The urban canopy meteorological forcing and its impact on ozone and PM2.5: role of the vertical turbulent transport
Abstract
Urban surfaces due to specific geometry and physical properties bring modified transport of momentum, moisture and heat between them and the air above and perturb the radiative, thermal and mechanical balance resulting in changed meteorological condition (e.g. the UHI - urban heat island phenomenon). From an air quality perspective, many studies argue that one of the most important changes is the increased turbulence enhancing vertical mixing of pollutants above cities, although increased temperatures and wind stilling play an important role too.
Using the regional climate model RegCM4 coupled to chemistry transport model CAMx over central Europe we study how urban surfaces affect the vertical turbulent transport of selected pollutants through modifications of the vertical eddy diffusion coefficient (Kv). For the period of 2007-2011 and over central Europe numerous experiments are carried out in order to evaluate the impact of six different methods for Kv calculation on the surface concentrations as well as vertical profiles of ozone and PM2.5 over selected cities (Prague and Berlin).
Three cascading domains are set up at 27 km, 9 km and 3 km resolutions, which further enables to analyze the sensitivity to model grid resolution. Numerous experiments are performed where urban surfaces are considered or replaced by rural ones in order to isolate the urban canopy meteorological forcing.
Apart from the well pronounced and expected impact on temperature (increases up to 2 °C) and wind (decreases up to -2 m s-1) there is strong impact on vertical eddy diffusion in all of the six Kv methods. The Kv enhancement ranges from a few 0.5 up to 30 m2 s-1 at the surface and from 1 to 100 m2 s-1 at higher levels depending on the methods, while the turbulent kinetic energy (TKE) based methods produce the largest impact.
The range of impact on the vertical eddy diffusion coefficient propagates to a range of ozone (O3) increase of 0.4 to 4 ppbv near the surface in both summer and winter, while at higher levels, decreases occur from a few -0.4 ppbv to as much as -2 ppbv. In case of PM2.5, enhanced vertical eddy diffusion leads to decrease of near surface concentrations ranging from almost zero to -1 μg m-3 in summer and to decreases from -0.5 to -2 μg m-3 in winter.
Comparing these results to the total-impact, i.e. to the impact of all considered urban meteorological changes, we can conclude that much of the overall urban meteorological forcing is explained by acting of the enhanced vertical eddy diffusion, which counterweights the opposing effects of other components of this forcing (temperature, humidity and wind impact). The results further show that this conclusion holds regardless of the resolution chosen and in both the warm and cold part of the year.
Our study demonstrates the dominant role of turbulent transport of pollutants above urban areas and stresses the need for further investigation how variation of urban land-use influence the pollutant transport from the urban canopy.