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Title: CCN activity and organic hygroscopicity of aerosols downwind of an urban region in central Amazonia: Seasonal and diel variations and impact of anthropogenic emissions

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) 60km 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. Also, 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 organicmore » 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. Our 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.« less
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  1. Brookhaven National Lab. (BNL), Upton, NY (United States). Environmental and Climate Science Dept.
  2. Harvard Univ., Cambridge, MA (United States). School of Enginering and Applied Sciences
  3. Univ. of Colorado, Boulder, CO (United States). Dept. of Chemistry and Biochemistry, Cooperative Inst. for Research in Environmental Sciences (CIRES)
  4. Univ. of Sao Paulo (Brazil). Physics Inst.
  5. Max Planck Society, Mainz (Germany). Max Planck Inst. for Chemistry, Biogeochemistry and Multiphase Chemistry Depts.
  6. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
  7. National Inst. of Amazonian Research, Manaus (Brazil)
  8. Georgia Inst. of Technology, Atlanta, GA (United States). School of Chemical and Biomolecular Engineering, School of Earth and Atmospheric Sciences
  9. Amazonas State Univ., Manaus (Brazil)
  10. Max Planck Society, Mainz (Germany). Max Planck Inst. for Chemistry, Biogeochemistry and Multiphase Chemistry Depts. ; Univ. of California, San Diego, CA (United States). Scripps Institution of Oceanography
  11. Harvard Univ., Cambridge, MA (United States). School of Enginering and Applied Sciences, Dept. of Earth and Planetary Sciences
Publication Date:
Report Number(s):
Journal ID: ISSN 1680-7316; R&D Project: 2016-BNL-EE630EECA-Budg; KP1701000
Grant/Contract Number:
Accepted Manuscript
Journal Name:
Atmospheric Chemistry and Physics
Additional Journal Information:
Journal Volume: 17; Journal Issue: 19; Journal ID: ISSN 1680-7316
European Geosciences Union
Research Org:
Brookhaven National Laboratory (BNL), Upton, NY (United States)
Sponsoring Org:
USDOE Office of Science (SC), Biological and Environmental Research (BER) (SC-23)
Country of Publication:
United States
OSTI Identifier: