Ultrafine particle formation across urban and background sites: Insights on Midday Pollution events through analysis of locally emitted pollutants

Ultrafine particles (UFPs, <100 nm) are associated with health and environmental risks. Reducing exposure to these particles requires identifying their origin. This study classified days into five categories based on dominant sources of UFP during the day: Strong Nucleation, Midday Pollution, Traffic Pollution, Baseline, and Mixed. The primary focus was on Midday Pollution days; days that showed higher UFP concentrations around noon, without evidence of particle formation. The classification was performed manually using particle size distribution data (6–523 nm) collected from 2006 to 2021 at the Southern Ontario Center for Atmospheric Aerosol Research (SOCAAR). The same types of days were also identified based on total particle number concentration data using ensemble learning models at three additional sites with differing concentrations of UFP: Hanlan’s Point (a background site with no local traffic), Downsview (an urban background site 250 meters from the nearest roadway), and Highway 401 (a site located near a major highway). The classified days at these locations had patterns consistent with those at SOCAAR. The noon-to-morning ratio of UFP at Hanlan’s Point was roughly twice that of SOCAAR and Highway 401, indicating more intense particle amplification at the background site. Midday Pollution days occurred more frequently during colder months. Higher sodium to chloride molar ratios on Midday Pollution days indicated that the atmospheric chemistry was different compared to other types of days. These results indicate that concentrations of UFP may rise midday due to a process other than new particle formation events. • Days were classified based on the dominant sources of UFP at four sites. • Particle amplification happened on the identified Midday Pollution (MP) days. • Particle amplification increased with distance from nearby traffic. • Na/Cl molar ratios were higher on MP days than on other days in winter.