Impact of organic molecular structure on the estimation of atmospherically relevant physicochemical parameters
<p>Many methods are currently available for estimating physicochemical properties of atmospherically relevant compounds. Though a substantial body of literature has focused on the development and intercomparison of methods based on molecular structure, there has been an increasing focus on met...
| Main Authors: | , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2021-04-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/21/6541/2021/acp-21-6541-2021.pdf |
| Summary: | <p>Many methods are currently available for estimating physicochemical properties of atmospherically relevant compounds. Though a substantial body of literature has focused on the development and intercomparison of methods based on molecular structure, there has been an increasing focus on methods based only
on molecular formula. However, prior work has not quantified the extent to
which isomers of the same formula may differ in their properties or,
relatedly, the extent to which lacking or ignoring molecular structure
degrades estimates of parameters. Such an evaluation is complicated by the
fact that structure-based methods bear significant uncertainty and are
typically not well constrained for atmospherically relevant molecules. Using
species produced in the modeled atmospheric oxidation of three representative
atmospheric hydrocarbons, we demonstrate here that estimated differences
between isomers are greater than differences between three widely used
estimation methods. Specifically, isomers tend to differ in their estimated
vapor pressures and Henry's law constants by a half to a full order of
magnitude greater than differences between estimation methods, and they differ in
their rate constant for reaction with OH radicals (<span class="inline-formula"><i>k</i><sub>OH</sub></span>) by a
factor of 2. Formula-based estimation of these parameters, using certain
methods, is shown to agree with structure-based estimates with little bias and
approximately normally distributed error. Specifically, vapor pressure can be
estimated using a combination of two existing methods, Henry's law constants
can be estimated based on vapor pressure, and <span class="inline-formula"><i>k</i><sub>OH</sub></span> can be
approximated as a constant for all formulas containing a given set of
elements. Formula-based estimation is, therefore, reasonable when applied to a
mixture of isomers but creates uncertainty commensurate with the lack of
structural information.</p> |
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| ISSN: | 1680-7316 1680-7324 |