Ellermann, T., Massling, A., Bossi, R., & Nordstrøm, C. 2022. The Particle Project 2021. Aarhus University, DCE – Danish Centre for Environment and Energy, 38 pp. Scientific Report from DCE – Danish Centre for Environment and Energy No. 500. http://dce2.au.dk/pub/SR500.pdf
The Particle Project 2021 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. In order to increase knowledge on the sources of these particulate air pollutants the 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 street (HCAB).
Particle number
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 station HVID and the urban background station HCØ and an even smaller fraction at the street 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 periods of the order of a decade. A general decreasing trend is observed for the rural background station RISØ, the urban background station HCØ and the street station 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 station 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 street station HCAB for these two size fractions in the last five years where also local contributions from traffic are expected. At the suburban station HVID only measurements covering six years are available.
Data for particle numbers in the size range 11 – 41 nm 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 station RISØ and the urban background station HCØ. A decreasing trend can be observed at the street station HCAB which levels more and more out in the last years.
The total number of particles for the size range 11 – 478/550nm shows a small decrease at the rural background station RISØ and the urban background station HCØ while the decrease is much more pronounced at the street station HCAB, especially between 2002 and the following ten years. It is still not possible to make a firm conclusion for the suburban station HVID due to the limited length of the data series.
Elemental carbon, EC
The annual mean concentration of EC in 2021 at the urban background station HCØ in Copenhagen was 0.24 µg/m3, which is about 10% lower than measured in 2020. At the rural station RISØ the change was smaller (3%), while the decrease was more pronounced at the urban street station HCAB (18%). The highest decrease is expected at the street station where the impact from road traffic is largest. The suburban station HVID had to be relocated in 2021 and therefore there were only data for about 6.5 months.
Since 2015, EC has decreased with about 35% 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 45% at the urban background station RISØ over the period 2010 to 2021. At the suburban station HVID there has been a reduction from 2016 to 2020 of about 29%. Since 2016, the decreasing trends have been quite similar at urban background, suburban and rural stations, 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 76% (1.8 µg/m3) in the annual average concentrations over the period 2010 to 2021, predominantly due to a reduction in local road traffic emissions, caused by introduction of cars with newer and cleaner engine technology and exhaust after treatment as particle filters.
Temporal variations
This annual report presents results from an analysis of the temporal variations of the particulate air pollution. The diurnal variation of particulate mass is analysed based on measurements of PM2.5 and PM10 using the TEOM method (Tapered Element Oscillating Microbalance) that measures particulate mass with hourly time resolution at the rural station RISØ (PM10) and the street station HCAB (PM2.5 and PM10). The measurements of EC are carried out with a time resolution of 24 h and these data are used to determine the monthly variations at all four stations. Particle number is measured with hourly variation and these data are used to study both the monthly and diurnal variation at all four stations.
The analysis of the monthly variations of EC show that EC has highest concentrations during winter and lowest concentrations during summer. This is because EC mainly originates from emissions from road traffic and household warming using wood burning. The smallest variation is at the street station HCAB and the highest variation is at the suburban station HVID followed by the urban background HCØ and regional background station RISØ.
In contrast to EC variation, the particle number is higher during summer than during winter at the urban background station HCØ and regional background station RISØ. At the street station HCAB, the particle number is roughly equal summer and winter. The reason for these monthly variations are the particle number originates from road traffic and wood burning and additionally from photochemical reactions in the atmosphere. These reactions require sunlight and therefore present an additional source to the particle number during summer. At the street station HCAB the particle number is strongly dominated by the road traffic emissions and the contribution from the chemical formation of particles is most likely too small to be seen in the monthly variations.
The diurnal variations of PM10 at the street station HCAB shows the expected patterns consistent with road traffic being the main source of PM10 although there also was an unexpected broad peak from late morning to middle afternoon. This peak was due to winter salting of the street during the first part of February. PM2.5 at HCAB showed a less pronounced diurnal variation in agreement with the main part of PM2.5 originating from long-range transport and only minor contributions comes from road traffic emissions. At the rural background station RISØ, there were no clear diurnal variation consistent with the main source being long-range transport.
The diurnal variation of particle number showed many interesting patterns that to a large part could be assigned to the sources. Examples on these are the strong rush hour peaks at the street station HCAB and the impact from social activities during Saturday evenings. At the urban background station HCØ there also exits a large peak during midday and afternoon during the summer half year and that peak could not be seen during the winter half year. This peak can best be explained by the atmospheric photochemical reactions that leads to particle formation during sunlight periods in the summer half year. This peak was also seen at the rural background station RISØ but not at the street station HCAB. At the suburban station HVID there were also relatively high concentrations during winter evenings that most likely are due to household warming using wood burning. Due to relocation of HVID in 2021, it was not possible to analyze the difference between summer and winter season at this station. The analysis of the diurnal variations in particle number will be refined during the next annual report where data from 2022 will be available for the entire year for all four stations.
