Exposure to ambient PM_2.5 is the leading global environmental risk factor for mortality and disease burden, with associated annual global welfare costs of trillions of dollars, However, no one knows what city has the highest level of the pollution.

In an open-access paper recently published in Atmospheric Environment: X, an international team of researchers outline the extent of the gap between what researchers know and don’t know about on-the-ground levels of fine particulate matter.

The health impacts of PM_2.5 start with cardiovascular damage and get worse. The Organization for Economic Co-operation and Development (OECD) estimates worldwide, the cost associated with premature deaths from pollution will rise from $3 trillion in 2015 to $18-25 trillion by 2060.

Understanding the amount of pollution in the air requires on-the-ground monitoring, but many cities—ostensibly some with high PM_2.5 levels—have no monitoring at all. In India, for example, there is just one monitor for every 6.8 million people. Such sparse monitoring fails to represent pollution variability.

Despite the importance of PM_2.5, we find that insufficient monitoring data exist to answer this basic question about the spatial pattern of PM_2.5 at the global scale. Only 24 of 234 countries have more than 3 monitors per million inhabitants, while density is an order of magnitude lower in the vast majority of the world’s countries, with 141 having no regular PM_2.5 monitoring at all. The global mean population distance to nearest PM_2.5 monitor is 220 km, too large for exposure assessment.

Efforts to fill in monitoring gaps with estimates from satellite remote sensing, chemical transport modeling, and statistical models have biases at individual monitor locations that can exceed 50 μg m⁻³. Progress in advancing knowledge about the global distribution of PM_2.5 will require a harmonized network that integrates different types of monitoring equipment (regulatory networks, low-cost monitors, satellite remote sensing, and research-grade instrumentation) with atmospheric and statistical models.

—Martin et al.

The researchers found that just about 9% of the world’s population lives in areas that have more than three monitors per million people. About 18% of the population live in areas without monitoring.

Although the health effects of PM_2.5 are valued at a few percent of global GDP, PM_2.5 is grossly under-monitored. The difference between the importance of this measure and the level of ground-based monitoring is shocking.

—Randall Martin, professor in the Department of Energy, Environmental ＆ Chemical Engineering in the McKelvey School of Engineering at Washington University in St. Louis and lead author

Beyond mitigating health concerns, a better understanding of pollution on the ground is important for several reasons. For Martin, whose research sits at the intersection of remote sensing and global modeling, having accurate data is the only way to develop accurate models.

Part of our analysis involves relating what satellites measure to ground-level PM_2.5. If we relied only on ground-based monitors, we would have insufficient information.

—Professor Martin

In order to improve the ground-level data, the researchers propose a robust “integrated monitoring framework” composed of different types of monitoring equipment focused on the most densely populated areas, or even on those most prone to variability. They calculate that in order to achieve a goal of one monitor per million people, thousands of new monitors will need to come online.

In addition, they suggest this framework should include ground-based aerosol measurements, innovative low-cost monitors, mobile monitoring and measurements at different points along the vertical (by aircraft, for example). Integrating these systems would not only help researchers gain a more comprehensive picture of the PM_2.5 levels on the ground, but would assist with air quality forecasting and help to improve atmospheric models.

Resources

• Randall V. Martin, Michael Brauer, Aaron van Donkelaar, Gavin Shaddick, Urvashi Narain, Sagnik Dey (2019) “No one knows which city has the highest concentration of fine particulate matter,” Atmospheric Environment: X, Volume 3, 100040 doi: 10.1016/j.aeaoa.2019.100040