Effect of vehicular traffic, remote sources and new particle formation on the activation properties of cloud condensation nuclei in the megacity of São Paulo, Brazil

摘要

Atmospheric aerosol is the primary source of cloud condensation nuclei (CCN).The microphysics and chemical composition of aerosols can affect clouddevelopment and the precipitation process. Among studies conducted in LatinAmerica, only a handful have reported the impact of urban aerosol on CCNactivation parameters such as activation ratio (AR) and activation diameter(). With over 20 million inhabitants, the Metropolitan Area ofSão Paulo (MASP) is the largest megacity in South America. To ourknowledge, this is the first study to assess the impact that remote sourcesand new particle formation (NPF) events have on CCN activation properties ina South American megacity. The measurements were conducted in the MASPbetween August and September 2014. We measured the CCN within the0.2–1.0 % range of supersaturation, together with particle numberconcentration (PNC) and particle number distribution (PND), as well astrace-element concentrations and black carbon (BC). NPF events wereidentified on 35 % of the sampling days. Combining multivariate analysisin the form of positive matrix factorization (PMF) with an aerosol profilefrom lidar and HYSPLIT model analyses allowed us to identify the maincontribution of vehicular traffic on all days and sea salt and biomassburning from remote regions on 28 and 21 % of the sampling days,respectively. The AR and parameters showed distinct patternsfor daytime with intense vehicular traffic and nighttime periods. Forexample, CCN activation was lower during the daytime than during thenighttime periods, a pattern that was found to be associated mainly withlocal road-traffic emissions. A decrease in CCN activation was observed onthe NPF event days, mainly due to high concentrations of particles withsmaller diameters. We also found that aerosols from sea salt, industrialemissions, and biomass burning had minor effects on . Forexample, nights with biomass burning and vehicular emissions showed slightlylower CCN activation properties than sea-salt, industrial and non-eventnights. Our results show that particulate matter from local vehicularemissions during the daytime has a greater effect on CCN activationparameters than that from remote sources.