Drought does or does not make air pollution worse; the verdict

This post picks up where the “Drought does or does not make air pollution worse; which is correct and why?” post left off.

Findings

I’ll get right to it.

Between Oct. 1, 2011 and Sept. 30, 2015, a time otherwise known as the drought years in the San Joaquin Valley, the daily maximum average fine particulate matter pollution (PM 2.5) exceeded the U.S. Environmental Protection Agency (EPA) air standard of 35.5 micrograms per cubic meter of air 267 times which means that on 1,195 days in a 1,461-day span (four-year duration), there were no standard exceedances at all. The number of days in which air was good totaled 216 and moderate air-quality days numbered 980. Remember: The greatest numbers of PM 2.5 exceedances are concentrated typically in the months of Nov., Dec., Jan. and Feb., the highest number of exceedances are very often recorded in Dec. and Jan. An exceedance corresponds to unhealthy air.

Correspondingly, between Oct. 1, 2007 and Sept. 30, 2011, the four-year period that preceded the drought (what I will refer to as the non-drought years), there were (in the Valley) 302 exceedances of the daily average PM 2.5. On the other side of the coin were 134 days in which air quality was good. Days on which the quality of Valley air was moderate registered 1,023. That is to say:

For PM 2.5 – drought (D) vs. non-drought (N-D) years:

  • 267 (D) vs. 302 (N-D) = the number of exceedances
  • 216 (D) vs. 134 (N-D) = the number of good air-quality days
  • 980 (D) vs. 1,023 (N-D) = the number of moderate air-quality days
  • 1,247 (D) vs. 1,325 (N-D) = the number of non-good air-quality days (moderate and unhealthy days combined)

That’s just for fine particulate matter pollution. Then there is ozone (O3), measured over an 8-hour time period. O3 is a warm-weather phenomenon which means it is most prevalent in the Valley from mid-to-late spring, all throughout the summer months (with rare exception) extending well into mid-autumn. The ozone standard is breached when concentrations exceed 75 parts per billion (ppb) or 0.075 parts per million (ppm) of air. As for O3 (daily maximum averaged over 8 hours), here are the numbers.

For O3 – drought (D) vs. non-drought (N-D) years:

  • 408 (D) vs. 419 (N-D) = the number of exceedances
  • 686 (D) vs. 790 (N-D) = the number of good air-quality days
  • 370 (D) vs. 252 (N-D) = the number of moderate air-quality days
  • 778 (D) vs. 671 (N-D) = the number of non-good air-quality days (moderate and unhealthy days combined)

That’s but the half of it. The other half of course is whether the air was more concentrated with the two pollution (PM 2.5 and O3) types.

– Oct. 1, 2011 to Sept. 30, 2015 avg. for days of unhealthy PM 2.5 = 54.76 micrograms per cubic meter.

– Oct. 1, 2007 to Sept. 30, 2011 avg. for days of unhealthy PM 2.5 = 54.45 micrograms per cubic meter.

– Oct. 1, 2011 to Sept. 30, 2015 avg. for non-good days (unhealthy and moderate combined) PM 2.5 = 28.17 micrograms per cubic meter.

– Oct. 1, 2007 to Sept. 30, 2011 avg. for non-good days (unhealthy and moderate combined) PM 2.5 = 28.87 micrograms per cubic meter.

– Oct. 1, 2011 to Sept. 30, 2015 avg. for days of unhealthy ozone = 84.84 ppb (0.08484 ppm).

– Oct. 1, 2007 to Sept. 30, 2011 avg. for days of unhealthy ozone = 88.65 ppb (0.08865 ppm).

– Oct. 1, 2011 to Sept. 30, 2015 avg. for non-good days (unhealthy and moderate combined) ozone = 76.96 ppb (0.07696 ppm).

– Oct. 1, 2007 to Sept. 30, 2011 avg. for non-good days (unhealthy and moderate combined) ozone = 78.8 ppb (0.0788 ppm).

Background

In the San Joaquin Valley it used to be that in a typical wet winter, when it was not raining, the likelihood of there being widespread fog was strong. Of course, with the passing of a rain event there was a very good chance that in the rain event’s wake a clearing of the atmosphere would follow. In other words, clear skies would replace wet weather, the former, more often than not, would be supplanted by fog, a cycle that could and did repeat itself many times over, typically from late October-early November to early-to-mid-March.

With that rain would be the preponderance of Valley fog during these months, which went without saying, especially during the period from 1978 to 2000. Between the latter year and the 2010s, it seems the fog has not been nearly as pronounced as it was in the latter part of the 20th century and, by pronounced, I mean thick and/or frequent. In fact, these past few years, where I live in Fresno, such misty conditions have been virtually nil. And, a lot of the Pacific-Ocean-originating rains that would normally make their way into the Valley, well, due to the phenomenon known as the “ridiculously resilient ridge” (of high pressure), this has caused these rains off the west coast to be sent northward into the Pacific Northwest and Canada. Now, why the change, one can only surmise.

There has been noticeable change in summertime weather in Fresno too. Comparing summers between 1978 and 2010, weather-wise, one summer compared with any other, one pretty much looks identical to the next. It is difficult telling them apart – weather during those summers is really that consistent. But there has been a change in the wind: now, the hottest days don’t seem to be nearly as hot as those typically encountered before the century odometer rolled over from the 20th to the 21st, nor does there appear to be the long strings of 100-plus degree days from 2010 on like I remember there being in the 1970s, 1980s, 1990s and perhaps even into the early part of the first half of the 21st century’s first decade even.

