2020 Ozone Season and Wildfire Impacts

2020 was an unusual year for air quality. After a relatively clean winter, frequent storms and reduced emissions from the COVID-19 response led to a period of extremely clean air in March and early April. On April 23rd, the ozone (smog) season began in earnest with almost daily exceedances of the federal ozone standard until November 1 with 158 exceedance days in total. At times, ozone reached levels not seen in more than 25 years, which is highly unusual considering how far overall ozone levels have declined in that period.

Residents of the South Coast Air Basin also experienced elevated fine particulate matter levels due to widespread use of Independence Day “personal” fireworks and from several wildfire smoke events across the west coast. Wildfire smoke was particularly bad in California this year as more than four million acres burned throughout the state, more than double the previous record of approximately two million recorded in 2018. These events provide insight into why the summer saw such high concentrations of fine particulate matter.

Another record-setting event led to elevated levels of ozone this season. Ozone formation is particularly sensitive to temperature, with higher temperatures leading to elevated ozone levels. The May through October period was the hottest six months on record for the South Coast region. Conditions were also atypically stagnant, with calm conditions trapping pollution near the ground and within the South Coast Air Basin. Emission changes stemming from the COVID-19 pandemic also may have played a role in the pattern of high ozone levels experienced this summer.

South Coast AQMD scientists and other researchers are still investigating the relative influence of each of these factors. Although these factors have led to an unusually bad year for air pollution, South Coast AQMD remains committed to continuing our long history of reducing emissions, which is the key to achieving long-term, permanent improvements in air quality.

Fig 1 - Ozone exceedence

Figure 1: Ozone exceedance days were abnormally hot and stagnant. These graphs show the trend in average daily high temperature and ventilation rate in Central Los Angeles on the highest ozone days each year. Average temperatures were hotter on high ozone days in 2020 and the air was more stagnant than previous years, creating ideal conditions for high ozone levels

Fig 2 - Wildfire Smoke

Figure 2: Wildfire smoke from local fires and fires throughout the State of California led to poor air quality this summer. Image source: Sept. 13th, 2020, Suomi NPP/VIIRS imagery

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