A Geographically Weighted Gaussian Process Regression Emulator of the GCHP 13.0.0 Global Air Quality Model

Short Summary: We developed a fast and accurate computer tool that predicts how air pollution levels will change around the world under different climate and policy choices. Using machine learning and real model data, our tool can estimate changes in harmful fine particulate pollution in seconds instead of thousands of hours. This makes it easier for researchers and policymakers to explore future air quality and health impacts under a wide range of scenarios.

Abstract: Air quality modelling has been an essential tool to study the impacts of socio-economic changes and policies on air quality and associated social costs due to human health impacts. However, high computational and human resource demands limit the use of state-of-the-art air quality models outside of the atmospheric science community. 

We address this limitation by training Geographically Weighted Gaussian Process Regressors (GW-GPR) on the outputs of a series of perturbation experiments from the high-fidelity GEOS-Chem High Performance global chemical transport model (GCHP 13.0.0). The Gaussian Process Regressor relates changes in annual mean surface anthropogenic PM_2.5 to changes in short-lived air pollutant emissions and atmospheric CH₄ and CO₂ levels for each GCHP model grid cell. In comparison to existing widely adopted linearized and regionalized approaches, our method can account for sub-regional changes in air pollutant emission patterns and incorporates the non-linear response of secondary air pollutants to precursor and greenhouse gas emissions. 

We evaluate and demonstrate the utility of our model by predicting the global distribution of PM_2.5 in 2050 (relative to 2014) under 4 sets of climate and air pollution control policy scenarios. The emulator reproduces grid cell-level changes in anthropogenic PM_2.5 (R² = 0.94 – 0.99 over the 4 scenarios tested), and associated global changes in premature mortalities at 95 % confidence level, while requiring < 10 seconds of CPU time (vs. ~3000 CPU hours for GCHP) for each scenario. The emulator is also able to capture projected global trends of population-weighted PM_2.5 from the AerChemMIP ensemble within the ensemble range. 

To our knowledge, the GW-GPR emulator is the first global-scale emulator operating at grid cell level with explicit consideration of non-linearities in atmospheric chemistry, climate change, and uncertainties resulting from both chemistry and climate variability. The accuracy, speed and simplicity of the emulator also show the capability of machine learning algorithms in emulating global air quality models, and make air quality modelling accessible for global climate/air pollution scenario analysis and integrated assessment.