Mesoscale meteorological and air quality impacts of increased urban albedo and vegetation

The large scale implementation of high-albedo building materials and urban surfaces and the reforestation of low vegetation urban areas are being encouraged as energy-saving measures. These strategies will result in modification of the physical properties of millions of buildings (e.g., roof reflectance) and their microclimates (e.g., shading, wind, and evapotranspiration effects of trees). This paper is about the atmospheric impacts of regional scale changes in building properties, paved-surface characteristics, and their microclimates. It discusses the possible meteorological and ozone air quality impacts of increases in surface albedo and urban trees in California's South Coast Air Basin (SoCAB). The photochemical model simulations of a late August period indicate that implementing high-albedo materials in the SoCAB would have a net effect of reducing ozone concentrations. Domain-wide population-weighted exceedance exposure to ozone above the California Ambient Air Quality Standard would be decreased by up to 12% during peak afternoon hours. With respect to the National Standard, exceedance exposure would be reduced by up to 17%. The simulations also indicate that the net effect of increased urban vegetation is a decrease in ozone concentrations if the additional vegetation (trees) are low emitters of biogenic hydrocarbons. With respect to the California standard, domain-wide population-weighted exceedance exposure to ozone above this threshold would be decreased by up to 14% during peak afternoon hours. With respect to the National Standard, the reduction would be up to 22%. In terms of total daytime exposure, these strategies can decrease exceedance exposure by up to 12% with respect to the California Standard and up to 20% with respect to the National Standard. Comparing the simulated air quality impacts of increased albedo and vegetation cover with the impacts of other strategies reveals that they are of the same order of magnitude. For instance, the simulations for this episode, using updated 1987 emission inventories for the SoCAB, indicate that the air quality benefits of albedo and vegetation increase strategies are comparable to those of converting at least 50% of the mobile sources operating in 1987 in the SoCAB to zero emitting vehicles (these findings are for ozone reductions only; removing or converting motor vehicles has several other advantages as well). At this time, this comparison is preliminary as there are uncertainties in the modeling system and emission inventories. In particular, mobile source emissions may be underestimated by as much as two-fold. These findings will be updated when other episodes are modeled and more representative emission inventories become available.