Impact of urban canopy meteorological forcing on aerosol concentrations

Abstract

Abstract. The regional climate model RegCM4 extended with the land surface model CLM4.5 was coupled to the chemistry transport model CAMx to analyze the impact of urban meteorological forcing on surface fine aerosol (PM2.5) concentrations for summer conditions over the 2001–2005 period, focusing on the area of Europe. Starting with the analysis of the meteorological modifications caused by urban canopy forcing, we found a significant increase in urban surface temperatures (up to 2–3 K), a decrease of specific humidity (by up to 0.4–0.6 gkg−1), a reduction of wind speed (up to −1 ms−1) and an enhancement of vertical turbulent diffusion coefficient (up to 60–70 m2s−1). These modifications translated into significant changes in surface aerosol concentrations that were calculated by a “cascading” experimental approach. First, none of the urban meteorological effects were considered. Then, the temperature effect was added, then the humidity and the wind, and finally, the enhanced turbulence was considered in the chemical runs. This facilitated the understanding of the underlying processes acting to modify urban aerosol concentrations. Moreover, we looked at the impact of the individual aerosol components as well. The urbanization-induced temperature changes resulted in a decrease of PM2.5 by −1.5 to −2 µg m−3, while decreased urban winds resulted in increases by 1–2 µg m−3. The enhanced turbulence over urban areas resulted in decreases of PM2.5 by −2 µg m−3. The combined effect of all individual impact depends on the competition between the partial impacts and can reach up to −3 µg m−3 for some cities, especially when the temperature impact was stronger in magnitude than the wind impact. The effect of changed humidity was found to be minor. The main contributor to the temperature impact is the modification of secondary inorganic aerosols, mainly nitrates, while the wind and turbulence impact is most pronounced in the case of primary aerosol (primary black and organic carbon and other fine particle matter). The overall as well as individual impacts on secondary organic aerosol are very small, with the increased turbulence acting as the main driver. The analysis of the vertical extent of the aerosol changes showed that the perturbations caused by urban canopy forcing, besides being large near the surface, have a secondary maximum for turbulence and wind impact over higher model levels, which is attributed to the vertical extent of the changes in turbulence over urban areas. The validation of model data with measurements showed good agreement, and we could detect a clear model improvement in some areas when including the urban canopy meteorological effects in our chemistry simulations.

Urban canopy meteorological forcing and its impact on ozone and PM<sub>2.5</sub>: role of vertical turbulent transport

“…Higher urban temperatures in connection with UHI directly modify chemical reaction rates and aerosol nucleation as well as indirectly modify dry deposition and wet scavenging rates (Seinfeld and Pandis, 1998). Although higher temperatures favor ozone formation (Im et al, 2011), in urban areas, the situation can be different. Huszar et al (2018a) showed that due to higher temperatures alone, surface ozone in urban areas is reduced, while the main contribution is given by increased dry deposition velocities and increased flux of nitrogen oxides (NO x ) towards nitric acid (HNO 3 ).…”

The impact of urban land-surface on extreme air pollution over central Europe

“…As the urban canopy meteorological forcing (UCMF) induced NO 2 changes are caused primary by vertical turbulence transport (Huszar et al, 2020), the amount of removed material (i.e., the NO 2 ) is expected to be proportional to the absolute amount of that material. This could explain the larger change for the high end of the distribution.…”

“…In summary, the urban canopy layer forces the air within and above the canopy layer towards modified physical properties (temperature, humidity, wind speed, etc. ), and therefore we adopt here the term "urban canopy meteorological forcing" (UCMF) introduced recently by Huszar et al (2020).…”

“…While it has been clear that a very strong link must exist between air pollution, vertical eddy diffusion, and, in general, the structure of the urban PBL (Masson et al, 2008), it has now been shown too by many authors that the component that explains much of the UCMF-induced concentration changes is the vertical eddy transport and its urban-induced modifications (e.g., Wang et al, 2007Wang et al, , 2009Zhu et al, 2015;Huszar et al, 2020). Using regional-scale modeling techniques, Martilli et al (2003), Sarrat et al (2006), Struzewska and Kaminski (2012), and Wang et al (2007Wang et al ( , 2009 showed that enhanced vertical eddy transport over cities results in a decrease in primary gaseous pollutants (NO x , CO) but leads to an increase in ozone due to reduced titration.…”

Authors response on Referee #3's comments

“…Authors response: Our paper is based on previous research made to investigate the role the urban canopy plays in controlling the city scale meteorological conditions and consequently the chemistry and transport of pollutants. The mutual links between meteorology and gas-phase chemistry (NOx-ozone) in urban areas is detailed in Huszar et al(2018a), whereas in Huszar et al(2018b) we extended this analysis to primary and secondary aerosols. Finally in Huszar et al(2020), after identifying that vertical eddy transport plays the most important role, we focused on this aspect of the urban canopy meteorological forcing on chemistry.…”