Effects of urbanization on regional meteorology and air quality in Southern California

Abstract

Abstract. Urbanization has a profound influence on regional meteorology and air quality\nin megapolitan Southern California. The influence of urbanization on\nmeteorology is driven by changes in land surface physical properties and land\nsurface processes. These changes in meteorology in turn influence air quality\nby changing temperature-dependent chemical reactions and emissions,\ngas–particle phase partitioning, and ventilation of pollutants. In this study\nwe characterize the influence of land surface changes via historical\nurbanization from before human settlement to the present day on meteorology and\nair quality in Southern California using the Weather Research and Forecasting\nModel coupled to chemistry and the single-layer urban canopy model\n(WRF–UCM–Chem). We assume identical anthropogenic emissions for the\nsimulations carried out and thus focus on the effect of changes in land\nsurface physical properties and land surface processes on air quality.\nHistorical urbanization has led to daytime air temperature decreases of up to\n1.4\u2009K and evening temperature increases of up to 1.7\u2009K. Ventilation of air\nin the LA basin has decreased up to 36.6\u2009% during daytime and increased\nup to 27.0\u2009% during nighttime. These changes in meteorology are mainly\nattributable to higher evaporative fluxes and thermal inertia of soil from\nirrigation and increased surface roughness and thermal inertia from\nbuildings. Changes in ventilation drive changes in hourly\n NOx concentrations with increases of up to 2.7\u2009ppb during\ndaytime and decreases of up to 4.7\u2009ppb at night. Hourly O3 \nconcentrations decrease by up to 0.94\u2009ppb in the morning and increase by up\nto 5.6\u2009ppb at other times of day. Changes in O3 concentrations are\ndriven by the competing effects of changes in ventilation and precursor\n NOx concentrations. PM 2.5 concentrations show slight\nincreases during the day and decreases of up to 2.5\u2009 µ g\u2009m −3 \nat night. Process drivers for changes in PM 2.5 include modifications\nto atmospheric ventilation and temperature, which impact gas–particle phase\npartitioning for semi-volatile compounds and chemical reactions.\nUnderstanding process drivers related to how land surface changes effect\nregional meteorology and air quality is crucial for decision-making on urban\nplanning in megapolitan Southern California to achieve regional climate\nadaptation and air quality improvements.