Abstract
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 land surface changes via historical urbanization from before human settlement to the 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–UCM–Chem). 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 1.4 K and evening temperature increases of up to 1.7 K. 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 and thermal inertia of soil from irrigation and increased surface roughness and thermal inertia from buildings. Changes in ventilation drive changes in hourly NOx concentrations with increases of up to 2.7 ppb during daytime and decreases of up to 4.7 ppb at night. Hourly O3 concentrations decrease by up to 0.94 ppb in the morning and increase by up to 5.6 ppb 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 2.5 µg m−3 at night. Process drivers for changes in PM2.5 include modifications to atmospheric ventilation and temperature, which impact gas–particle phase partitioning for semi-volatile compounds and chemical reactions. Understanding process drivers related to 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.
Highlights
The world has been undergoing accelerated urbanization since the industrial revolution in the 19th century (Grimm et al, 2008; Seto et al, 2012)
The comparison between modeled PM2.5 concentrations and daily averaged observations derived from hourly beta attenuation monitoring (BAM) measurements is discussed in the Supplement Sect
We have characterized the impact of land surface changes via urbanization on regional meteorology and air quality in Southern California using an enhanced version of WRF–UCM–Chem
Summary
The world has been undergoing accelerated urbanization since the industrial revolution in the 19th century (Grimm et al, 2008; Seto et al, 2012). Urbanization leads to profound human modification of the land surface and its associated physical properties such as roughness, thermal inertia, and albedo (Fan et al, 2017) and land surface processes like irrigation (Vahmani and Hogue, 2014). These changes in land surface physical properties and processes alter corresponding surface–atmosphere coupling including exchange of water, momentum, and energy in urbanized regions (Vahmani and Ban-Weiss, 2016a; Li et al, 2017), which exerts an important influence on regional meteorology and air quality (Vahmani et al, 2016; Civerolo et al, 2007).
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