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Atmospheric Chemistry and Physics









Secondary organic aerosol (SOA) formation from ambient air was studied using an oxidation flow reactor (OFR) coupled to an aerosol mass spectrometer (AMS) during both the wet and dry seasons at the Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5) field campaign. Measurements were made at two sites downwind of the city of Manaus, Brazil. Ambient air was oxidized in the OFR using variable concentrations of either OH or O3, over ranges from hours to days (O3) or weeks (OH) of equivalent atmospheric aging. The amount of SOA formed in the OFR ranged from 0 to as much as 10 µg m−3, depending on the amount of SOA precursor gases in ambient air. Typically, more SOA was formed during nighttime than daytime, and more from OH than from O3 oxidation. SOA yields of individual organic precursors under OFR conditions were measured by standard addition into ambient air and were confirmed to be consistent with published environmental chamber-derived SOA yields. Positive matrix factorization of organic aerosol (OA) after OH oxidation showed formation of typical oxidized OA factors and a loss of primary OA factors as OH aging increased. After OH oxidation in the OFR, the hygroscopicity of the OA increased with increasing elemental O : C up to O : C ∼ 1.0, and then decreased as O : C increased further. Possible reasons for this decrease are discussed. The measured SOA formation was compared to the amount predicted from the concentrations of measured ambient SOA precursors and their SOA yields. While measured ambient precursors were sufficient to explain the amount of SOA formed from O3, they could only explain 10–50 % of the SOA formed from OH. This is consistent with previous OFR studies, which showed that typically unmeasured semivolatile and intermediate volatility gases (that tend to lack C = C bonds) are present in ambient air and can explain such additional SOA formation. To investigate the sources of the unmeasured SOA-forming gases during this campaign, multilinear regression analysis was performed between measured SOA formation and the concentration of gas-phase tracers representing different precursor sources. The majority of SOA-forming gases present during both seasons were of biogenic origin. Urban sources also contributed substantially in both seasons, while biomass burning sources were more important during the dry season. This study enables a better understanding of SOA formation in environments with diverse emission sources.


Brett B. Palm1,2, Suzane S. de Sá3, Douglas A. Day1,2, Pedro Campuzano-Jost1,2, Weiwei Hu1,2, Roger Seco4,Steven J. Sjostedt5, Jeong-Hoo Park6,a, Alex B. Guenther4,7, Saewung Kim4, Joel Brito8,b, Florian Wurm8, Paulo Artaxo8,Ryan Thalman9,c, Jian Wang9, Lindsay D. Yee10, Rebecca Wernis11, Gabriel Isaacman-VanWertz10,d, Allen H. Goldstein10,11,Yingjun Liu3,e, Stephen R. Springston9, Rodrigo Souza12, Matt K. Newburn13, M. Lizabeth Alexander13, Scot T. Martin3,14,and Jose L. Jimenez1,2

1Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA
2Dept. of Chemistry, University of Colorado, Boulder, CO, USA
3School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
4Dept. of Earth System Science, University of California, Irvine, CA, USA
5Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, USA
6National Center for Atmospheric Research, Boulder, CO, USA
7Div. of Atmospheric Sciences & Global Change, Pacific Northwest National Laboratory, Richland, WA, USA
8Institute of Physics, University of São Paulo, São Paulo, Brazil
9Environmental and Climate Sciences Department, Brookhaven National Laboratory, Upton, NY, USA
10Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA, USA
11Department of Civil and Environmental Engineering, University of California, Berkeley, CA, USA
12University of the State of Amazonas, Manaus, Brazil
13Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA, USA
14Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA
anow at: Climate and Air Quality Research Department, National Institute of Environmental Research (NIER), Incheon, 22689, Republic of Korea
bnow at: Laboratory for Meteorological Physics (LaMP), Université Clermont Auvergne, 63000 Clermont-Ferrand, France
cnow at: Department of Chemistry, Snow College, Richfield, UT, USA
dnow at: Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA, USA
enow at: Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA, USA

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This work is licensed under a Creative Commons Attribution 4.0 License.