TEAMPACCC
Trends in Earth's Atmospheric Makeup: Pollution of the Air - Chemistry - Climate Connections
Chemistry - Climate Connections
We seek to understand factors controlling chemistry-climate interactions including the two-way couplings between air pollutants and climate. Reducing emissions of some near-term climate forcing agents (NTCFs) such as methane, ozone, and some aerosols offers the potential to address jointly climate and air quality goals. We study the impacts of climate change on air pollution and the meteorology that drives regional pollutant responses as well as climate responses to changes in NTCFs.
Climate responses to regional air pollution controls
Air pollutant emissions in the US have declined following controls established by the US Clean Air Act (CAA) of 1970. The CAA reduced emissions of air pollutants from mobile and industrial sources, including nitrogen oxides and carbon monoxide, two key precursors to ozone, and sulfur dioxide, which forms sulfate aerosol and acid rain. Beyond their air quality impacts, ozone, aerosols, and their precursors directly and indirectly influence the radiative budget of the atmosphere, acting as short-lived climate forcers. We assessed tropospheric composition and regional climate responses to US air pollution controls by simulating a “world avoided” scenario in which US air pollution controls were never implemented. Using a pair of initial-condition ensembles generated by a fully-coupled chemistry-climate model, CESM2(WACCM6), we quantified the anthropogenically forced signal due to avoided US air pollutant emissions relative to the noise of internal variability (i.e., signal-to-noise ratio) (Elkins et al., 2025). We found increases in tropospheric column ozone, robust to natural internal variability, extending across the Northern Hemisphere. Robust aerosol increases, dominated by changes in sulfate, were localized near the US. While an ensemble-mean US surface cooling signal (-0.4°C) implies that aerosol-driven cooling prevails over any ozone-induced warming, we found that large regional internal variability may confound its detection in any single transient realization. [Steph]
Climate and tropospheric oxidative capacity
The hydroxyl radical (OH) largely controls the tropospheric self-cleansing capacity by reacting with gases harmful to the environment and human health. OH concentrations are determined locally by competing production and loss processes. Lacking strong observational constraints, global models differ in how they balance these processes, such that the sign of past and future OH changes is uncertain. In a warmer climate, OH production will increase due to its water vapor dependence, partially offset by faster OH-methane loss. Weather-sensitive emissions will also likely increase, although their net impact on global mean OH depends on the balance between source (nitrogen oxides) and sink (reactive carbon) gases. Lightning activity increases OH, but its response to climate warming is of uncertain sign. To enable confident projections of OH, we recommend efforts to reduce uncertainties in kinetic reactions, in measured and modeled OH, in proxies for past OH concentrations, and in source and sink gas emissions. (Fiore et al., 2024).
The impact of individual processes on OH is difficult to resolve in fully coupled chemistry-climate models due to their high dimensionality. We use AquaChem, an intermediate in the model hierarchy with full chemical complexity and simplified atmospheric dynamics (and emissions), to attribute the sensitivity of OH to changes in emissions and idealized climate change. [Qindan, Isabella]
