Along with much of the eastern U.S, we’re living through another round of air pollution alerts here in Northern Virginia. Best guess seems to be that those Canadian forest fires will be burning for months yet, so this will be occurring sporadically all summer.
I decided to see how the current situation looks, compared to historical air pollution levels in this area. To do that, I downloaded a little over two decades of daily data on fine particulates (PM 2.5) in Fairfax County.
I got some real surprises. Mainly, as high as the PM 2.5 levels have been, this June, that’s not a monthly record. In the 2000s (and presumably earlier) we routinely exceeded the monthly average level of PM 2.5 that we’ve seen in this smoky June 2023. Best guess, that was due to a toxic interaction of air-conditioning and coal-fired electrical generation.
It is exactly as I recall. Summertime air quality in the DC area was always bad. It had only recently gotten materially better. And then, along came these fires.
Details follow.
Long-term trend toward cleaner air
The EPA allows you to look up historical AQI data, at this website. For Fairfax County, and PM 2.5 (fine particulates), the earliest complete year of data is 2000. So that’s where this analysis starts. (Although the cutoffs for the AQI scale changed over this period, it appears that the website delivers AQI data uniformly using the current cutoffs.)
Source: Analysis of daily data from EPA website cited above.
The air got materially cleaner over this period. That’s clearly visible when I plot the annual average AQI for fine particulates (PM 2.5) from 2000 to June 2023. Back in 2000, the average was a bit over 50. By the time you get to 2015, the average was a bit over 30.
Best guess, around here, that was mostly a consequence of replacing coal with natural gas in our electricity generation mix. In 2000, half the power consumed in Virginia was coal-fired power. By 2020, that had fallen to just 4 percent. Almost certainly, the oldest and dirtiest plants were retired first. But this is also the era when regulation of particulates from diesels went into effect.
Source: Underlying data from the U.S. Energy Information Administration.
But August was always hazy, hot, and humid.
So far so good. But here’s where things turn weird. Let me now plot the same data as monthly averages, from January 2000 to June 2023.
Source: Analysis of daily data from EPA website cited above.
Surprise. Every year, in the 2000s, in the heat of summer, monthly-average particulate levels rose to the level they reached for June 2023.
I didn’t expect that.
I knew that we always had terrible ground-level ozone in the summer, but there are good reasons for that. Ground level ozone forms from the interaction of oxides of nitrogen and volatile organic compounds, acted on by sunlight and heat. We naturally got peak ozone during the peak of the summer season.
But what caused these August peaks in PM 2.5, that somehow was fully-phased-out by 2010 or so, I cannot quite fathom. Because July and August are the peak months for electricity use (in the U.S. and presumably in Virginia), I’m guessing this also has to do with electricity generation and the change in the generation mix of the Virginia grid.
And, by inference, about half the improvement in the yearly averages was due to getting rid of those July-August peaks. You can see that the annual minimums declined from about 40 to about 30, or half the decline in the annual averages.
My only real point is that, two decades ago, every summer, monthly average particulate levels in this area exceeded what they were in June 2023.
Plot the worst day in each month.
When I plot the worst day in each month, then June 2023 finally stands out against the historical background. In the 2000s, we routinely had Code Orange y AQI days for fine particulates (AQI > 100). But we never had a Code Red day, that is, AQI over 150. By the 2010s, Code Orange days had become rare.
In any case, since the start of recordkeeping in 2000, we hadn’t had anything close to the AQI of 198, for particulates, that we saw in June 2023.
Summary: We’re just having a series of bad days.
So that’s how to characterize this situation around here. We have occasional days with incredibly awful air quality (for particulates), compared to historical averages. But the average for the month isn’t even as bad as it was back in the days of air-conditioners running on coal-fired electricity.
