Each year since 2000 the American Lung Association has released its annual “State of the Air” report. The purpose of such is to bring to the attention of the public and others the condition or “state” of air in America. Part of the report deals with American cities known for having the poorest quality air in the nation for fine particulate matter pollution over a period of 24 hours (short-term PM 2.5), over a period of a year (annual or long-term PM 2.5) as well as ozone in terms of average taken over an eight-hour time-frame. In fact, for average 8-hour ozone in 2018, of the top 10 most polluted cities, seven were in California.
One of the most striking statements related to air pollution in the southern California counties of Los Angeles, Orange, Riverside and San Bernardino – the jurisdiction for stationary-source-pollution regulatory control or oversight for the South Coast Air Quality Management District (AQMD) – in fact, came courtesy of AQMD Executive Officer Wayne Nastri in the Apr. 23, 2019 edition of The Daily Breeze in the op-ed: “What’s the air quality in SoCal Really Like?”
Related to climatic effects on air quality in the South Coast Air Basin, here is what Nastri wrote: “Although the South Coast AQMD has implemented policies and strategies to reduce emissions, most don’t realize that the changing climate plays a crucial role in air quality. Hotter temperatures, stagnant weather, and large wildfires throughout California have directly led to increases in high levels of ozone and fine particulate matter during certain parts of the year.”
Now for a closer look at the two air pollutants mentioned – ozone and fine particulate matter.
It is important to note that ozone is primarily a warm weather pollutant in that it predominates in many cities and regions during spring, summer and fall when temperatures are warmer and conditions for its formation are also ripe. Ozone in the stratosphere is considered to be beneficial in terms of it helping prevent some skin-damaging ultraviolet solar radiation from reaching earth’s surface. As for ozone in the troposphere, also known as ground-level ozone, this is the bad ozone and when breathed in is damaging to the lungs. When ozone combines with certain other chemicals in air, smog forms, hence the reason ozone is also known as a smog-forming pollutant or emission or smog precursor.
As for fine particles or particulates, there are basically two different types: solid and liquid-droplet matter. The tiny solid particles are also referred to as soot, black carbon or even elemental carbon. A second kind is brown carbon or organic carbon. Both types can hang or be suspended in air. Unlike ozone, fine particulate matter pollution – otherwise known as PM 2.5, named such because the aerodynamic diameter of each particle is less than 2.5 micrometers – is unhealthful at any concentration in air. It can lodge itself deep in the lung where it can cause damage, but because of its extremely small size, PM 2.5 can enter the bloodstream and get carried to other locations and organs in the body, even to the brain.
In terms of the latter pollutant’s impact on city and regional air quality, the higher concentrations, generally, are found in cities. PM 2.5 comes from mobile and stationary sources and even area sources like aerosols. Wood-burning activity and driving contributes the most but so does construction equipment, farm equipment and operations, and business, industry and energy generation.
It is not uncommon for metropolitan regions to have inordinately high or elevated PM 2.5 concentrations while soot levels in air outside city limits are typically markedly lower.
For ozone – in many places it is more commonly known as ozone smog or just smog – it’s an entirely different story (read: “It doesn’t necessarily have to be but it can be and sometimes is different from the PM 2.5 story, that is).
That is to say that the level of ozone in air can be higher outside city boundaries than within due to prevailing air currents which can move such pollution around. Two examples where this happens are Arvin located south of Bakersfield and Parlier southeast of Fresno in Kern and Fresno counties, respectively. Kings, Sequoia and Yosemite National Parks are others.
So, regionally, ozone could be bad, but one location in a given region could be worse in this regard.
Interesting to note are the exceptions to the general rules. For example, there have been cases where all of the conditions for smog formation are present, yet, because of one reason or another, smog failed to form, such as is a situation where the temperature exceeded a threshold through which no noticeable smog in air was evident. If a key ingredient to either ozone or particulate matter pollution, if an insufficient amount of oxides of nitrogen (NOx) in this case is absent, then quite possibly particulate matter concentrations or ozone levels in the atmosphere are lower on account of such.
A situation may arise like during fireworks celebrations that because of high pressure creating an inversion layer, this allows for fine PM to rise, so much so that precautionary statements or air or health alerts are issued. Though not common, nevertheless, the events have happened.
Of course, there are locations where emissions from area industries impact both cities and entire regions or air basins. And, as mentioned above, items such as temperature, weather and wildfires can and do factor in, in the broader context. There is ongoing debate as to whether climate is a contributing factor or not.
Images: U.S. Environmental Protection Agency (upper); National Park Service (lower)