At 48 days, the figure was 9.8 percent below the 10-year average, as lower solar radiation and more rainy days curbed ozone formation

[123RF]
[123RF]

The number of days on which ozone advisories were issued in South Korea this summer totaled 48, the second fewest in the past 11 years, the National Institute of Environmental Research said Tuesday.

The institute analyzed nationwide ozone advisory days from May through August — the period covered by its intensive high-ozone management program — and found advisories were issued on 13 days in May, 15 in June, and 10 each in July and August, for a total of 48 days. That is the second fewest since 2016, trailing only 2020, when advisories were issued on 40 days.

The total was 5.2 days, or 9.8 percent, below the 10-year average from 2016 to 2025 of 53.2 days. August showed the largest gap from the 10-year average, with 3.7 fewer advisory days than normal.

By region, the southern area — encompassing Gwangju's Dong-gu, Seo-gu, Nam-gu, Buk-gu and Gwangsan-gu, along with Mokpo, Yeosu, Suncheon, Naju, Gwangyang and Yeongam-gun in South Jeolla Province — recorded only 17 advisory days from May through August, the largest shortfall from its 10-year average of 20.6 days.

Advisory days in the southern region during the May–August period have been declining, from 32 days two years ago to 24 days last year.

An ozone advisory is issued when the hourly average concentration reaches 0.12 ppm or above. Heat wave days are defined as days when the national average daily high temperature reaches 33 degrees Celsius or above. [Korea Meteorological Administration data]
An ozone advisory is issued when the hourly average concentration reaches 0.12 ppm or above. Heat wave days are defined as days when the national average daily high temperature reaches 33 degrees Celsius or above. [Korea Meteorological Administration data]

Lower solar radiation and more frequent rainfall this summer were key factors behind the reduced number of ozone advisories.

Ozone forms when nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the atmosphere undergo photochemical reactions in sunlight, meaning concentrations rise with higher temperatures, stronger sunlight and stagnant air.

In fact, an institute analysis of meteorological data from May through August going back to 2016 confirmed a positive correlation between total global solar radiation — the cumulative amount of solar energy reaching the surface over a given period — and the number of ozone advisory days (Pearson correlation coefficient: 0.55), and a negative correlation with the number of rainy days (Pearson correlation coefficient: -0.57).

This August, compared with 2024 — the year with the most frequent ozone advisories — the national average temperature was 0.7 degrees Celsius lower and there were 4.9 fewer heat wave days, defined as days when the daily high reaches 33 degrees Celsius or above. Total global solar radiation was lower by 86.3 MJ/㎡, and there were three more rainy days. Relative humidity was also higher, and average wind speeds were faster.

Levels of ozone precursor substances were also lower.

The national average concentration of nitrogen dioxide, a key ozone precursor, stood at 0.0077 ppm from May through August — 36 percent below the 10-year average.


thlee@heraldcorp.com