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Discussion papers
https://doi.org/10.5194/acp-2018-889
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-889
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 04 Oct 2018

Research article | 04 Oct 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Effects of Urbanization on Regional Meteorology and Air Quality in Southern California

Yun Li1, Jiachen Zhang1, David J. Sailor2, and George A. Ban-Weiss1 Yun Li et al.
  • 1Department of Civil and Environmental Engineering, University of Southern California, Los Angeles, 90007, USA
  • 2School of Geographical Science and Urban Planning, Arizona State University, Tempe, 85281, USA

Abstract. Urbanization has a profound influence on regional meteorology and air quality in megapolitan Southern California. The influence of urbanization on meteorology is driven by changes in land surface physical properties and land surface processes. These changes in meteorology in turn influence air quality by changing temperature-dependent chemical reactions and emissions, gas-particle phase partitioning, and ventilation of pollutants. In this study we characterize the influence of historical urbanization from before human settlement to present-day on meteorology and air quality in Southern California using the Weather Research and Forecasting Model coupled to chemistry and the single-layer urban canopy model (WRF/Chem-UCM). We assume identical anthropogenic emissions for the simulations carried out, and thus focus on the effect of changes in land surface physical properties and land surface processes on air quality. Historical urbanization has led to daytime air temperature decreases of up to 1.4K, and evening temperature increases of up to 1.7K. Ventilation of air in the LA basin has decreased up to 36.6% during daytime and increased up to 27.0% during nighttime. These changes in meteorology are mainly attributable to higher evaporative fluxes from irrigation, higher thermal inertia from irrigation and building materials, and increased surface roughness from buildings. Changes in ventilation drive changes in hourly NOx concentrations with increases of up to 2.7ppb during daytime and decreases of up to 4.7ppb at night. Hourly O3 concentrations decrease by up to 0.94ppb in the morning, and increase by up to 5.6ppb at other times of day. Changes in O3 concentrations are driven by the competing effects of changes in ventilation and precursor NOx concentrations. PM2.5 concentrations show slight increases during the day, and decreases of up to 2.5μg/m3 at night. Processes drivers for changes in PM2.5 include modifications to atmospheric ventilation, and temperature, which impacts gas-particle phase partitioning for semi-volatile compounds and chemical reactions. Understanding processes drivers for how land surface changes effect regional meteorology and air quality is crucial for decision making on urban planning in megapolitan Southern California to achieve regional climate adaptation and air quality improvements.

Yun Li et al.
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Short summary
In this study, we find that land surface changes via urbanization have caused reductions (increases) in air temperature and ventilation during the day (night) in Southern California. Changes in meteorology have consequently caused (a) increases in NOx and PM2.5 during the day, vice versa at night, and (b) increases in O3 for most times of day. These results are informative for sustainable urban planning to achieve a balance between climate mitigation/adaptation and air quality improvements.
In this study, we find that land surface changes via urbanization have caused reductions...
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