due to the continuing improvements of the vehicle fleet and variations in meteorology are important for the annual reduction in the concentrations of EC at the urban curbside station.     

The temporal analysis of the particle number concentrations at the urban curbside station (HCAB) indicated that the impact of Covid-19 was not very large for the particle number concentration or that the complex physical and chemical processes determining the particle number concentrations counteracted the impact from the Covid-19 restrictions. Unfortunately, the data coverage for the first months in 2019 is not sufficient to carry out a similar temporal analysis for the urban background, rural and suburban stations.