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.

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