Although an accurate evaluation of the distribution of ultrafine particulate matter in air is of utmost significance to public health,the usually used PM2.s index fails to provide size distribution information.Here we demonstrate a low-profile and cavity-free size spectrometer for probing fine and ultrafine particulate matter by using the enhanced particle-perturbed scattering in strong optical evanescent fields of a nanofiber array.The unprecedented size resolution reaches 10 nm for detecting single 100-nm-diameter nanoparticles by employing uniform nanofibers and controlling the polarizations of the probe light.This size spectrometry was tested and used to retrieve the size distribution of particulate matter in the air of Beijing,yielding mass concentrations of nanoparticles,as a secondary exercise,consistent with the officially released data.This nanoflber-array probe shows potential for the full monitoring of air pollution and for studying early-stage haze evolution and can be further extended to explore nanoparticle interactions.