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

Sullivan, Amy P. and Hodas, Natasha and Turpin, Barbara J. and Skog, Kate and Keutsch, Frank N. and Gilardoni, Stefania and Paglione, Marco and Rinaldi, Matteo and Decesari, Stefano and Facchini, Maria Cristina and Poulain, Laurent and Herrmann, Hartmut and Wiedensohler, Alfred and Nemitz, Eiko and Twigg, Marsailidh M. and Collett, Jeffrey L., Jr. (2016) Evidence for ambient dark aqueous SOA formation in the Po Valley, Italy. Atmospheric Chemistry and Physics, 16 (13). pp. 8095-8108. ISSN 1680-7324. doi:10.5194/acp-16-8095-2016.

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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 (R^2 = 0.84), aerosol liquid water (R^2 = 0.65), RH (R^2 = 0.39), and aerosol nitrate (R^2 = 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 R^2 = 0.83 and OOA-4 R^2 = 0.04, whereas Period C WSOC vs. OOA-2 R^2 = 0.03 and OOA-4 R^2 = 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.

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Herrmann, Hartmut0000-0001-7044-2101
Additional Information:© Author(s) 2016. This work is distributed under the Creative Commons Attribution 3.0 License. Received: 31 Oct 2015 – Published in Atmos. Chem. Phys. Discuss.: 16 Dec 2015. Revised: 25 Mar 2016 – Accepted: 08 Jun 2016 – Published: 04 Jul 2016. We acknowledge funding from the National Science Foundation under projects AGS-1050052, AGS-1052611, and AGS-1051338. Measurements at SPC were also funded by the European Union FP7 project PEGASOS (FP7-ENV-2010/265148) and by the Regione Emilia Romagna (project SUPERSITO DRG no. 428/10). The authors thank the European Union FP7 ÉCLAIRE (FP7-ENV-2011/282910) project for funding the ammonia measurements in Bologna, the Energy Research Centre of the Netherlands (ECN) for providing the MARGA instrument at SPC, and C. DiMarco, M. Blom, S. Leeson, T. Hutchings, C. Braban, and L. Giulianelli for supporting the ammonia measurements. The authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and/or READY website ( used in this publication. Edited by: R. McLaren
Funding AgencyGrant Number
European Research Council (ERC)FP7-ENV-2010/265148
Regione Emilia Romagna428/10
European Research Council (ERC)FP7-ENV-2011/282910
Issue or Number:13
Record Number:CaltechAUTHORS:20160930-155334053
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Official Citation:Sullivan, A. P., Hodas, N., Turpin, B. J., Skog, K., Keutsch, F. N., Gilardoni, S., Paglione, M., Rinaldi, M., Decesari, S., Facchini, M. C., Poulain, L., Herrmann, H., Wiedensohler, A., Nemitz, E., Twigg, M. M., and Collett Jr., J. L.: Evidence for ambient dark aqueous SOA formation in the Po Valley, Italy, Atmos. Chem. Phys., 16, 8095-8108, doi:10.5194/acp-16-8095-2016, 2016.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:70734
Deposited By: George Porter
Deposited On:03 Oct 2016 18:07
Last Modified:11 Nov 2021 04:33

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