The long and short of this is that I have tried to create a general weather profile for California’s roughly 24,000-square-mile central interior area between years 1978 and today. For that entire period the most pronounced weather changes seem to have occurred between 2010 and 2015. That said it is the weather picture year-to-year between 2007 and 2015 that I want to focus on as it has to do with its possible impact on air quality, specifically the period of drought from 2011 to 2015. I want to know if there is a drought-air quality-worsening cause-and-effect in the San Joaquin Valley as it has been suggested.

Analysis

If drought does in fact exacerbate pollution in the air, then one would expect either a higher total number of exceedance-hours (or a greater number hours where pollution in the air exceeds the standard for a given pollutant, in other words) or there are increased concentrations of that pollutant in the air during a period in which drought conditions exist compared to a period in which drought conditions don’t exist – that is the thinking.

The two pollutants of concern for purposes of this analysis are ozone (O3) and fine particulate matter (PM 2.5) or particulates less than 2.5 micrometers in diameter or roughly from between one-twentieth and one-thirtieth the width of a human hair. It is important to bear in mind that whether PM 2.5 or O3 for any amount of either that exists in the air, air can be considered anything less than completely PM 2.5- or O3-free, respectively. For annual PM 2.5, the standard set by the EPA is 12 micrograms per cubic meter of air. If, for example, a particular area has as its total PM 2.5, 10 micrograms per cubic meter, while being in the good range, there still remains the presence of fine particulate matter pollution in the air. The point at which fine particulate matter pollution poses a danger to health is when levels exceed 35 micrograms per cubic meter of air – period. For ozone, the air becomes unhealthy if O3 concentrations exceed 75 parts per billion (ppb) of air. (Note: On Oct. 1, 2015, the EPA established a new eight-hour standard for ozone of 70 ppb of air).

And, therefore my hypothesis: If the drought which has persisted through four years (2011-2015) has in fact had any effect in terms of exacerbating pollution in the air (read: “making air more polluted”), this should be reflected in an increase in the total number of exceedances and/or by there being greater concentrations of a given pollutant in the air, that is, compared to non-drought times. According to the San Joaquin Valley Air Pollution Control District the dry period has influenced in a negative way, fine particulate matter pollution.

Conclusions

During the non-drought years (Oct. 1, 2007 to Sept. 30, 2011) there were both more PM 2.5 and O3 exceedances than in the drought years (Oct. 1, 2011 to Sept. 30, 2015) in the San Joaquin Valley, that is, regarding the pollutant-specific data. Regarding the same pollutant-specific data, there were more good-air-quality days in the drought years than in the non-drought years for PM 2.5, but not for O3.

What’s more during the drought years, the average for unhealthy days for PM 2.5 was higher than the average for unhealthy days for PM 2.5 in the non-drought years, but not by much (a 0.31 micrograms per cubic meter difference). Similarly, the average for non-good days for PM 2.5 (combining unhealthy and moderate air-quality days) in the non-drought years was only slightly higher than said non-good days during the drought years – the difference being 0.7 micrograms per cubic meter.

Turning now to O3, the average for days of unhealthy ozone was somewhat higher for the non-drought period than it was during the drought period – a 3.81 ppb difference. The same for the ozone average for non-good days (combining unhealthy and moderate air-quality days) in the non-drought years was somewhat higher than during the years of drought – a 1.84 ppb difference.

Conditions that can influence pollution

It is worth noting that differences between the drought and non-drought times considered in this evaluation were more than just meteorological in nature. For instance, the period between 2007 and 2011 was in a state of economic drought, speaking of droughts, whereas the period between 2011 and 2015 was in a state of economic recovery. Moreover, the recessionary times had a definite impact on driving: between 2007 and 2008, light-duty vehicle miles were in sharp decline continuing a trend that began in 2006. In 2009, light-duty vehicle miles rose slightly after hitting a low the previous year.1 Since 2012, on the other hand, the number of miles driven has been on the upswing. In Fresno County, in fact, vehicle miles traveled (VMT) increased from 22,205,530 in 2010 to 22,972,750 in 2013.2

Each of these conditions and more can have an influence on air pollution and no doubt has. The point is, any number of factors can be an influence on Valley air pollution at different times.

Chief sources of information used for this evaluation

California Environmental Protection Agency Air Resources Board, AQ2MIS San Joaquin Valley Air Basin, Daily Average PM25 at Highest Site, for years 2007-2015.

California Environmental Protection Agency Air Resources Board, AQ2MIS San Joaquin Valley Air Basin, Daily Max 8 Hr Overlapping Avg Ozone – Natl at Highest Site, for years 2007-2015.

Notes

  1. Michael Sivak, “Has Motorization in the U.S. Peaked? Part 2: Use of Light-Duty Vehicles,” University of Michigan, Transportation Research Institute, UMTRI-2013-20, July 2013, pp. 5, 6.
  2. Tim Sheehan, “More Valley commuters roll solo than most of California,” The Fresno Bee, July 10, 2015.

1 thought on “Drought does or does not make air pollution worse; the verdict

  1. Your analysis does not include the most critical factor involved in assessing changes in air quality – changes in total emissions responsible for air quality levels. Between 2007 and 2015, directly-emitted PM2.5 (diesel exhaust PM, smoke particles, a portion of dust emissions) declined by 18% due to regulatory controls implemented by the San Joaquin Valley Unified Air Pollution Control District and the California Air Resources Board, and oxides of nitrogen (NOx) emissions – a contributor to ambient PM2.5 concentrations – declined by 41%. I will send by separate email a spreadsheet containing the 2007/2011/2015 emission inventories for PM2.5 and NOx on which these reduction fractions are based.

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