High levels and behavior of tropospheric ozone in the Andes Mountains, Central Chile

Tropospheric ozone is generated by photochemical reactions by precursor pollutants, or it can also proceed from stratospheric intrusion. In addition to affecting the population's health, ozone exposure threatens biodiversity conservation and the production of food and forest products.

A prior study by this research group assessed public ozone monitoring stations across Chile, revealing that only stations near the Andes Mountains in central Chile exceed the national standard of 61 ppb. The highest concentrations were observed at Las Condes station (790 m.a.s.l.) and Los Andes station (830 m.a.s.l.).

In this context, the present study aimed to determine whether tropospheric ozone exists in high concentrations in the Andes Mountains range at higher altitudes and its behavior. For this reason, an ozone monitoring station was installed at the NUNATAK-1 refuge laboratory, located in Portillo (32.844ºS, 70.129ºW), east of the Los Andes station, at 3,000 m.a.s.l. The monitoring period for this study spanned from October 6, 2022, to October 18, 2023, with minute resolution using Thermo Scientific™ 49iQ Ozone Monitor, which employs a dual cell UV photometry (sample and reference) to measure the amount of ozone in the air from ppb levels up to 200 ppm, (detection limit: 0.50 ppb in 60 second averaging time; precision: ±1.0 ppb; response time: 20 seconds).

During the monitoring period, hourly averages of up to 80 ppb were observed, and the 99th percentile of the maximum daily 8-hour moving average was 61.56 ppb, exceeding the Chilean standard. The Portillo records were compared with Chile's most contaminated monitoring stations, Las Condes and Los Andes, using an ANOVA test, and a p-value ≤ 0.05 was obtained. Therefore, it is concluded that there are statistically significant differences between the stations. The LSD test further determined that Portillo exhibited a significantly higher average ozone concentration, with levels averaging 55% higher than those in Los Andes and 58% higher than those in Las Condes.

In the northern hemisphere, elevated tropospheric ozone levels have been observed in high-altitude regions, including the western US, western Europe, central Japan, central China, Himalayas, Greenland, southern Algeria, and Izaña. However, in the southern hemisphere, ozone in mountainous areas has not been studied as extensively due to poor data coverage. This study unveils ozone records spanning a year in the Andes Mountains, revealing even higher concentrations than those observed in urban areas in Chile. Therefore, tropospheric ozone could be harming the biodiversity of the Andes Mountain range in central Chile. Furthermore, ozone could adversely affect the health of climbers and hikers exploring the Andes Mountain range.

Subsequent studies should determine whether the tropospheric ozone recorded in the Andes Mountains range results from long-distance ozone or precursor pollutants transport from urban source regions, or ozone intrusion from the stratosphere. Photochemical modeling is recommended for a more comprehensive understanding and monitoring campaigns of the vertical ozone profile using radiosonde.

Acknowledgment to ANID Project: Anillo ACONCAGUA ACT210021, Fondecyt Regular 1221526, FOVI230167, and ANID National PhD Scholarship 21202033.