Urban canopy meteorological forcing and its impact on ozone and PM<sub>2.5</sub>: role of vertical turbulent transport
<p>It is well known that the urban canopy (UC) layer, i.e., the layer of air corresponding to the assemblage of the buildings, roads, park, trees and other objects typical to cities, is characterized by specific meteorological conditions at city scales generally differing from those over rural...
| Main Authors: | , , , , , , , , |
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2020-02-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/20/1977/2020/acp-20-1977-2020.pdf |
| Summary: | <p>It is well known that the urban canopy (UC) layer, i.e., the layer of air corresponding to the assemblage of the buildings, roads, park, trees and other objects typical to cities, is characterized by specific meteorological conditions at city scales generally differing from those over rural surroundings. We refer to the forcing that acts on the meteorological variables over urbanized areas as the urban canopy meteorological forcing (UCMF). UCMF has multiple aspects, while one of the most studied is the generation of the urban heat island (UHI) as an excess of heat due to increased absorption and trapping of radiation in street canyons. However, enhanced drag plays important role too, reducing mean wind speeds and increasing vertical eddy mixing of pollutants. As air quality is strongly tied to meteorological conditions, the UCMF leads to modifications of air chemistry and transport of pollutants. Although it has been recognized in the last decade that the enhanced vertical mixing has a dominant role in the impact of the UCMF on air quality, very little is known about the uncertainty of vertical eddy diffusion arising from different representation in numerical models and how this uncertainty propagates to the final species concentrations as well as to the changes due to the UCMF.</p>
<p>To bridge this knowledge gap, we set up the Regional Climate Model version 4 (RegCM4) coupled to the Comprehensive Air Quality Model with Extensions (CAMx) chemistry transport model over central Europe and designed a series of simulations to study how UC affects the vertical turbulent transport of selected pollutants through modifications of the vertical eddy diffusion coefficient (<span class="inline-formula"><i>K</i><sub>v</sub></span>) using six different methods for <span class="inline-formula"><i>K</i><sub>v</sub></span> calculation. The mean concentrations of ozone and PM<span class="inline-formula"><sub>2.5</sub></span> in selected city canopies are analyzed. These are secondary pollutants or having secondary components, upon which turbulence acts in a much more complicated way than in the case of primary pollutants by influencing their concentrations not only directly but indirectly via precursors too. Calculations are performed over cascading domains (of 27, 9, and 3 km horizontal resolutions), which further enables to analyze the sensitivity of the numerical model to grid resolution. A number of model simulations are carried out where either urban canopies are considered or replaced by rural ones in order to isolate the UC meteorological forcing. Apart from the well-pronounced and expected impact on temperature (increases up to 2 <span class="inline-formula"><sup>∘</sup></span>C) and wind (decreases by up to 2 <span class="inline-formula">ms<sup>−1</sup></span>), there is a strong impact on vertical eddy diffusion in all of the six <span class="inline-formula"><i>K</i><sub>v</sub></span> methods. The <span class="inline-formula"><i>K</i><sub>v</sub></span> enhancement ranges from less than 1 up to 30 <span class="inline-formula">m<sup>2</sup> s<sup>−1</sup></span> at the surface and from 1 to 100 <span class="inline-formula">m<sup>2</sup> s<sup>−1</sup></span> at higher levels depending on the methods. The largest impact is obtained for the turbulent kinetic energy (TKE)-based methods.</p>
<p>The range of impact on the vertical eddy diffusion coefficient propagates to a range of ozone (<span class="inline-formula">O<sub>3</sub></span>) increase of 0.4 to 4 ppbv in both summer and winter (5 %–10 % relative change). In the case of PM<span class="inline-formula"><sub>2.5</sub></span>, we obtained decreases of up to 1 <span class="inline-formula">µg m<sup>−3</sup></span> in summer and up to 2 <span class="inline-formula">µg m<sup>−3</sup></span> in winter (up to 30 %–40 % relative change). Comparing these results to the “total-impact”, i.e., to the impact of all meteorological<span id="page1978"/> modifications due to UCMF, we can conclude that much of UCMF is explained by the enhanced vertical eddy diffusion, which counterbalances the opposing effects of other components of this forcing (temperature, humidity and wind). The results further show that this conclusion holds regardless of the resolution chosen and in both the warm and cold parts of the year.</p> |
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| ISSN: | 1680-7316 1680-7324 |