The concentration of airborne particulate matter (PM2.5) and sulphur dioxide (SO2) decreased from 2015 to 2017 at more than half of China’s 1000 air quality monitoring stations. But amounts of ground level ozone increased at half the sites over the same period, according to new analysis.

The lowest PM2.5 concentrations — 20-25 µg m-3 — were in Hong Kong, Taiwan and Tibet. While the downward trend is encouraging, these figures are still above World Health Organization guidelines. Reductions in the concentration of sulphur dioxide were also widespread, with 59% of monitoring stations in the study reporting a significant reduction. On average, levels decreased by 3.4 µg m-3 per year.

Falling levels of PM2.5 are in line with satellite studies but the latest analysis, which includes data from more than 1000 sites, provided surprises too.

“Satellite observations show a decreasing trend in NO2 concentrations across China, but we find that large decreases in NO2 in some regions of the country are being offset by increases in other regions,” says Ben Silver of the University of Leeds, UK.

Rising NO2 concentrations could help explain why ozone values have increased, although Silver points out that in heavily polluted cities decreasing NO2 emissions can also push up ozone levels. Ozone forms from complex reactions in the atmosphere involving NO2 and volatile organic compounds; at ground level it can be detrimental to health as well as crop yields.

The researchers are also investigating the impact of the weather. “Day-to-day variation in air quality is most strongly affected by weather conditions rather than emissions – so a year which had a higher than average occurrence of weather conditions conducive to poor air quality, followed by a year which had favourable weather conditions could make it look like air quality has improved, while emissions have stayed constant,” says Silver.

Next the researchers will use an air quality computer model to probe the 2015 – 2017 measurements in detail. They hope to establish whether decreases in PM2.5 and SO2 can be ascribed to emission controls, meteorology or a combination of both.

Picturing a scene of rapid industrialization and urbanization brings home the challenge of improving air quality. But access to ground-level data provides a tool for tracking progress and identifying future opportunities.

“Much of China’s air quality control has focused on the ‘low hanging fruit’ of large sources of air pollution, namely their coal fired power stations,” says Silver. “Our research shows that a large fraction — around one third — of PM2.5 pollution can be attributed to heating and cooking emissions from households.”

The team published the findings in Environmental Research Letters (ERL).

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