Water-soluble ions species of size-resolved aerosols: Implications for the atmospheric acidity in São Paulo megacity, Brazil Marcelo Vieira-Filho a, ⁎, Jairo J. Pedrotti b , Adalgiza Fornaro c a Departamento de Engenharia, Universidade Federal de Lavras, Campus da UFLA, 37200-000, Lavras, Minas Gerais, Brazil b Escola de Engenharia, Universidade Presbiteriana Mackenzie, Rua Consolação, 930, 01302-907, Consolação, São Paulo, SP, Brazil c Departamento de Ciências Atmosféricas, Instituto de Astronomia, Geofísica e Ciências, Atmosféricas da Universidade de São Paulo, Rua do Matão, 1226, 05508-090, Cidade, Universitária, São Paulo, SP, Brazil abstract article info Article history: Received 25 February 2016 Received in revised form 28 June 2016 Accepted 5 July 2016 Available online 06 July 2016 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 3.40 μgm -3 , which accounted for over 25% of PM 0.56 mass. Regarding the relative distribution of ionic species, 90% of NH 4 + levels, were restricted to smaller than 1 μm diameter range. The average neutralization index for PM b 1 μm 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 PM 0.56 . The total neutralization index registered the lowest value for PM 0.56 , 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 b 0.020 μm). 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. © 2016 Elsevier B.V. All rights reserved. Keywords: Urban air pollution Megacity Size-resolved aerosols Acidity Ammonium 1. Introduction Airborne particulate matter (PM) is a complex system and its com- position is highly dependent on local characteristics like air pollutants sources, topography and meteorology. In addition, PM could be related with urban air pollution issues resulting in public health problems (Martins, 2004; Cirera et al., 2009; Tadano et al., 2014). Furthermore, due to physical-chemical characteristics and different removal mecha- nisms of PM, multiple scales studies are required to improve the knowl- edge about its environmental fate (McMurry, 2000; Zhang and Vet, 2006). Atmospheric deposition of PM delivers loads of many harmful com- pounds to ecosystems, which could lead to soil acidification, eutrophica- tion, poor quality of water and sediment (Bouwman et al., 2002; Allen et al., 2011). Aerosol acidity is an important physical-chemical parame- ter that affects many atmospheric chemical processes (Huo et al., 2012). For instance, the aerosol acid equilibrium could lead to volatilization of HCl (Brimblecombe and Clegg, 1988) and NH 3 (Huang et al., 2011), and to oxidation of sulfur and volatile organic compounds (VOCs) (Chameides, 1984; Keene et al., 2004), influencing on the gas-particle processes (Nenes et al., 1998; Hennigan et al., 2015). Even though the pH value of 5.6 was defined as the acidic buffering capacity of particulate matter (Xu et al., 1996), this definition does not account for several processes therein, which hinder the convoluted aerosol system. Several weak acid-basic components have important role in acidity, however most concentrations of these species are neglected due to the lack of data and accurate methods to quantify its concentrations. Another issue that hinders acidity evaluation of aerosol system is quantitative contribution of aerosol buffering capacity in the atmospheric deposition (wet and dry) (McMurry, 2000; Huang et al., 2011). São Paulo Metropolitan Area (MASP) is the sixth largest area in the world with over 20 million inhabitants and the highest vehicle fleet in Brazil (over 20 million), responsible for over 30% of Brazilian Gross Domestic Product (GDP). MASP has been subjected to serious pollution episodes due to industrialization and increase of vehicular fleet (CETESB, 2015). Studies on wet deposition in São Paulo State have reported that H + volume-weighted-mean concen- trations ranged from 16.9 to 40.6 μmol L -1 (Paiva et al., 1997; Rocha et al., 2003; Fornaro and Gutz, 2003; Lara et al., 2005; dos Atmospheric Research 181 (2016) 281–287 ⁎ Corresponding author. E-mail addresses: vieira.filho@live.com (M. Vieira-Filho), fornaro@model.iag.usp.br (J.J. Pedrotti), jpedrotti@mackenzie.br (A. Fornaro). http://dx.doi.org/10.1016/j.atmosres.2016.07.006 0169-8095/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Atmospheric Research journal homepage: www.elsevier.com/locate/atmosres