Chemical competition in nitrate and sulfate formations and its effect on air quality

Abstract

The competition for bases in nitrate and sulfate aerosol formations significantly affects the concentration of nitrate aerosols. Sensitivity experiments with reduced sulfate precursor emissions, achieved through the use of the CAM-Chem model, show that nitrate concentration is particularly affected by sulfate precursor emissions in winter when their changes are linearly correlated. As a result, the nitrate concentration in the atmosphere is less affected by its own precursor emissions. The concentration-precursor emission relationship for sulfate is not significantly affected by the competition. The future air quality projection following the IPCC A1B emission change scenario shows that the decrease in sulfate precursor emissions and increase in ammonia emissions by 2050 will lead to adverse changes in nitrate concentrations. This will be in response to the reduction in its precursor emissions over the major industrial region of the United States. Due to the difference between nitrate and sulfate aerosols in physical properties and efficiency in cloud condensation, the visibility reduction caused by sulfate emission control occurs over industrial regions in winter. The resulting adverse changes in visibility to emission control will further increase the uncertainty in assessing air quality change. The nonlinear relationship between precursor emissions and changes in pollution levels will increase the difficulty in making effective air quality control strategies.