Abstract
Urban air chemistry is characterized by measurements of gas and aerosol composition. These measurements are interpreted from a long history for laboratory and theoretical studies integrating chemical processes with reactant (or emissions) sources, meteorology and air surface interaction. The knowledge of these latter elements and their changes have enabled chemists to quantitatively account for the averages and variability of chemical indicators. To date, the changes are consistent with dominating energy-related emissions for more than 50 years of gas phase photochemistry and associated reactions forming and evolving aerosols. Future changes are expected to continue focusing on energy resources and transportation in most cities. Extreme meteorological conditions combined with urban surface exchange are also likely to become increasingly important factors affecting atmospheric composition, accounting for the past leads to projecting future conditions. The potential evolution of urban air chemistry can be followed with three approaches using observations and chemical transport modeling. The first approach projects future changes using long term indicator data compared with the emission estimates. The second approach applies advanced measurement analysis of the ambient data. Examples include statistical modeling or evaluation derived from chemical mechanisms. The third method, verified with observations, employs a comparison of the deterministic models of chemistry, emission futures, urban meteorology and urban infrastructure changes for future insight.
Highlights
Atmospheric chemistry began in the mid-18th century with Rutherford’s discovery of nitrogen dioxide (NO2 ) and Priestley’s discovery of oxygen in air
A special case focusing on urban conditions emerged driven by increasing concerns for adverse health effects associated with air pollution exposure
The extrapolation of future conditions’ trends directly from changes in the ambient concentrations depends on the assumptions that meteorological conditions are constant on average, background concentrations have minimal variability and local source change is captured by emission inventories
Summary
Atmospheric chemistry began in the mid-18th century with Rutherford’s discovery of nitrogen dioxide (NO2 ) and Priestley’s discovery of oxygen in air. During the course of studies after the 1970s, estimates of the change in ambient composition using models combining emissions, meteorology and chemistry along with measurements were made to characterize the spatial and temporal distributions of gas and aerosol properties These studies created a large data base for forecasting changes in the composition of the urban atmosphere mainly with changes in a relatively constrained “business-as-usual” range of pollutant emissions. A response to such changes can be described using three modes of study These include investigation, based on trends in gas and particle composition, the interpretation of mechanisms through atmospheric measurement campaigns, and the application of modeling with expanded hypothetical meteorological and emissions changes. It is anticipated that aerosol source and formation mechanisms will continue to evolve with advancing knowledge
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