Exploring the role of climate action in transboundary air pollution inequality using GEOS-Chem adjoint sensitivities

Introduction

Given its atmospheric lifetime of days to a week, fine particulate matter (PM_2.5) – an air pollutant responsible for adverse health effects – can be advected far beyond its sources and across political borders. This uncomfortable exchange leads to inequality, in which a country may bear air pollution-related health impacts associated with anthropogenic emission activity beyond its borders. Climate action is irrevocably linked to this inequality. Anthropogenic sources of greenhouse gases often co-emit air pollutants and their chemical precursors; thus, action targeting greenhouse gases can result in air pollution-related co-benefits that are realized through improvements in air quality.

 

Methods

Despite this close linkage, much of the research on co-benefits has focused on quantifying benefits in terms of magnitude or absolute number. This ignores how atmospheric composition source-receptor dynamics could be affected by climate action. In this study, we quantify how air pollution exchanges within and between regions (e.g., Africa and Europe) could vary across different shared socioeconomic pathways (SSPs) and representative concentration pathways (RCPs). We perform thousands of GEOS-Chem adjoint simulations to calculate the sensitivities of country-scale PM_2.5 exposures to their precursor emissions. We then combine these simulated sensitivities with gridded emission projections for the SSPs and RCPs to determine how source-receptor relationships between specific countries and regions may differ across different climate futures. Lastly, we leverage methods from the Global Burden of Disease study to assess the impacts of transboundary air pollution exchange on human health.

 

Results and Discussion

We find that reductions in anthropogenic emissions from climate action in more sustainable futures (SSP1-19) results in more co-benefits (480 thousand deaths avoided) than worst-case scenario fragmentation futures (SSP3-60) (140 thousand deaths avoided). In sustainable futures (SSP1), African countries have more influence on climate co-benefits; they contribute between 2.3-2.8 times as many benefits to European countries than vice-versa. However, in fragmented futures (SSP3), this dynamic flips and African countries become more dependent on European climate action as they contribute between 1.7-0.4 times as many benefits. Ultimately, this suggests that changes in anthropogenic emissions associated with climate action have the capacity to not only affect atmospheric composition through improved air quality but also to modify exchange relationships between individual countries and regions.