Traffic and nucleation events as main sources of ultrafine particles in high-insolation developed world cities
Brines, M and Dall'Osto, M and Beddows, DCS and Harrison, RM and Gomez-Moreno, F and Nunez, L and Artinano, B and Costabile, F and Gobbi, GP and Salimi, F and Morawska, L and Sioutas, C and Querol, X, Traffic and nucleation events as main sources of ultrafine particles in high-insolation developed world cities, Atmospheric Chemistry and Physics, 15 pp. 5929-5945. ISSN 1680-7316 (2015) [Refereed Article]
Road traffic emissions are often considered the
main source of ultrafine particles (UFP, diameter smaller
than 100 nm) in urban environments. However, recent studies
worldwide have shown that – in high-insolation urban regions
at least – new particle formation events can also contribute
to UFP. In order to quantify such events we systematically
studied three cities located in predominantly sunny environments:
Barcelona (Spain), Madrid (Spain) and Brisbane
(Australia). Three long-term data sets (1–2 years) of fine
and ultrafine particle number size distributions (measured
by SMPS, Scanning Mobility Particle Sizer) were analysed.
Compared to total particle number concentrations, aerosol
size distributions offer far more information on the type, origin
and atmospheric evolution of the particles. By applying
k-means clustering analysis, we categorized the collected
aerosol size distributions into three main categories: "Traffic"
(prevailing 44–63 % of the time), "Nucleation" (14–19 %)
and "Background pollution and Specific cases" (7–22 %).
Measurements from Rome (Italy) and Los Angeles (USA)
were also included to complement the study. The daily variation
of the average UFP concentrations for a typical nucleation
day at each site revealed a similar pattern for all cities,
with three distinct particle bursts. A morning and an evening
spike reflected traffic rush hours, whereas a third one at midday
showed nucleation events. The photochemically nucleated
particles’ burst lasted 1–4 h, reaching sizes of 30–40 nm.
On average, the occurrence of particle size spectra dominated
by nucleation events was 16 % of the time, showing the importance
of this process as a source of UFP in urban environments
exposed to high solar radiation. Nucleation events
lasting for 2 h or more occurred on 55 % of the days, this extending
to > 4 h in 28 % of the days, demonstrating that atmospheric
conditions in urban environments are not favourable
to the growth of photo chemically nucleated particles. In summary, although traffic remains the main source of UFP in urban
areas, in developed countries with high insolation urban
nucleation events are also a main source of UFP. If traffic related
particle concentrations are reduced in the future, nucleation
events will likely increase in urban areas, due to the
reduced urban condensation sinks.