Sulfuric Acid and Nanocluster Aerosol Measured in an Urban Street Canyon of Helsinki, Finland

New particle formation (NPF) in urban air is important for human health and various atmospheric processes. Gaseous sulfuric acid (GSA) is often shown to be associated with NPF. However, its connection to 1‐3 nm sized particles, called nanocluster aerosol (NCA), has not been widely studied in traffic‐related environment. NCA concentration is an important quantity in terms of NPF because it is connected to the initial steps of the particle growth process. However, according to recent findings, NCA can be a direct result of road traffic emissions in urban areas rather than being connected to the atmospheric nucleation process often induced by photochemistry. While GSA and sub‐3 nm particles have been concurrently measured in continental pristine and background areas, we present the first simultaneous measurement of GSA and NCA at urban roadside areas. This enables better understanding of the formation process of NCA and its connections to, e.g., traffic density.

The GSA concentration was measured in a street canyon (traffic density: 28 000 vehicles/weekday, 10% heavy duty vehicles) using a nitrate‐based chemical‐ionization atmospheric‐pressure‐interface time‐of‐flight mass spectrometer (CI‐APi‐TOF) in May 2017. The measurement was performed in a Supersite air quality monitoring station located near the city center of Helsinki, Finland. The continuous time series of GSA concentration provides information on the diurnal variation of the GSA concentration, and the influence of solar radiation, traffic density, and other environmental variables on the GSA concentration. The NCA concentration was measured simultaneously at the same location using an Airmodus Particle Size Magnifier (PSM) and a TSI Condensation Particle Counter (CPC). In addition, also other aerosol and gas measurement devices were used in the campaign.

The highest GSA concentrations were observed during the rush hours but increased concentrations were also observed after midnight, when the traffic density was the lowest. The diurnal variation of the GSA concentration had a correlation with the diurnal variation of the NCA concentration.