Observation-based constraints on modeled aerosol surface area: implications for heterogeneous chemistry
<p>Heterogeneous reactions occurring at the surface of atmospheric aerosol particles regulate the production and lifetime of a wide array of atmospheric gases. Aerosol surface area plays a critical role in setting the rate of heterogeneous reactions in the atmosphere. Despite the central role...
| Main Authors: | , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2022-12-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/22/15449/2022/acp-22-15449-2022.pdf |
| Summary: | <p>Heterogeneous reactions occurring at the surface of
atmospheric aerosol particles regulate the production and lifetime of a wide
array of atmospheric gases. Aerosol surface area plays a critical role in
setting the rate of heterogeneous reactions in the atmosphere. Despite the
central role of aerosol surface area, there are few assessments of the
accuracy of aerosol surface area concentrations in regional and global
models. In this study, we compare aerosol surface area concentrations in the
EPA's Community Multiscale Air Quality (CMAQ) model with commensurate
observations from the 2011 NASA flight-based DISCOVER-AQ (Deriving
Information on Surface Conditions from COlumn and VERtically Resolved
Observations Relevant to Air Quality) campaign. The study region includes
the Baltimore and Washington, D.C. metropolitan area. Dry aerosol surface area
was measured aboard the NASA P-3B aircraft using an ultra-high-sensitivity
aerosol spectrometer (UHSAS). We show that modeled and measured dry aerosol
surface area, <span class="inline-formula"><i>S</i><sub>a,mod</sub></span> and <span class="inline-formula"><i>S</i><sub>a,meas</sub></span> respectively, are
modestly correlated (<span class="inline-formula"><i>r</i><sup>2</sup>=0.52</span>) and on average agree to within a
factor of 2 (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>S</mi><mrow><mi mathvariant="normal">a</mi><mo>,</mo><mi mathvariant="normal">mod</mi></mrow></msub><mo>/</mo><msub><mi>S</mi><mrow><mi mathvariant="normal">a</mi><mo>,</mo><mi mathvariant="normal">meas</mi></mrow></msub><mo>=</mo><mn mathvariant="normal">0.44</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="105pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="c4791e9b42e6dd70c4016273f7907404"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-15449-2022-ie00001.svg" width="105pt" height="15pt" src="acp-22-15449-2022-ie00001.png"/></svg:svg></span></span>) over the course of
the 13 research flights. We show that <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>S</mi><mrow><mi mathvariant="normal">a</mi><mo>,</mo><mi mathvariant="normal">mod</mi></mrow></msub><mo>/</mo><msub><mi>S</mi><mrow><mi mathvariant="normal">a</mi><mo>,</mo><mi mathvariant="normal">meas</mi></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="71pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="53e3c246b1ae5238395b67cd66f5152d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-15449-2022-ie00002.svg" width="71pt" height="15pt" src="acp-22-15449-2022-ie00002.png"/></svg:svg></span></span> does
not depend strongly on photochemical age or the concentration of secondary
biogenic aerosol, suggesting that the condensation of low-volatility
gas-phase compounds does not strongly affect model–measurement agreement. In
comparison, there is strong agreement between measured and modeled aerosol
number concentration (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>N</mi><mi mathvariant="normal">mod</mi></msub><mo>/</mo><msub><mi>N</mi><mi mathvariant="normal">meas</mi></msub><mo>=</mo><mn mathvariant="normal">0.87</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="94pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="c2b479e796d070903c50af20686550bd"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-15449-2022-ie00003.svg" width="94pt" height="14pt" src="acp-22-15449-2022-ie00003.png"/></svg:svg></span></span>, <span class="inline-formula"><i>r</i><sup>2</sup>=0.63</span>). The
persistent underestimate of <span class="inline-formula"><i>S</i><sub>a</sub></span> in the model, combined with strong
agreement in modeled and measured aerosol number concentrations, suggests
that model representation of the size distribution of primary emissions or
secondary aerosol formed at the early stages of oxidation may contribute to
the observed differences.</p>
<p>For reactions occurring on small particles, the rate of heterogeneous
reactions is a linear function of both <span class="inline-formula"><i>S</i><sub>a</sub></span> and the reactive uptake
coefficient (<span class="inline-formula"><i>γ</i></span>). To assess the importance of uncertainty in modeled
<span class="inline-formula"><i>S</i><sub>a</sub></span> for the representation of heterogeneous reactions in models, we
compare both the mean and the variance in
<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>S</mi><mrow><mi mathvariant="normal">a</mi><mo>,</mo><mi mathvariant="normal">mod</mi></mrow></msub><mo>/</mo><msub><mi>S</mi><mrow><mi mathvariant="normal">a</mi><mo>,</mo><mi mathvariant="normal">meas</mi></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="71pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="b67f626ae553d4a3e96b43b9e492730d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-15449-2022-ie00004.svg" width="71pt" height="15pt" src="acp-22-15449-2022-ie00004.png"/></svg:svg></span></span> to those in <span class="inline-formula"><i>γ</i></span>(N<span class="inline-formula"><sub>2</sub></span>O<span class="inline-formula"><sub>5</sub></span>)<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi/><mi mathvariant="normal">mod</mi></msub><mo>/</mo><mi mathvariant="italic">γ</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="30pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="90bd37049090e6d148d97c5b49d8b62a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-15449-2022-ie00005.svg" width="30pt" height="14pt" src="acp-22-15449-2022-ie00005.png"/></svg:svg></span></span>(N<span class="inline-formula"><sub>2</sub></span>O<span class="inline-formula"><sub>5</sub></span>)<span class="inline-formula"><sub>meas</sub></span>. We find that
the uncertainty in model representation of heterogeneous reactions is
primarily driven by uncertainty in the parametrization of reactive uptake
coefficients, although the discrepancy between <span class="inline-formula"><i>S</i><sub>a,mod</sub></span> and
<span class="inline-formula"><i>S</i><sub>a,meas</sub></span> is not insignificant. Our analysis suggests that model
improvements to aerosol surface area concentrations, in addition to more
accurate parameterizations of heterogeneous kinetics, will advance the
representation of heterogeneous chemistry in regional models.</p> |
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| ISSN: | 1680-7316 1680-7324 |