Articles | Volume 16, issue 13
Atmos. Chem. Phys., 16, 8095–8108, 2016
Atmos. Chem. Phys., 16, 8095–8108, 2016

Research article 04 Jul 2016

Research article | 04 Jul 2016

Evidence for ambient dark aqueous SOA formation in the Po Valley, Italy

Amy P. Sullivan1, Natasha Hodas2, Barbara J. Turpin3, Kate Skog4, Frank N. Keutsch4,5, Stefania Gilardoni6, Marco Paglione6, Matteo Rinaldi6, Stefano Decesari6, Maria Cristina Facchini6, Laurent Poulain7, Hartmut Herrmann7, Alfred Wiedensohler7, Eiko Nemitz8, Marsailidh M. Twigg8, and Jeffrey L. Collett Jr.1 Amy P. Sullivan et al.
  • 1Colorado State University, Department of Atmospheric Science, Fort Collins, Colorado 80523, USA
  • 2California Institute of Technology, Division of Chemical Engineering, Pasadena, California 91125, USA
  • 3Rutgers University, Department of Environmental Sciences, New Brunswick, New Jersey 08901, USA
  • 4University of Wisconsin – Madison, Department of Chemistry, Madison, Wisconsin 53706, USA
  • 5Harvard University, Department of Chemistry and Chemical Biology, Cambridge, Massachusetts 02138, USA
  • 6Istituto di Scienze dell'Atmosfera e del Clima, Consiglio Nazionale delle Ricerche, 40129 Bologna, Italy
  • 7Leibniz Institute for Tropospheric Research, 04318 Leipzig, Germany
  • 8Centre for Ecology and Hydrology, Bush Estate, Penicuik, EH26QB, UK

Abstract. Laboratory experiments suggest that water-soluble products from the gas-phase oxidation of volatile organic compounds can partition into atmospheric waters where they are further oxidized to form low volatility products, providing an alternative route for oxidation in addition to further oxidation in the gas phase. These products can remain in the particle phase after water evaporation, forming what is termed as aqueous secondary organic aerosol (aqSOA). However, few studies have attempted to observe ambient aqSOA. Therefore, a suite of measurements, including near-real-time WSOC (water-soluble organic carbon), inorganic anions/cations, organic acids, and gas-phase glyoxal, were made during the PEGASOS (Pan-European Gas-AeroSOls-climate interaction Study) 2012 campaign in the Po Valley, Italy, to search for evidence of aqSOA. Our analysis focused on four periods: Period A on 19–21 June, Period B on 30 June and 1–2 July, Period C on 3–5 July, and Period D on 6–7 July to represent the first (Period A) and second (Periods B, C, and D) halves of the study. These periods were picked to cover varying levels of WSOC and aerosol liquid water. In addition, back trajectory analysis suggested all sites sampled similar air masses on a given day. The data collected during both periods were divided into times of increasing relative humidity (RH) and decreasing RH, with the aim of diminishing the influence of dilution and mixing on SOA concentrations and other measured variables. Evidence for local aqSOA formation was only observed during Period A. When this occurred, there was a correlation of WSOC with organic aerosol (R2 = 0.84), aerosol liquid water (R2 = 0.65), RH (R2 = 0.39), and aerosol nitrate (R2 = 0.66). Additionally, this was only observed during times of increasing RH, which coincided with dark conditions. Comparisons of WSOC with oxygenated organic aerosol (OOA) factors, determined from application of positive matrix factorization analysis on the aerosol mass spectrometer observations of the submicron non-refractory organic particle composition, suggested that the WSOC differed in the two halves of the study (Period A WSOC vs. OOA-2 R2 = 0.83 and OOA-4 R2 = 0.04, whereas Period C WSOC vs. OOA-2 R2 = 0.03 and OOA-4 R2 = 0.64). OOA-2 had a high O ∕ C (oxygen ∕ carbon) ratio of 0.77, providing evidence that aqueous processing was occurring during Period A. Key factors of local aqSOA production during Period A appear to include air mass stagnation, which allows aqSOA precursors to accumulate in the region; the formation of substantial local particulate nitrate during the overnight hours, which enhances water uptake by the aerosol; and the presence of significant amounts of ammonia, which may contribute to ammonium nitrate formation and subsequent water uptake and/or play a more direct role in the aqSOA chemistry.

Short summary
This paper presents the results from our measurements and approach for the investigation of aqueous secondary organic aerosol (aqSOA) formation in the ambient atmosphere. When local aqSOA formation was observed, a correlation of water-soluble organic carbon with organic aerosol, aerosol liquid water, relative humidity, and aerosol nitrate was found. Key factors of local aqSOA production include air mass stagnation, formation of local nitrate overnight, and significant amounts of ammonia.
Final-revised paper