A study by the Spanish National Research Council (CSIC), an agency dependent on the Ministry of Science, Innovation and Universities of Spain, concludes that the most intense Saharan dust events ever recorded in the air quality monitoring networks of Spain and Portugal occurred between 2020 and 2022.

This research, published in Atmospheric Chemistry and Physics, confirms the record-breaking nature of the dust storms that occurred in February 2020 in the Canary Islands and March 2022 in mainland Spain and continental Portugal, associated with such high dust concentrations that turned the skies orange.

Dust concentrations were so high that the air quality monitors were unable to measure such a high concentration of respirable PM₁₀ particles, so Sergio Rodríguez and Jessica López Darias, CSIC researchers at the Institute of Natural Products and Agrobiology in Tenerife (Canary Islands), developed a data reconstruction methodology. The concentration of respirable PM₁₀ and PM_2.5 particles, i.e., particles with a diameter smaller than 10 and 2.5 microns, respectively, is a key record in the air quality networks, in the European Union standards.

Since early February 2020, the two scientists of CSIC have observed that during Saharan dust events, the PM₁₀ monitors of the air quality network of the Canary Islands experienced "saturation," i.e., "PM₁₀ concentrations increased up to reach 1,000 µg/m³, the highest concentration that many of the commercial instruments can measure, a PM₁₀ concentration much higher than the 20 to 30 µg/m³ of PM₁₀ characteristic of the Canary Islands under dust-free conditions," Rodríguez explained.

During these events, PM₁₀ monitors were saturated, whereas PM_2.5 monitors showed high but variable concentrations, and this allowed them to develop a new data reconstruction methodology that could be validated by the few instruments able to measure more than 1,000 µg/m³. This reconstruction methodology was applied to the dust events that affected the Canary Islands in February 2020, February 2021, January and February 2022, and the Iberian Peninsula in mid-March 2022.

The reconstructed data evidenced that "during these dust events, the 1-hour average concentrations of PM₁₀ respirable particles reached values close to 5,000 micrograms per cubic meter in Tenerife and Almería and exceeded 5,250 micrograms per cubic meter in Gran Canaria," Jessica Lopez said.

Historic records

The researchers analyzed the 2000–2022 data of 341 air quality monitoring stations in Spain and Portugal and reconstructed 1,690 pieces of (1-h average) data of PM₁₀ concentrations in 55 stations. Then, the 24-hour average PM₁₀ concentrations were calculated, since this is the parameter used by the World Health Organization, which recommends that the population not be exposed to concentrations higher than 45 µg/m³. Results show that during the Saharan dust events of the period 2020–2022, PM₁₀ concentrations experienced a significant increase with respect to Saharan dust events of the period 2000–2019.

In the Canary Islands, intense dust events have usually (2000–2019) had PM₁₀ concentrations between 200 and 400 µg/m³—24-hour average; however, in the period 2020–2022 there have been 6 extreme dust events with PM₁₀ concentrations within the range 600–1,840 µg/m³; the latter value recorded on Gran Canaria island.

Mainland Spain and continental Portugal reached record-breaking PM₁₀ concentrations during the 15–16 of March 2022 dust event, when a massive dusty air mass from Algeria crossed the Iberian Peninsula from the southeast to the northwest.

"In mainland Spain and continental Portugal, PM₁₀ concentrations during Saharan dust events are usually lower than 100 micrograms per cubic meter; however, during the extreme dust event of the 15–16 March 2022 the 24h average PM₁₀ concentrations were similar to those typical of the Sahara desert, with values within the ranges 1,500–3,100 in Almería, 800–950 in Salamanca, Ávila and Valladolid, 600–650 in Central Portugal and 440–480 in Orense and northern Portugal," Rodríguez said.

These are the highest dust concentrations ever recorded in the air quality network since the measurements were standardized in the European Union in 2005. Researchers also analyzed previous data.

Meteorology and climate change

These extreme dust events are occurring in an anomalous meteorological scenario, characterized by a blocking anticyclonic situation in the Iberian Peninsula and Western Europe, which deviate southward, to the region of the Canary Islands–Cabo Verde; the cyclones were regularly linked to the mid-latitude westerly circulation.

The anticyclone over the Iberian Peninsula and the cyclone south of the Canary Islands–Cabo Verde creates a meteorological dipole (clockwise over the anticyclone and anticlockwise in the cyclone) with intense winds that results in massive emission and transport of dust.

This study doesn't address whether these extreme dust events are caused by climate change. However, the researchers highlight that all these extreme dust events occurred during meteorological anomalies in the Northern Hemisphere characterized by subtropical anticyclones shifting to higher latitudes, a widened tropical belt and amplified mid-latitude Rossby waves, features that resemble the anomalies in the atmospheric circulation linked by climate change, according to previous studies.

The extreme dust events, to which the authors of this study refer as "duxt episodes," are an emerging meteorological phenomenon. In 2018, a duxt event impacted the eastern Mediterranean. In June 2020, the so-called Godzilla episode affected the Caribbean and North America. In March 2021, two duxt events occurred in China and in November 2021 a new duxt episode occurred in Uzbekistan, all events induced by meteorological dipoles.

These duxt events are occurring in a paradoxical context in which dust emissions are lessening in north Africa and Asia due to a decrease in wind speed linked to climate change, a feature that makes it extraordinarily difficult to make a long-term forecast of these phenomena.