Local sources of particles and precursor gases have long been considered as the major control for the ground‐level particle number concentration in an urban environment. Here we show the existence of two distinct sources. The first source was detectable during morning and afternoon rush hours and was defined by high black carbon concentrations. Particle number concentration inversely correlated with the local planetary boundary layer height. The particle size distributions were characterized by a wide range of modal diameters and did not exhibit detectable modal growth. This source was attributed to vehicular emissions. The second source yielded particle number concentration comparable to those during the rush hours and was detected six times over the 3‐week measurement campaign. Small particles produced by this source were recorded during the midday after the diminishment of the rush‐hour traffic effects. The particles exhibited prolonged modal growth over 8 hr, which may indicate a regional scale nucleation event. The data suggest that these particles were likely formed above the nocturnal boundary layer after sunrise and were subsequently transported to the surface through convective mixing. Overall, the nocturnal and convective boundary layer evolution was found to be closely associated with the of small particle event and the most important factor affecting the ground‐level particle number concentration. Shallow nocturnal boundary layers trapped pollution near the ground leading to particle number concentrations over 104 cm−3.

Ultrafine particles with diameters less than 100 nm suspended in the air are a topic of interest in air quality and climate sciences. Sub-10 nm particles are of additional interest due to their health effects and contribution to particle growth processes. Ambient measurements were carried out at North Carolina State University in Raleigh, NC between April to June 2019 and November 2019 to May 2020 to investigate the temporal variability of size distribution and number concentration of ultrafine particles. A mobile lab was deployed between March and May 2020 to characterize the spatial distribution of sub-10 nm particle number concentration. New particle formation and growth events were observed regularly. Also observed were direct emissions of sub-10 nm particles. Analysis against meteorological variables, gas-phase species, and particle concentrations show that the sub-10nm particles dominated number concentration during periods of low planetary boundary layer height, low solar radiation, and northeast winds. The spatial patterns observed during mobile deployments suggest that multiple temporally stable and spatially confined point sources of sub-10 nm particles are present within the city. These sources likely include the campus utility plants and the Raleigh-Durham International Airport. Additionally, the timing of data collection allowed for investigation of variations in the urban aerosol number size distribution due to reduced economic activity during the COVID-19 pandemic.

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