During the period analyzed, 92% of the population of China experienced >120 hours of unhealthy air (as defined by the US EPA standard), and 38% experienced average concentrations that were unhealthy. China’s population-weighted average exposure to PM_2.5 was 52 μg/m³.

Average air pollution maps. Maps of average pollutant concentration for PM2.5, PM10, and O3 for eastern China (top row) and the Beijing to Shanghai corridor (bottom row). Concentrations are shown using color gradients and contour lines; the colors (green, yellow, etc.) represent US EPA qualitative health impacts. Pollution concentrations were computed as described in the text using hourly data and then the hourly concentration fields were averaged over the four month study duration. Rohde and Muller. Click to enlarge.

China began development of a national Air Reporting System in 2012; the system includes 945 sites in 190 cities that report hourly via the Internet. Provincial governments perform air quality monitoring at 600 additional locations that are not yet integrated into the national system.

The Berkeley Earth authors used two different third-party sources to gain access to the 4-month data set: PM25.in and AQICN.org. PM25.in is a direct mirror of data from the 945 stations in China’s national network, while AQICN.org is the world’s largest aggregator of real-time air quality data and included many additional sites in China and surrounding areas.

After removing stations with a high percentage of missing values or with other quality control problems, the Berkeley Earth study used 880 national network sites, 640 other sites in China and Taiwan, and 236 sites in other countries within 500 km of China (mostly South Korea). The air quality network is skewed towards urban areas.

The authors calculated the change in mortality due to PM_2.5 air pollution by adopting the integrated exposure response function approach—this considers the relative risk of death for five disease classes (stroke, ischemic heart disease, lung cancer, chronic obstructive pulmonary disease, and lower respiratory infection) and which was adopted by World Health Organization.

Pollution emission is often localized, they noted, especially in the Beijing to Shanghai corridor where many of the highest PM concentrations also occur. Most of the largest emissions appear in or near urban areas (e.g. Handan, Shijazhuang, Zibo, Tangshan, Linyi, Hangshou), though not all major cities have high pollution fluxes (e.g. Chongqing, Chengdu, Wuhan).

Within the study region, 10% of the area is responsible for 34% of the PM_2.5 emissions, and 5% of the area is responsible for 22% of emissions. However, small and moderate sources are also important. Approximately 37% of the study region had PM_2.5 fluxes >0.5 μg/m³/hr, sufficient to exceed US EPA standards after only 3 days of stagnant air.

… Though most of China is subject to potentially harmful levels of PM^2.5, some large population centers (Chongqing, Wuhan, Chengdu) emit less than half the PM^2.5 of others. Among northeastern cities, Beijing has relatively low emissions except for NO₂. Low SO₂ fluxes may indicate cities that benefit from lower coal usage or better smokestack pollution controls. Compared to natural gas, coal produces 150 to 400 times more PM for the same energy delivered. China has plans for new coal plants in the next decade that could effectively double their coal consumption, potentially exacerbating the problem of air pollution.

—Rohde and Muller

Berkeley Earth hopes to expand geographic coverage to include more of Asia, the US, and Europe, and to study how sources of air pollution change with time.

Resources

• Robert A. Rohde, Richard A. Muller (2015) “Air Pollution in China: Mapping of Concentrations and Sources”