CCN activity and organic hygroscopicity of aerosols downwind of an urban region in central Amazonia: Seasonal and diel variations and impact of anthropogenic emissions
Ryan Thalman1,a, Suzane S. de Sá2, Brett B. Palm3, Henrique M. J. Barbosa4, Mira L. Pöhlker5, M. Lizabeth Alexander7, Joel Brito4,b, Samara Carbone4, Paulo Castillo1, Douglas A. Day3, Chongai Kuang1, Antonio Manzi8, Nga Lee Ng9,10, Arthur J. Sedlacek III1, Rodrigo Souza11, Stephen Springston1, Thomas Watson1, Christopher Pöhlker5, Ulrich Pöschl5,6, Meinrat O. Andreae5,12, Paulo Artaxo4, Jose L. Jimenez3, Scot T. Martin2,13, and Jian Wang11Environmental and Climate Sciences Department, Brookhaven National Laboratory, Upton, NY, USA 2School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA 3Department of Chemistry and Biochemistry and Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado Boulder, Boulder, CO, USA 4Physics Institute, University of São Paulo, São Paulo, Brazil 5Biogeochemistry and Multiphase Chemistry Departments, Max Planck Institute for Chemistry, Mainz, Germany 6Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, USA 7Pacific Northwest National Laboratory, Richland, WA, USA 8National Institute of Amazonian Research, Manaus, Amazonas, Brazil 9School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA, USA 10School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA 11Amazonas State University, Manaus, Amazonas, Brazil 12Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, USA 13Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA anow at: Department of Chemistry, Snow College, Richfield, UT, USA bnow at: Laboratory for Meteorological Physics, University Clermont Auvergne, Clermont-Ferrand, France
Received: 17 Mar 2017 – Accepted for review: 24 Mar 2017 – Discussion started: 28 Mar 2017
Abstract. During the Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5) campaign, size-resolved cloud condensation nuclei (CCN) spectra were characterized at a research site (T3) 60 km downwind of the city of Manaus, Brazil, in central Amazonia for one year (12 March 2014 to 3 March 2015). Particle hygroscopicity (κCCN) and mixing state were derived from the size-resolved CCN spectra, and the hygroscopicity of the organic component of the aerosol (κorg) was then calculated from κCCN and concurrent chemical composition measurements. The annual average κCCN increased from 0.13 at 75 nm to 0.17 at 171 nm, and the increase was largely due to an increase in sulfate volume fraction. During both wet and dry seasons, κCCN, κorg, and particle composition under background conditions exhibited essentially no diel variations. The constant κorg of ~ 0.15 is consistent with the largely uniform and high O : C value (~ 0.8), indicating that the aerosols under background conditions are dominated by the aged regional aerosol particles consisting of highly oxygenated organic compounds. For air masses strongly influenced by urban pollution and/or local biomass burning, lower values of κorg and organic O : C atomic ratio were observed during night, due to accumulation of freshly emitted particles, dominated by primary organic aerosol (POA) with low hygroscopicity, within a shallow nocturnal boundary layer. The O : C, κorg, and κCCN increased from the early morning hours and peaked around noon, driven by the formation and aging of secondary organic aerosol (SOA) and dilution of POA emissions into a deeper boundary layer, while the development of the boundary layer, which leads to mixing with aged particles from the residual layer aloft, likely also contributed to the increases. The hygroscopicities associated with individual organic factors, derived from PMF analysis of AMS spectra, were estimated through multi-variable linear regression. For the SOA factors, the variation of the κ value with O : C agrees well with the linear relationship reported from earlier laboratory studies of SOA hygroscopicity. On the other hand, the variation in O:C of ambient aerosol organics is largely driven by the variation in the volume fractions of POA and SOA factors, which have very different O : C values. As POA factors have hygroscopicity values well below the linear relationship between SOA hygroscopicity and O:C, mixtures with different POA and SOA fractions exhibit a steeper slope for the increase of κorg with O : C, as observed during this and earlier field studies. This finding helps better understand and reconcile the differences in the relationships between κorg and O : C observed in laboratory and field studies, therefore providing a basis for improved parameterization in global models, especially in a tropical context.
Thalman, R., de Sá, S. S., Palm, B. B., Barbosa, H. M. J., Pöhlker, M. L., Alexander, M. L., Brito, J., Carbone, S., Castillo, P., Day, D. A., Kuang, C., Manzi, A., Ng, N. L., Sedlacek III, A. J., Souza, R., Springston, S., Watson, T., Pöhlker, C., Pöschl, U., Andreae, M. O., Artaxo, P., Jimenez, J. L., Martin, S. T., and Wang, J.: CCN activity and organic hygroscopicity of aerosols downwind of an urban region in central Amazonia: Seasonal and diel variations and impact of anthropogenic emissions, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-251, in review, 2017.