Possible long-term changes in climate will affect regional meteorology and biogenic emissions, which may lead to either increases or decreases in important urban and regional scale air pollutants including ozone, fine particulates and regional haze. This may result in changes in human exposure to pollutants and changes in the effectiveness of currently proposed air quality management strategies. Key goals of this study are to quantify interactions and feedback effects among complex meteorological and chemical processes and to develop datasets that can be used for both continental and high-resolution urban modeling studies. By developing and evaluating a coupled modeling system for simulating effects of climate on air quality, this study will lead to better tools for developing optimal air pollution control strategies and for protecting human health and quality of life in a changing climate.
Hypothesis #1, Meteorological Dynamics: Future climate scenarios may exhibit changes in meteorological processes including wind speed and turbulent/convective mixing in the planetary boundary layer, and frequency and intensity of synoptic scale events such as stagnation and frontal passage. These changes may lead to greater dispersion and dilution of urban air pollutants, leading to fewer exceedences of air quality standards in urban areas. The increased transport from urban to rural areas is expected to cause increases in rural O3 and PM2.5, possibly leading to increases in exceedences of the 8-hour averaged O3 standard and visibility progress goals.
Hypothesis #2, Hydrological Effects on Ambient Photochemistry: There may be changes in atmospheric water vapor (H2O) concentration and formation of clouds or fog. Increasing concentrations of H2O will lead to increased formation of free radicals that will accelerate photochemical reactions in urban areas. This will lead to increases in rates of production of O3 and PM2.5 and a shift in the urban chemistry toward relatively more NOx-sensitive conditions for control of O3 and PM2.5. Increases in water vapor and clouds will also lead to increased rates of heterogeneous conversion of reactive NOx to uncreative nitric acid (HNO3), which should contribute to a shift toward more NOx-sensitive conditions for O3 and PM2.5 formation.
Hypothesis #3, Actinic Flux: Changes in cloud cover will affect actinic flux, which drives the photochemical formation of secondary air pollutants. Increases in cloud cover would result in reduced formation of O3 and PM2.5. Increased cloud cover would contribute to a shift toward relatively more VOC-sensitive conditions for pollutant formation.
Hypothesis #4, Temperature Effects: Increases in temperature will cause increases in the rates of chemical reactions that produce O3 and PM2.5, thereby accelerating the formation of pollutants in urban areas. Higher temperatures also lead to increased thermal decomposition of peroxyacylnitrates (PAN), which act as short-term reservoirs of reactive NOx. This may lead to decreases in long-range transport of PAN and increases in O3 and PM2.5 formation near urban source areas. Temperature increases may affect the thermodynamic equilibrium of ammonia, nitrates and sulfates resulting in a shift toward lower PM2.5 concentrations. Higher temperature may also reduce condensation of low volatility organic compounds to form secondary organic aerosols (SOA).
Hypothesis #5, Biogenic Emissions: Predicted changes in biogenic emissions associated with possible future climates can substantially alter the composition of the regional atmosphere. Biogenic emissions from vegetation are extremely temperature-sensitive and so are sensitive to the factors controlling leaf temperature: air temperature, solar radiation, wind speed, humidity, and stomatal conductance. Conductance is influenced by water availability (i.e. drought), CO2 and other factors. Other factors that can influence at least some biogenic emissions include air toxics, nutrient availability, and photosynthetic photon flux density. Biogenic emissions are expected to increase with rising temperature and decrease with reductions in incident solar radiation, but the impact of other factors is less clear. Air quality effects of increased biogenic emissions could include increased formation of O3, CO, formaldehyde (HCHO) and SOA. This may cause increased transport of O3 and HCHO from rural regions to suburban and urban regions, thereby causing VOC reductions to become relatively less effective than NOx reductions for attaining urban O3 air quality standards.