Abstract: With the rapid increase in motor vehicle numbers, nano- to sub-micron particle pollution in urban street canyons has become an important air-quality concern because of its high spatial heterogeneity and proximity to emission sources. In China, however, high-resolution investigations that simultaneously integrate particle number size distributions, chemical composition, and the source-related formation processes within street-canyon environments remain limited. To address this gap, a 16-day high-time-resolution field campaign was conducted at the Sanli'an section of Changjiang West Road in Hefei, a typical urban street canyon influenced by traffic and commercial activities. Particle number size distributions were measured using a Scanning Mobility Particle Sizer (SMPS), while the non-refractory submicron aerosol chemical composition (NR-PM₁) was characterized using an Aerosol Chemical Speciation Monitor (ACSM). These measurements were combined with meteorological observations and source apportionment analysis to investigate the emission characteristics, atmospheric processing, and controlling mechanisms of nano- to sub-micron particles in this microenvironment. The results show a pronounced tri-modal diurnal pattern in particle number concentrations. Three distinct peaks were observed during the morning traffic rush (07:00–09:00), lunchtime cooking activities (11:00–13:00), and the evening traffic rush (18:00–21:00), indicating the combined influence of vehicular exhaust and cooking-related emissions. Number concentrations were dominated by particles in the nucleation and Aitken modes during traffic and cooking periods, while accumulation-mode particles increased under stagnant meteorological conditions. Organic aerosols were the dominant component of NR-PM₁, accounting for 55%–75% of the total mass concentration, followed by nitrate (15%–25%). Both organic aerosol and nitrate exhibited higher concentrations during the morning and evening, and lower levels around noon, reflecting the combined effects of emission intensity, photochemical oxidation, and boundary-layer evolution. A positive matrix factorization (PMF) analysis resolved five contributing factors for particle number concentrations: combustion-related emissions, new particle formation, traffic emissions, regional transport, and aged mixed aerosols. For chemical species, three major factors were identified: secondary nitrate, secondary organic aerosol (SOA), and primary organic aerosol (POA). Meteorological conditions played a critical role in regulating particle formation and evolution. Low temperature (< 10 °C) and low relative humidity (< 40%) under relatively clean background conditions favored new particle formation in the 7–20 nm size range. In contrast, high temperature (> 30 °C) and high relative humidity (> 70%) promoted particle growth and secondary aerosol accumulation. Additionally, low wind speeds (< 1.5 m/s), particularly under southwesterly wind conditions, enhanced pollutant accumulation within the street canyon and shifted the particle size distributions toward the accumulation mode. Overall, the results systematically elucidate the emission characteristics, nucleation processes, and aging behavior of nano- to sub-micron particles in an urban street-canyon environment. The findings provide quantitative evidence for refined urban air-quality management and the coordinated control of vehicular exhaust and cooking-related emissions.