Atmospheric new particle formation has been studied in many different environments on Earth. In rural areas, particle formation processes are frequently initiated by sunlight, which can convert gaseous compounds to compounds with lower volatility. One of the most important compounds capable of forming new particles is sulfuric acid. The traditional view of sulfuric acid formation is via photochemical oxidation of sulfur dioxide, originating from marine and road transport, industry, volcanoes, etc.
Particle formation rates in urban areas have also been found to correlate with solar irradiance levels. However, because the diurnal variation of traffic levels nearly overlaps with radiation levels, the contribution of traffic to sulfuric acid and nanoparticle formation may have been hidden in many studies related to particle formation in traffic-influenced areas. Our previous results (Rönkkö et al., 2017, PNAS) already showed that traffic is a major source of nanocluster aerosol—nanoparticles smaller than 3 nanometers—in urban areas.
In May 2017, we conducted a measurement campaign at the curbside of a street canyon near the city center of Helsinki together with Helsinki Region Environmental Services Authority and Finnish Meteorological Institute. We used state-of-the-art instruments to measure concentrations of airborne sulfuric acid and nanocluster aerosol together with environmental parameters, such as solar irradiance and wind direction, to distinguish the contributions of traffic and photochemistry to sulfuric acid and particle formation.
For the first time, our results (Olin et al., 2020, Atmos. Chem. Phys.) quantitatively connect traffic emissions to urban sulfuric acid levels. The results also imply that the directly emitted sulfuric acid converts rapidly into particle phase, but direct sulfur dioxide emissions are converted to gaseous sulfuric acid, which can remain in the urban atmosphere. Additionally, we observed that this remaining sulfuric acid is not the key compound behind nanocluster formation in the urban environment.
Miska Olin et al. (2020): Traffic-originated nanocluster emission exceeds H2SO4-driven photochemical new particle formation in an urban area. Atmospheric Chemistry and Physics 20, 1-13, https://doi.org/10.5194/acp-20-1-2020
Topi Rönkkö et al. (2017): Traffic is a major source of atmospheric nanocluster aerosol, P. Natl. Acad. Sci. USA 114, 7549–7554, https://doi.org/10.1073/pnas.1700830114