Ozone Air Quality Implications of Large-Scale Albedo and Vegetation

Modifications in the Los Angeles Basin

H. Taha

Increasing the urban surface albedo (solar reflectance) and urban reforestation are two innovative measures that have the potential to reduce the formation of photochemical smog. Implementing these strategies can reduce regional air temperatures, the need for cooling energy, and associated emissions of oxides of nitrogen from power plants. Lowered air temperatures would also reduce temperature-dependent emissions of reactive organic gases from vegetation, reduce anthropogenic evaporative losses from mobile sources, and slow down the chemical reactions responsible for producing photochemical smog. One region targeted for the implementation of these strategies is the Los Angeles Basin, whose summer mesoscale meteorology and air quality are modeled in this project.

The meteorological and photochemical modeling work performed in this study indicates that implementing high-albedo materials and urban vegetation in the Los Angeles Basin would have a net effect of reducing ozone concentrations (see Figure). For example, if the albedo of roofs is increased by 0.35 and that of pavements by 0.25, and two trees were added to each house, the domain-wide population-weighted exceedance exposure to ozone above the National Ambient Air Quality Standard (120 parts per billion (ppb) ozone) would be decreased by up to 20% (or more) during a typical summer day in late August.

Figure. Changes in ozone concentrations (ppb) at 3 p.m. on August 27, due to increased vegetation cover in the Los Angeles Basin. The simulations were based on the assumption that between 16 and 20 million trees have been planted in the region.

These improvements in air quality are on the same order of magnitude as those gained from implementing other major strategies, such as mobile source emission control. Based on 1987 emission inventories, the simulations indicate that the air quality benefits of albedo and vegetation increase strategies are comparable to those of converting all mobile sources operating in 1987 in the Basin to zero-emitting vehicles. These findings are for ozone reductions only; removing or converting motor vehicles have several other advantages as well. At this time, this comparison is preliminary since there are uncertainties in the modeling system and emission inventories. These findings will be updated as other episodes are studied and more representative emission inventories become available. This study also identified those tree species (hydrocarbon-emitting trees) that should not be planted because they would worsen the air quality.

In its 1994 Air Quality Management Plan the South Coast Air Quality Management District has considered high-albedo materials and urban vegetation as potential strategies for improving the ozone air quality. Our modeling studies are currently paving the way for the development of a RECLAIM equivalent for high-albedo materials and low-emitting urban trees. (RECLAIM is an acronym for the Regional Clean Air Incentive Market, developed for the California South Coast Air Basin.)

References

Taha H. Modeling the impacts of increased urban vegetation on the ozone air quality in the South Coast Air Basin. Accepted for publication in Atmospheric Environment, 1995.

Taha H. Modeling the impacts of large-scale albedo changes on ozone air quality in the South Coast Air Basin. Submitted to Atmospheric Environment, 1995.

Taha H, Douglas S, Haney J. Mesoscale meteorological and air quality impacts of increased urban albedo and vegetation. Submitted to Energy and Buildings-Special Issue on Urban Heat Islands and Cool Communities, 1995.

Taha H, Douglas S, Haney J, Winer A, Benjamin M, Hall D, Hall J, Liu X, Fishman B. Modeling the Ozone Air Quality Impacts of Increased Albedo and Urban Forest in the South Coast Air Basin. Lawrence Berkeley National Laboratory Report No. LBL-37316, 1995.

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