Simultaneous measurements of particle number size distributions at ground level and 260 m on a meteorological tower in urban Beijing, China

摘要

Despite extensive studies into the characterization of particle numbersize distributions at ground level, real-time measurements above the urbancanopy in the megacity of Beijing have never been performed to date. Here weconducted the first simultaneous measurements of size-resolved particlenumber concentrations at ground level and 260 m in urban Beijing from 22August to 30 September. Our results showed overall similar temporalvariations in number size distributions between ground level and 260 m, yetperiods with significant differences were also observed. Particularly,accumulation-mode particles were highly correlated ( = 0. 85) at thetwo heights, while Aitken-mode particles presented more differences. Detailedanalysis suggests that the vertical differences in number concentrationsstrongly depended on particle size, and particles with a mobility diameterbetween 100 and 200 nm generally showed higher concentrations at higheraltitudes. Particle growth rates and condensation sinks were also calculated,which were 3.2 and 3.6 nm h, and 2.8  ×  10 and2.9  ×  10 s, at ground level and 260 m, respectively.By linking particle growth with aerosol composition, we found that organicsappeared to play an important role in the early stage of the growth (09:00–12:00 LT) while sulfate was also important during the later period. Positivematrix factorization of size-resolved number concentrations identified threecommon sources at ground level and 260 m, including a factor associated withnew particle formation and growth events (NPEs), and two secondary factorsthat represent photochemical processing and regional transport. Cooking emission was found to have a large contribution tosmall particles and showed much higher concentration at ground level than260 m in the evening. These results imply thatinvestigation of NPEs at ground level in megacities needs to consider theinfluences of local cooking emissions. The impacts of regional emissioncontrols on particle number concentrations were also illustrated. Ourresults showed that regional emission controls have a dominant impact onaccumulation-mode particles by decreasing gas precursors and particulatematter loadings, and hence suppressing particle growth. In contrast, theinfluences on Aitken particles were much smaller due to the enhanced newparticle formation (NPF) events.