Water-soluble ions species of size-resolved aerosols: Implications for the atmospheric acidity in São Paulo megacity, Brazil

Publication date: 15 November 2016
Source:Atmospheric Research, Volume 181
Author(s): Marcelo Vieira-Filho, Jairo J. Pedrotti, Adalgiza Fornaro
Over the last decade, an increase of ammonium salts in atmospheric deposition has been reported worldwide, especially in megacities. The present study aims to give a better comprehension analysis about particulate matter acidity in São Paulo megacity (MASP), Brazil. Size-resolved aerosols were sampled in MASP, during 2012 winter, showing a bimodal mass concentration distribution, with sulfate concentration exceeding 3.40μgm⁻³, which accounted for over 25% of PM0.56 mass. Regarding the relative distribution of ionic species, 90% of NH4⁺ levels, were restricted to smaller than 1μm diameter range. The average neutralization index for PM<1μm was 0.62, which indicated an ammonia-limiting atmosphere due to partial neutralization of atmospheric acids. Particles of the accumulation mode presented more acid behavior than other aerosol fractions, with pH value as low as 4.15 in PM0.56. The total neutralization index registered the lowest value for PM0.56, but it did not respond promptly to aerosol variations as the E-AIM model predictions. The highest discrepancies between the acidity proxies occurred in the smaller fractions of particulate matter, especially in the after-filter (AF) stage (diameter<0.020μm). In addition, AF stage had the highest contribution to PM total mass, about 14% for all the stages analyzed. Such contribution indicates that acidity in ultrafine particles are still mixed for the MASP and need further investigation.



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