Historical Changes in Air Chemistry and Their Influence on Cloud Condensation Nuclei Formation in the Eastern United States (Invited Presentation)

Gases such as sulfur compounds, nitrogen oxides, and hydrocarbons are key precursors of primary aerosols that, depending on their chemical composition, can activate cloud condensation nuclei and thus influence precipitation processes. In the United States (USA), environmental regulation has led to major decreases in sulfur and nitrogen oxides emissions during the last 50 years. At the same time, land-use changes and atmospheric warming have contributed to greater emissions of plant-emitted reactive hydrocarbons, which are precursors to secondary organic aerosols (SOAs). As a result, it is plausible that detectable changes in aerosol chemical composition and subsequent impacts on cloud formation, precipitation, and aerosol wet scavenging have likely occurred in the eastern USA. Thus, in this presentation, we will summarize research results associated with the hypotheses that historical changes in biogenic organic sources, inorganic aerosols such as sulfate and SOAs, have influenced the aerosol composition and cloud condensation nuclei efficiencies during the last decades. We will present and discuss historical hydrocarbon emissions in the context of vegetation distribution patterns in the eastern USA. We will summarize the magnitude of decreases in aerosol precursors such as sulfur dioxide and nitrogen oxides. We will provide evidence on the chemical composition characteristics, hygroscopicity and size distributions of aerosols formed from the oxidation of biogenic hydrocarbons such as isoprene and monoterpenes that produce SOAs, and changes in inorganic aerosols due to reduced levels of sulfur dioxide, and nitrogen oxides from photochemical numerical models. These models will also provide evidence on the emerging cloud condensation nuclei efficiencies of aerosols due to land use changes, and emissions reductions in the eastern USA over the past decades. The main conclusion derived from the current research confirms that aerosol chemical attributes have changed in the last few decades, and such changes have influenced the cloud condensation nuclei properties as a function of both size and relative abundance of sulfate versus organics.