I'm writing this quick blog for a few reasons. One, I teach environmental science and many of my students ask why code red days occur and why they occur most often in the summer months, and two, I saw a large number of people exercising (running and biking) last evening, presumably to avoid the excessive heat that has engulfed our region. Lets start with the first point.
Poor air quality days, like today, occur because of the emission of certain chemical compounds into the atmosphere that often times react with light from the sun to produce pollutants. The emission of these chemical compounds, primarily through the burning of fossil fuels for electricity production (coal) and for transportation (gasoline and diesel) couple with other pollutants to give us bad air to breath. The two most prevalent pollutants that give us poor air quality in the D.C region are ground-level ozone and incredibly tiny particles of materials like dust and carbon (among other things).
The reason for more poor air quality days in the summer, like today, has to do with several factors:
1. Ground-level ozone, the bad ozone, forms when our power plants and cars emit a chemical compounds called nitrogen oxides and volatile organic compunds that react with sunlinght to produce this form of unhealthy ozone. We use more electricity right now to power more air conditioning systems, that allows for more emmisions of these aforementioned chemicals into the atmosphere, that increases ground-level ozone concentrations and degrades the air.
2. Our summer months have days that are filled with much more light from the sun and it is this sunlight that works in tandem with the chemical compounds we emit from fossil fuel use (again electricity production and transportation) to create more atmospheric pollutants than at any other time of the year.
3. From a weather standpoint in the summer much of the North American continent is heated up in an almost semi-uniform manner and thus we often do not get very strong frontal systems (like cold fronts) to pass by and deliver big changes in air. These frontal systems are often accompanied by strong winds, think of the winds our region gets in the fall, winter, and spring, and because of the lack of strong winds over a large geographic area during the summer months the air in our atmosphere can stick around for days and become stagnant. We keep emitting pollutants into our atmosphere in the summertime and without strong winds to move these pollutants away from our region they accumulate more and more into the air we breath and degrade the air quality.
Now to my second point about the people exercising last evening outside.
Out of our two primary pollutants that cause bad air quality, ground-level ozone and particle matter, its ground-level ozone that accumulates and can be very detrimental to our health. Remember, it takes sunlight to make ground-level ozone, and we are getting sunlight all day long that works to form this harmful air pollutant, so it is the afternoon and evening that has the largest amount of ground-level ozone present in the atmosphere (with the highest concentrations). So exercising in the evening will expose you to more harmful pollution that at most other times of the day. Don't do it. Exercise in the morning or inside at a gym on these poor air quality days.
Monitor our air quality be visiting this web site from the Metropolitan Washington Council of Governments.