Sponsor
This work was funded by the Natural Science and Engineering Research Council of Canada, DOE (ASR/BER) DE-SC0016559 and EPA STAR 83587701-0.
Published In
Atmospheric Chemistry and Physics
Document Type
Article
Publication Date
11-6-2017
Subjects
Atmospheric aerosols -- Mathematical models, Air quality -- Mathematical models
Abstract
When simulating the formation and life cycle of secondary organic aerosol (SOA) with chemical transport models, it is often assumed that organic molecules are well mixed within SOA particles on the timescale of 1 h. While this assumption has been debated vigorously in the literature, the issue remains unresolved in part due to a lack of information on the mixing times within SOA particles as a function of both temperature and relative humidity. Using laboratory data, meteorological fields, and a chemical transport model, we estimated how often mixing times are < 1 h within SOA in the planetary boundary layer (PBL), the region of the atmosphere where SOA concentrations are on average the highest. First, a parameterization for viscosity as a function of temperature and RH was developed for α-pinene SOA using room-temperature and low-temperature viscosity data for α-pinene SOA generated in the laboratory using mass concentrations of ∼ 1000 µg m−3. Based on this parameterization, the mixing times within α-pinene SOA are < 1 h for 98.5 % and 99.9 % of the occurrences in the PBL during January and July, respectively, when concentrations are significant (total organic aerosol concentrations are > 0.5 µg m−3 at the surface). Next, as a starting point to quantify how often mixing times of organic molecules are < 1 h within α-pinene SOA generated using low, atmospherically relevant mass concentrations, we developed a temperature independent parameterization for viscosity using the room temperature viscosity data for α-pinene SOA generated in the laboratory using a mass concentration of ~70 μg m-3. Based on this temperature-independent parameterization, mixing times within α-pinene SOA are < 1 h for 27 and 19.5% of the occurrences in the PBL during January and July, respectively, when concentrations are significant. However, associated with these conclusions are several caveats, and due to these caveats we are unable to make strong conclusions about how often mixing times of organic molecules are < 1 h within pinene SOA generated using low, atmospherically relevant mass concentrations. Finally, a parameterization for viscosity of anthropogenic SOA as a function of temperature and RH was developed using sucrose–water data. Based on this parameterization, and assuming sucrose is a good proxy for anthropogenic SOA, 70 and 83% of the mixing times within anthropogenic SOA in the PBL are < 1 h for January and July, respectively, when concentrations are significant. These percentages are likely lower limits due to the assumptions used to calculate mixing times.
DOI
10.5194/acp-17-13037-2017
Persistent Identifier
http://archives.pdx.edu/ds/psu/22177
Citation Details
Maclean, A. M., Butenhoff, C. L., Grayson, J. W., Barsanti, K., Jimenez, J. L., and Bertram, A. K.: Mixing times of organic molecules within secondary organic aerosol particles: a global planetary boundary layer perspective, Atmos. Chem. Phys., 17, 13037-13048, https://doi.org/10.5194/acp-17-13037-2017, 2017.
Description
© Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License.
Published by Copernicus Publications on behalf of the European Geosciences Union.