Making sense of air quality data – PM 2.5

Fine particulate matter or PM 2.5 (particulate matter less than 2.5 micrometers in diameter) is expressed in micrograms per cubic meter of air.

What this refers to is the concentration in the air of fine particulates. For instance, the U.S. Environmental Protection Agency (EPA) assigned a health standard for PM 2.5 of 35 micrograms per cubic meter (35 μg/m³ set by the EPA on Oct. 17, 2006). This particular threshold is in reference to daily ambient PM 2.5 or over a period of 24 hours. So, for every cubic meter of air, if the fine particulate matter concentration is greater than 35 micrograms, the standard is exceeded. This corresponds to an Air Quality Index (AQI) of 101 and places air at the unhealthy-for-sensitive-individuals or populations level.

Then there is the U.S. EPA annual fine particulate matter standard. This is set at 12 μg/m³ of air and became effective on Dec. 14, 2012.

Again, nothing too complicated about this.

Concentrations of PM 2.5 can vary, from a minimum of 1 μg/m³ of air to 75 μg/m³ and higher. For concentrations between 1 and 12, air is considered to be good. Above 12, the quality of the air worsens. For a more detailed description, see: The “Real-Time Air Advisory Network” (“RAAN”) on the Web site of the San Joaquin Valley Air Pollution Control District here.

Real-time observations, real-world conditions

On Dec. 12, 2013, in Central Fresno, fine particulate matter peaked at 74 μg/m³ at hour zero (the transition from Dec. 11^th to Dec. 12^th – midnight). Then there was a precipitous decline between 1 a.m. and 3 a.m. dropping from 75 micrograms to 40 micrograms, respectively. A low of 28 micrograms was reached at 8 a.m.

Between 8 a.m. and 7 p.m. it was all uphill from there – literally, downhill from there – figuratively.

By 10 p.m. on the 12^th, PM 2.5 concentrations reached a level of 76 μg/m³, finally dropping to 70 micrograms at hour 24 rising slightly from 69 micrograms at 11 p.m. The range for the day? It went from a low of 28 to a high of 76 micrograms per cubic meter of air. The 76 micrograms corresponds to air that is very unhealthy. Between 10 a.m. on the 12^th and 7 a.m. the following day when concentrations finally reached 35 micrograms, outdoor Central Fresno air was unhealthy to breathe.

It is important to understand that stagnant air conditions were present throughout the San Joaquin Valley basically between Dec. 2013 and Feb. 2014. There was much written in the media about this high-pressure ridge that had parked itself over California and which seemingly refused to leave. The term given to this condition: “Ridiculously Resilient Ridge,” also known variously as “Triple R” and “RRR.”

Meantime, check out the difference between conditions in Central Fresno and the Fresno County rural community of Huron in this regard. The highest reading for Dec. 12^th was 41. This occurred both at 1 p.m. and 9 p.m. On Dec. 13^th, concentrations never got above 40 micrograms per cubic meter of air, and that occurred at noon.

In the Oct. 10, 2014 “Making sense of air quality data – ozone” post and today’s post, “Making sense of air quality data – PM 2.5,” evident are several differences.

For one, whereas ozone by-and-large in the San Joaquin Valley is a warm-weather phenomenon, particulate matter, on the other hand, while present in the Valley year round, is far more pronounced and problematic in times when cooler weather is on hand. In addition, the presence of high levels of fine particulate matter pollution in cooler periods is very often associated with wood-burning among other activities. Stagnant air conditions serve to exacerbate conditions in this regard irrespective of whether it is ozone or fine particulate matter pollution we are talking about here.

One more point to touch upon with respect to PM 2.5. On July 4, 2011, in Central Fresno fine particle pollution spiked to 78 micrograms per cubic meter of air at 11 p.m. Air did not return to healthy levels until 1 a.m. on July 5^th when it reached 14 micrograms.

This compares to Independence Day PM 2.5 concentrations only reaching a maximum of 28 micrograms this year in Central Fresno and that was at 11 p.m. Having spent July 4^th this year on California’s Central Coast, I cannot speak to weather conditions present in the Valley that day, but apparently conditions were significantly more pleasant or comfortable or favorable weather-wise compared to the 4^th of three years prior. What a difference!

– Alan Kandel