On the diurnal cycle of urban aerosols, black carbon and the occurrence of new particle formation events in springtime São Paulo, Brazil
Large conurbations are a significant source of the anthropogenic\npollution and demographic differences between cities that result in a\ndifferent pollution burden. The metropolitan area of Sao Paulo (MASP,\npopulation 20 million) accounts for one fifth of the Brazilian vehicular\nfleet. A feature of MASP is the amount of ethanol used by the vehicular\nfleet, known to exacerbate air quality. The study describes the diurnal\nbehaviour of the submicron aerosol and relies on total particle number\nconcentration, particle number size distribution, light scattering and\nlight absorption measurements. Modelled planetary boundary layer (PBL)\ndepth and air mass movement data were used to aid the interpretation.\nDuring morning rush-hour, stagnant air and a shallow PBL height favour\nthe accumulation of aerosol pollution. During clear-sky conditions,\nthere was a wind shift towards the edge of the city indicating a heat\nisland effect with implications on particulate pollution levels at the\nsite. The median total particle number concentration for the submicron\naerosol typically varied in the range 1.6 x 10(4)-3.2 x 10(4) cm(-3)\nfrequently exceeding 4 x 10(4) cm-3 during the day. During weekdays,\nnucleation-mode particles are responsible for most of the particles by\nnumbers. The highest concentrations of total particle number\nconcentrations and black carbon (BC) were observed on Fridays. Median\ndiurnal values for light absorption and light scattering (at 637 nm\nwavelength) varied in the range 12-33 Mm(-1) and 21-64 Mm(-1),\nrespectively. The former one is equal to 1.8-5.0 mu g m(-3) of BC. The\ngrowth of the PBL, from the morning rush-hour until noon, is consistent\nwith the diurnal cycle of BC mass concentrations. Weekday hourly median\nsingle-scattering albedo (omega(0)) varied in the range 0.59-0.76.\nOverall, this suggests a top of atmosphere (TOA) warming effect.\nHowever, considering the low surface reflectance of urban areas, for the\ngiven range of omega(0), the TOA radiative forcing can be either\npositive or negative for the sources within the MASP. On the average,\nweekend omega(0) values were 0.074 higher than during weekdays. During\n11% of the days, new particle formation (NPF) events occurred. The\nanalysed events growth rates ranged between 9 and 25 nm h(-1). Sulphuric\nacid proxy concentrations calculated for the site were less than 5% of\nthe concentration needed to explain the observed growth. Thus, other\nvapours are likely contributors to the observed growth.