Fig. 4.

Unraveling NPF in urban air. Photochemical oxidation of vehicular exhaust (i.e., aromatic VOCs) yields abundant precursors for efficient nucleation and growth of UFPs. Nucleation produces freshly nucleated particles, which are subjected to coagulation capture by existing particles or growth to nucleation-mode particles (>3 nm). Under clean conditions with the lowest levels of preexisting particles and strong UV radiation, momentary nucleation of freshly nucleated particles (marked by red color) and rapid growth of nucleation-mode particles result in banana NPF. With low to intermediate levels of preexisting particles, continuous nucleation and relatively slow growth of nucleation-mode particles result in continuum NPF. Nucleation is suppressed by coagulation loss of freshly nucleated particles by nucleation-mode particles or preexisting particles (horizontal dashed arrows). During pollution evolution (from left to right), nucleation-mode particles successively grow to larger sizes to contribute to UFPs or PM_2.5. Under polluted conditions, preexisting (secondary and primary) particles inhibit nucleation, leading to masked NPF, although photochemistry is still sufficient for nucleation (arrows with dashed lines).