Led by researchers from the University of Washington (UW), the study reported that power plants and vehicles have successfully reduced the total amount of pollution they belch every year. However, the air is only truly cleaner during the summer months.
The study evaluated data taken by the 2015 Wintertime Investigation of Transport, Emissions and Reactivity (WINTER) survey. It was supported by both NASA and NOAA (National Oceanic and Atmospheric Administration).
Researchers flew over six major cities and the coal power plants in the Ohio River Valley region. After measuring the amount of particles in the air over those sites, they found that the toxic polluting particles travel through different routes during winter.
Smog particles can be divided into several groups. The most concerning types of particulate matter are sulfates and nitrates. Sulfates start out as sulfur dioxide (SO2) emitted by coal-fired power plants. Nitrates, on the other hand, originate from nitrogen oxides (NOx).
Strict implementation of air quality regulations have reduced the overall levels of both sulfates and nitrates over the years. In the eastern U.S., the amount of particulates in the air during the summer is particularly impressive.
However, the pollution levels during winter have not enjoyed the same level of reduction. Whereas summertime levels are down by around 33 percent, wintertime concentrations are only reduced by half. The UW researchers sought to determine the reason why wintertime remains more polluted. (Related: Air pollution found to increase emergency room admissions for bleeding peptic ulcers.)
During summer, part of the NOx and SO2 emitted by pollution sources stay in their gaseous states. They either react to sunlight or get deposited on the surface. The rest of the pollution become nitrate and sulfate particulates.
Cutting down on the emission of air pollution causes the levels of the particulates to go down as well. However, the chemistry of the atmosphere during winter is different from that during summer.
Winter is colder and experiences lower levels of sunlight. During this season, more chemical changes take place in the liquid phase of primary pollutants rather than in their gaseous state.
Furthermore, decreasing the amount of emissions released in the air increases the amounts of available oxidants. More oxidants means greater efficiency in converting SO2 into sulfate during the liquid phase.
As if that was not bad enough, the lower levels of sulfate reduces the acidity of the particulates. The lower acidity makes it easier for nitrogen oxides to convert into nitrates.
The UW study confirmed that air quality regulations have succeeded in reducing the amounts of nitrogen oxide and sulfur dioxide in the atmosphere. It added, however, that the regulations were not as effective on the particulates that also affect human health.
"It's not that the reductions aren't working," elaborated UW researcher Viral Shah, the primary author of the study. "It's just that the reductions have a cancelling effect, and the cancelling effect has a set strength."
Shah and his teammates believed that federal and state agencies must increase the reduction of primary air pollutant emission. Otherwise, it will take until 2023 at the earliest before the levels of toxic particulate matter during winter starts going down much faster.
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