Estimating ground-level PM<inf>2.5</inf> using micro-satellite images by a convolutional neural network and random forest approach

TitleEstimating ground-level PM2.5 using micro-satellite images by a convolutional neural network and random forest approach
Publication TypeJournal Article
Year of Publication2020
AuthorsT Zheng, MH Bergin, S Hu, J Miller, and DE Carlson
JournalAtmospheric Environment
Date Published06/2020

PM poses a serious threat to public health, however its spatial concentrations are not well characterized due to the sparseness of regulatory air quality monitoring (AQM) stations. This motivates novel low-cost methods to estimate ground-level PM at a fine spatial resolution so that PM exposure in epidemiological research can be better quantified. Satellite-retrieved aerosol products are widely used to estimate the spatial distribution of ground-level PM . However, these aerosol products can be subject to large uncertainties due to many approximations and assumptions made in multiple stages of their retrieval algorithms. Therefore, estimating ground-level PM directly from satellites (e.g. satellite images) by skipping the intermediate step of aerosol retrieval can potentially yield lower errors because it avoids retrieval error propagating into PM estimation and is desirable compared to current ground-level PM retrieval methods. Additionally, the spatial resolutions of estimated PM are usually constrained by those of the aerosol products and are currently largely at a comparatively coarse 1 km or greater resolution. Such coarse spatial resolutions are unable to support scientific studies that thrive on highly spatially-resolved PM . These limitations have motivated us to devise a computer vision algorithm for estimating ground-level PM at a high spatiotemporal resolution by directly processing the global-coverage, daily, near real-time updated, 3 m/pixel resolution, three-band micro-satellite imagery of spatial coverages significantly smaller than 1 × 1 km (e.g., 200 × 200 m) available from Planet Labs. In this study, we employ a deep convolutional neural network (CNN) to process the imagery by extracting image features that characterize the day-to-day dynamic changes in the built environment and more importantly the image colors related to aerosol loading, and a random forest (RF) regressor to estimate PM based on the extracted image features along with meteorological conditions. We conducted the experiment on 35 AQM stations in Beijing over a period of ~3 years from 2017 to 2019. We trained our CNN-RF model on 10,400 available daily images of the AQM stations labeled with the corresponding ground-truth PM and evaluated the model performance on 2622 holdout images. Our model estimates ground-level PM accurately at a 200 m spatial resolution with a mean absolute error (MAE) as low as 10.1 μg m (equivalent to 23.7% error) and Pearson and Spearman r scores up to 0.91 and 0.90, respectively. Our trained CNN from Beijing is then applied to Shanghai, a similar urban area. By quickly retraining only RF but not CNN on the new Shanghai imagery dataset, our model estimates Shanghai 10 AQM stations' PM accurately with a MAE and both Pearson and Spearman r scores of 7.7 μg m (18.6% error) and 0.85, respectively. The finest 200 m spatial resolution of ground-level PM estimates from our model in this study is higher than the vast majority of existing state-of-the-art satellite-based PM retrieval methods. And our 200 m model's estimation performance is also at the high end of these state-of-the-art methods. Our results highlight the potential of augmenting existing spatial predictors of PM with high-resolution satellite imagery to enhance the spatial resolution of PM estimates for a wide range of applications, including pollutant emission hotspot determination, PM exposure assessment, and fusion of satellite remote sensing and low-cost air quality sensor network information. 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 −3 −3

Short TitleAtmospheric Environment