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. Atmospheric aerosols play an important role in air pollution. Aerosol particle chemical composition is highly variable depending on the season, hourof the day, day of the week, meteorology, and location of the measurement site. Long measurement periods and highly time-resolved data arerequired in order to achieve a statistically relevant amount of data for assessing those variations and evaluating pollution episodes. In this study, wepresent continuous atmospheric PM1 (particulate matter &lt; 1 µm) concentration and composition measurements at an urbanstreet canyon site located in Helsinki, Finland. The study was performed for 4.5 years (2015–2019) and involved highly time-resolvedmeasurements by taking advantage of a suite of online state-of-the-art instruments such as an aerosol chemical speciation monitor (ACSM), a multi-angleabsorption photometer (MAAP), a differential mobility particle sizer (DMPS), and an Aethalometer (AE). PM1 consisted mostly of organics, withmean mass concentrations of 2.89 µg m−3 (53 % of PM1) followed by inorganic species (1.56 µg m−3,29 %) and equivalent black carbon (eBC, 0.97 µg m−3, 18 %). A trend analysis revealed a decrease in BC from fossil fuel(BCFF), organics, and nitrate over the studied years. Clear seasonal and/or diurnal variations were found for the measured atmosphericPM1 constituents. Particle number and mass size distributions over different seasons revealed the possible influence of secondary organicaerosols (SOAs) during summer and the dominance of ultrafine traffic aerosols during winter. The seasonality of measured constituents also impacted the particle's coating and absorptive properties. The investigation of pollution episodes observed at the site showed that a large fraction ofaerosol particle mass was comprised of inorganic species during long-range transport, while during local episodes eBC and organics prevailedtogether with elevated particle number concentration. Overall, the results increased knowledge of the variability of PM1 concentration andcomposition in a Nordic traffic site and its implications on urban air quality. Considering the effects of PM mitigation policies in northernEurope in the last decades, the results obtained in this study may be considered illustrative of probable future air quality challenges incountries currently adopting similar environmental regulations.

In-depth characterization of submicron particulate matter inter-annual variations at a street canyon site in northern Europe