Abstract
Abstract. Elevated ambient concentrations of sub-3 nm particles (nanocluster aerosol, NCA) are generally related to atmospheric new particle formation events, usually linked with gaseous sulfuric acid (H2SO4) produced via photochemical oxidation of sulfur dioxide. According to our measurement results of H2SO4 and NCA concentrations, traffic density, and solar irradiance at an urban traffic site in Helsinki, Finland, the view of aerosol formation in traffic-influenced environments is updated by presenting two separate and independent pathways of traffic affecting the atmospheric NCA concentrations: by acting as a direct nanocluster source and by influencing the production of H2SO4. As traffic density in many areas is generally correlated with solar radiation, it is likely that the influence of traffic-related nanoclusters has been hidden in the diurnal variation and is thus underestimated because new particle formation events also follow the diurnal cycle of sunlight. Urban aerosol formation studies should, therefore, be updated to include the proposed formation mechanisms. The formation of H2SO4 in urban environments is here separated into two routes: primary H2SO4 is formed in hot vehicle exhaust and is converted rapidly to the particle phase; secondary H2SO4 results from the combined effect of emitted gaseous precursors and available solar radiation. A rough estimation demonstrates that ∼85 % of the total NCA and ∼68 % of the total H2SO4 in urban air at noontime at the measurement site are contributed by traffic, indicating the importance of traffic emissions.
Highlights
Urban environments exhibit some of the highest aerosol particle concentrations encountered in the Earth’s atmosphere
Because current regional and global air quality models do not include particles in the sub-3 nm size range as primary emissions (Paasonen et al, 2016), the modeled nanocluster aerosol (NCA)-sized particle concentrations are mainly driven by photochemical new particle formation (NPF) processes, neglecting their origin from traffic as primary sources
A rough calculation gives that, on an annual basis, up to % of NCA can originate from traffic in Helsinki, and, according to the measured NCA concentrations, at typical weekday noontime, ∼ % of the total NCA concentration was contributed by traffic at our studied www.atmos-chem-phys.net/20/1/2020/
Summary
Urban environments exhibit some of the highest aerosol particle concentrations encountered in the Earth’s atmosphere. Recent studies on urban aerosol particles have focused attention on the formation process of sub-3 nm particles (Zhao et al, 2011; Kulmala et al, 2013; Kontkanen et al, 2017) called nanocluster aerosol (NCA) (Rönkkö et al, 2017). The importance of photochemical formation mechanisms, involving, e.g., sulfuric acid (H2SO4) and ammonia (Yao et al, 2018) or other photochemically produced vapors (Lehtipalo et al, 2018), has been highlighted. These studies omit the important role of direct emission of NCA-sized particles in their analysis, despite recent findings that traffic is a major source of such particles (Rönkkö et al, 2017). The proposed mechanisms assume that key precursor vapors are formed via photochemical oxidation (Paasonen et al, 2010; Lehtipalo et al, 2018)
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