Long-Term Trends in Fine Particulate Matter: Case Study of the Southeast Pennsylvania Region (2004-2021)

In order to understand long-term trends in air quality in the Mid-Atlantic region, maximum daily (24-hour average) observed fine particulate matter (PM_2.5; particles <2.5 µm in diameter) concentrations in Southeast Pennsylvania from 2004-2021 were analyzed. Southeast Pennsylvania encompasses Bucks, Chester, Delaware, Montgomery, and Philadelphia Counties, including the city of Philadelphia. This region is representative for studying air quality trends, as it lies downwind of air-polluting power plants concentrated in the Ohio River Valley and northern Mid-Atlantic states. PM_2.5 is one of six criteria pollutants regulated by the US Environmental Protection Agency (EPA) under the Clean Air Act. PM_2.5 is hazardous to human health because it can penetrate deep into the lungs and enter the bloodstream, increasing the risk of heart disease, lung cancer, and trouble breathing. The PM_2.5 data were analyzed by creating line and bar graphs that highlighted trends within individual seasons, as well as trends in “extreme” days – days with PM_2.5 levels ≥ 35 µg/m³. The results showed a consistent decline in concentrations from 2004-2018, signaling a general improvement in air quality. During this period, there was an average annual decrease in PM_2.5 concentrations of 0.60 µg/m³. Code Green days, days with “good” air quality as defined by the EPA’s Air Quality Index, became 116.8% more common. The downward trend was most prominent in the summer months (average season-by-season decrease of 0.952 µg/m³), less so in the spring and fall seasons, and almost non-existent during winter (average season-by-season decrease of 0.312 µg/m³), with the sharpest decrease in “extreme” days occurring between the summers of 2008 and 2009. The summer season decrease was attributed to a reduction in sulfate, historically a major component of PM_2.5 in the Mid-Atlantic region that peaked during the summer months. The decline in sulfate concentrations was expected since most power plants that burn coal, the main emitter of sulfur dioxide (SO2) (the chemical precursor of sulfate), have transitioned to natural gas usage in recent years to meet federal regulations and enhance cost efficiency. On the other hand, the PM_2.5 during winter was largely attributed to nitrate, a component of PM_2.5 that peaks during cold conditions. Nitrate derives from nitrogen oxides (NOx, NO + NO2) emissions from industry, mobile sources, and fuel combustion, which have decreased more gradually in the last two decades, thus explaining the weaker decline in winter PM_2.5 concentrations. Finally, the main downward trend gave way to an 0.85 µg/m³ per year increase in PM_2.5 concentrations from 2018-2021, the last three years of the analysis period. It is not clear whether this recent uptick marks the beginning of a new trend or a temporary departure from the overall decline, as three years is insufficient time to deduce a long-term pattern. One possible cause of the increase is wildfire smoke, which contains high concentrations of PM_2.5. Smoke from large wildfires in the Western US and Canada is often transported to the Mid-Atlantic during the summer and fall. Despite this uncertainty, the overarching results reveal a decline in PM_2.5 concentrations in the Mid-Atlantic, suggesting that air quality is gradually improving and will pose less threat to human health in the future.