10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Contribution of Primary and Secondary Particles to Mode-Segregated Aerosol Particle Number Concentrations in Four European Cities
IOAR RIVAS, Cristina Reche, David Beddows, David Green, Leena Järvi, Christoph Hueglin, Hilkka Timonen, Gary W. Fuller, Jarkko Niemi, Markku Kulmala, Roy M. Harrison, Andrés Alastuey, Xavier Querol, Frank J. Kelly, King's College London
Abstract Number: 1477 Working Group: Source Apportionment
Abstract Atmospheric ultrafine particles may have a primary (directly-emitted) or secondary origin (formed from gas-phase precursor compounds). Primary particle number concentrations are estimated by applying an empirical scaling factor to Black Carbon (BC) concentrations (Rodríguez and Cuevas, 2007). The factor corresponds to the slope of the lower edge of the correlation between BC and particle number concentration (N) at morning traffic rush hours. As the relative contribution of primary and secondary particles may vary by region and time, we investigated these contributions during 2009-2016 at urban background stations (UB) in four European cities: Barcelona, Helsinki, London, and Zurich. These cities are characterized by different climatic and emission patterns. Moreover, a street canyon site in Helsinki was also assessed during 2015-2016. We investigated separately the contribution to total (N, measured with a Condensation Particle Counter), nucleation (Nnuc, particles <25 nm), Aitken (Nait, 25-100 nm), and accumulation (Nacc, 100-500 nm) modes (from a Differential/Scanning Mobility Particle Sizer). The size ranges for N and Nnuc varies due to different instrumentation used. Total N includes particles from 5 nm to 1 µm (N5-1000) in Barcelona, N10-1000 in Helsinki, N7-1000 in London, and N4-3000 in Zurich.
Zurich and Barcelona registered the highest N (13000 cm-3 and 12500 cm-3, respectively) among the UB stations, followed by London (10500 cm-3) and Helsinki (5200 cm-3). Concentrations at the traffic site in Helsinki were higher than any of the UB (13700 cm-3). Regarding BC concentrations, London (1.45 µg m-3), Barcelona (1.28 µg m-3), and the traffic site in Helsinki (1.25 µg m-3) registered the highest levels, being much lower in Zurich (0.87 µg m-3) and Helsinki UB (0.39 µg m-3).
Preliminary results suggest similar primary and secondary relative contributions at all studied UB stations. For all modes, the contribution of secondary particles was higher than the contribution of primary particles. On average for all cities and modes, particles from secondary origin was estimated to be 57%. The highest secondary contribution was observed in the nucleation mode at all cities, except at Zurich where it was in the Aitken mode. Generally, higher secondary contributions were observed during the spring and summer months, when higher solar radiation and temperatures took place. This is particularly important in Barcelona when secondary contributions to N are on average of 80% during midday summertime due to photochemical nucleation processes.
When comparing the urban background and traffic site in Helsinki, a slightly higher contribution of primary particles to total N were observed in the traffic site. When looking at the different modes, primary contribution at the traffic site is particularly higher at the Aitken and accumulation modes when compared to the urban background site. Traffic sites in urban areas influenced by diesel vehicles emissions have reported a mode around 30-40 nm for ambient air ultrafine particles size distributions (Dall’Osto et al., 2012), which would mainly affect the Nait.
Positive Matrix Factorization (PMF) applied to particle number size distributions is an effective method to assess the sources contributing significantly to N. The next step is to combine and compare the results from both methods allowing us for a better estimation and understanding of the contribution of primary and secondary particles to number concentrations.
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 747882.