December 1, 2021
Journal Article

Urban NOx emissions around the world declined faster than anticipated between 2005 and 2019

Abstract

Emission inventory development for air pollutants, by compiling records from individual emission sources, takes many years and involves extensive multi-national effort. A complementary method to estimate air pollution emissions is in the use of satellite remote sensing. In this study, NO2 observations from the Ozone Monitoring Instrument (OMI) are combined with re-analysis meteorology to estimate urban nitrogen oxide (NOX) emissions for 80 global cities between 2005 – 2019. Satellite-based and inventory-based urban NOX emissions both show a 40 – 55% drop (3.5 – 5 % yr-1) in North America and Australasia during the 15-year period. In Europe, Korea, and Japan there is a less pronounced drop of 30 – 50% (2 – 4.5 % yr-1) over the same timeframe. In Europe, Korea, and Japan, inventories consistently underestimate the magnitude of NOX emissions; this disagreement in magnitude between the satellite and inventory may be partially related to the early afternoon satellite measurement and distinctive diurnal emission patterns. In China, inventories fail to capture the timing of urban emission reductions, which appear to have occurred faster (~8% yr-1) in the 2012 – 2015 timeframe than currently reported (0 – 4% yr-1). In developing nations (Latin America, Africa, India), it appears that large projected urban increases in NOX emissions (+1 – 4% yr-1) have not materialized to date. As a result, satellite-based measurements aggregated globally show a larger decrease in urban NOX emissions than currently reported in the inventories. While many of the discrepancies between satellite-based and inventory emissions estimates represent real differences, some of the discrepancies might be related to the assumptions made to compare the satellite-based estimates with inventory estimates. Our work identifies that the three largest uncertainties in the comparison are the tropospheric column measurements, wind speed and direction, and spatial definition of each city.

Published: December 1, 2021

Citation

Goldberg D., S.C. Anenberg, Z. Lu, D.G. Streets, L.N. Lamsal, E.E. Mcduffie, and S.J. Smith. 2021. Urban NOx emissions around the world declined faster than anticipated between 2005 and 2019. Environmental Research Letters 16, no. 11:Art. No. 115004. PNNL-SA-167181. doi:10.1088/1748-9326/ac2c34

Research topics