In 1997, the U.S. Environmental Protection Agency adopted a national health standard for ozone – averaged over eight hours – of 84 parts per billion (ppb) of air. Eleven years later, a new, revised, updated average ozone national health threshold was set – 75 ppb. And, in 2015, an even further tightening of the federal eight-hour ozone health standard – this one set at 70 ppb.
Okay, so in analyzing ozone data for the area in which I reside, I consulted different sources: “2015-16 Report to the Community,”1 prepared and published by the San Joaquin Valley Air Pollution Control District, the “San Joaquin Valley Air Basin Daily Max 8 Hr Overlapping Avg Ozone – Natl at Highest Site” and one additional California Environmental Protection Agency Air Resources Board (ARB) data table. For purposes of this analysis, the years of concern (or those being called into question), are 2013, 2014 and 2015. Also taken into consideration are the two national standards for 8-hour ozone of 75 and 84 ppb, respectively.
So, why investigate various ozone-data sources for the two different ozone standards and for my particular region – the San Joaquin Valley – this way at all? Would you believe because of the possibility of there being an inconsistency one ozone-data source relative to another? Hopefully, by comparing one source against another, this will shed some important and useful light in terms of helping one understand why the said seeming inconsistency, all of it having to do with the number of exceedances of the National Ambient Air Quality Standard for ozone averaged over a period of eight hours.
In the figure: “County Days over Federal 8-hour Ozone Standard,” the burgundy-colored line depicts the 2008 standard (75ppb) while the blue-colored line depicts the 1997 standard (84 ppb). In 2015, regarding the number of exceedance days throughout the 8 counties that make up the San Joaquin Valley, the figure shows a decrease in the number of such exceedances for the 2008 standard during that period while, at the same time, the figure also shows the number of said exceedances for the 1997 standard for the same period of time to increase, which seems highly contradictory.
Looking at the burgundy-colored line, for years 2013, 2014 and 2015 the number of exceedances jumps from what looks to be about 255 to around 280 before dropping to approximately 240 or, in other words, between 2013 and 2014, what is shown is an increase of about 25 exceedance days after which there is a decline of approximately 40 exceedance days regarding the 2008 (75 ppb) standard. Meanwhile, turning attention to the blue-colored line, the increase from 2013 to 2014 looks to go from somewhere in the neighborhood of 60 exceedance days, climbing to what appears to be around 70 before peaking at about 92 exceedance days representing an increase of roughly 32 exceedance days overall over the three years as it has to do with the 1997 (84 ppb) standard.
Wait – how is this possible?! In other words, how can the lower (75 ppb) threshold rise and then fall while the higher (84 ppb) threshold shows only a rise?
Regarding national 8-hour ozone readings for the 75 ppb standard, from the ARB’s “Latest Year’s (Annual) Ozone Summaries for Selected Regions (PST)” table, in the San Joaquin Valley Air Basin, table data indicates that the number of Exceedance Days in 2015 is 81, while the numbers for 2014 and 2013 are 95 and 89, respectively. The two different data sources at least agree in terms of the trend (first an increase followed by a decrease between years 2013 and 2015), that is where 2008 ozone data is concerned. It is appropriate to note here that the 2015 data, though not necessarily preliminary, may be. And then presented below the data table, there is this qualifying statement:
“The number of exceedance days equals the number of distinct days on which the relevant standard was exceeded at any monitoring site in the region. If the standard was exceeded at more than one site on a given day, it only counts as one exceedance day for the broader region.”
And, when it comes to comparing the higher 84 ppb, 1997 standard, referred to are the ARB’s “San Joaquin Valley Air Basin Daily Max 8 Hr Overlapping Avg Ozone – Natl at Highest Site 2013,” “2014” and “2015” data tables.
In 2013, from the ARB source in question, the number of exceedance days above the 84 ppb standard is shown to be 31. Furthermore, the number of days exceeding the 84 ppb threshold in 2014 is 73*. And, finally, for year 2015, exceedance days at the 84 ppb standard number 35. This is where information from the San Joaquin Valley Air Pollution Control District and the California Environmental Protection Agency Air Resources Board disagrees or, at least, would seem to, at any rate.
In taking all of this data into consideration, one is left wondering why it appears there is disagreement between the two different data sources, that is, between the regional and the state in regards to the 1997 (84 ppb) standard between reporting years 2013 and 2015. Can the apparent difference be because the ARB data for 2015 might be preliminary and not final?
People reading this might be asking if either or both sources can be trusted. Aside from that, here is an important question to ask: If the number of exceedance days regarding the 2008 (75 ppb) standard decreases for a given year, can the number of exceedance days regarding the 1997 (84 ppb) standard for the same given year increase and vice versa? Besides this, having two separate sources (resources) from which to compare like data is indeed helpful.
Anyway, more should be known upon checking this year’s figures next year. Until then …
* At the 84 ppb threshold level for year 2014, the number of ozone exceedance days in the San Joaquin Valley was determined (based off of the California Environmental Protection Agency Air Resources Board’s “San Joaquin Valley Air Basin Daily Max 8 Hr Overlapping Avg Ozone – Natl at Highest Site 2014” data table) to be 41 and not the 73 as is mentioned above. As to the latter (higher) number, it was arrived at based on data extracted from the table in question, not on March 21, 2016 at 7:01:50 p.m. (PDT) when the former (lower) number was arrived at, but on August 6, 2015 at 4:40:56 p.m. (PDT) instead. Meanwhile, at the 75 ppb threshold level for year 2014, the number of ozone exceedance days in the San Joaquin Valley was determined (based off of the ARB’s “San Joaquin Valley Air Basin Daily Max 8 Hr Overlapping Avg Ozone – Natl at Highest Site 2014” data table) to be 130 (as of the Aug. 6, 2015, 4:40:56 p.m. extraction date and time) as opposed to 95, this determination made on March 21, 2016 at 7:01:50 p.m., which would account for the difference. The ARB data in question for 2014 now appear to be confirmed whereas on Aug. 6, 2015 at 4:40:56 p.m. (PDT) said data appears to have been preliminary.
- “Ozone Trends,” the figure: “County Days over Federal 8-hour Ozone Standard,” as presented on page 3 of the “2015-16 Report to the Community” report, prepared and published by the San Joaquin Valley Air Pollution Control District, available and accessible here.