Ozone Alerts Decline Despite Summer Heat; Second Lowest Since 2016

Despite a nationwide heat wave this summer, the number of ozone alerts issued has decreased compared to previous years. Analysis suggests that weather conditions such as solar radiation and precipitation days have a greater impact on high ozone levels than temperature alone.

The National Institute of Environmental Research, under the Ministry of Climate Energy and Environment, reported on September 22 that the total number of ozone alerts issued nationwide from May to August this year was 48 days.

This marks the second lowest level since 2016, following 40 days in 2020, which experienced prolonged monsoon rains. The number of alert days has decreased over the past three years, from 61 days in 2024 to 54 days last year and 48 days this year.

Notably, August saw a significant decline. The number of ozone alerts in August this year was 10 days, a 27% decrease from the 10-year average of 13.7 days from 2016 to 2025. Compared to 21 days in 2024, this represents a reduction of over 52%.

Regionally, the number of alert days was 27 in the metropolitan area, 16 in the central region, 17 in the southern region, and 23 in the southeastern region. All areas experienced a decrease of 9.1% to 17.5% compared to the 10-year average. The southern region saw the largest decline, dropping from an average of 20.6 days to 17 days this year, following decreases from 32 days in 2024 and 24 days last year.

The institute attributes the reduction in ozone alerts to weather conditions such as solar radiation and precipitation days. Ozone is formed through photochemical reactions involving nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight, typically occurring at higher concentrations when temperatures are elevated, sunlight is strong, and the atmosphere is stagnant.

Analysis of monthly weather data and ozone alert days from May to August since 2016 shows a positive correlation (r=0.55) with total solar radiation and a negative correlation (r=-0.57) with precipitation days. This indicates that as sunlight increases, the number of alert days rises, while more rainy days tend to reduce them.

This August, the average national temperature was 0.7 degrees lower than in August 2024, with 4.9 fewer heat wave days. Total solar radiation decreased by 86.3 MJ/m², while the number of precipitation days increased by three. Higher average wind speeds and relative humidity also created conditions less favorable for ozone formation.

A reduction in precursor substances for ozone formation may have also played a role. The average concentration of nitrogen dioxide (NO₂) nationwide from May to August this year was 0.0077 ppm, approximately 36% lower than the 10-year average. The institute is further analyzing the relationship between changes in precursor substances, particularly high-reactivity VOCs in the southern region, and the decrease in ozone levels.

Seong Ji-won, head of the Air Quality Research Department at the National Institute of Environmental Research, stated, "This year's case shows that high temperatures do not necessarily lead to an increase in ozone alerts. Ozone levels are the result of a complex interplay between weather conditions, emissions of precursor substances, and photochemical reactions in the atmosphere. We will closely analyze the causes of the decrease in ozone alerts in the southern region to use as a scientific basis for tailored ozone management policies."