Pesticides in the clouds: the behind the scenes of a unique study

Paris, Oct 4 (The Conversation) A Franco-Italian team analyzed six cloud water samples taken at the summit of the Puy de Dôme massif in Auvergne during different seasons, between 2023 and 2024. The researchers detected 32 different pesticides, including several banned in Europe for more than a decade. A third of the samples also had a total pesticide concentration above the regulatory levels for drinking water.

Their pioneering work also made it possible to estimate, for the first time, the quantity of pesticides found in all low and medium clouds in mainland France, ranging from 6 to 139 tonnes. A look back at this unprecedented publication, with its first author.

The Conversation: What is the genesis of your research? Angelica Bianco : With my chemist colleague Pascale Besse-Hoggan, an expert at ICCF (UCA/CNRS) in the (bio)degradation of pesticides in soils, we have wanted, for several years, to quantify pesticides in clouds, because pesticides are contaminants of interest in all environmental matrices.

A recent study by researcher Ludovic Mayer and his colleagues had already reported the presence of pesticides in atmospheric aerosols collected at 29 sites in Europe, including several in the free troposphere, the first layer of the Earth's atmosphere which begins at an altitude of one to two kilometers and which is little or not affected by local emissions.

Furthermore, the presence of pesticides in precipitation has been known for a long time, with notable work in the late 1990s. We therefore took advantage of the Puy de Dôme Observatory, managed by the Observatoire de physique du globe (OPGC) in Clermont-Ferrand and by the Laboratoire de météorologie physique (LaMP) of the University of Clermont-Ferrand (UCA) and the CNRS, to quantify pesticides in a matrix that has remained unexplored until now: clouds.

Technically, this isn't the first cloud measurement. In 1991, German researcher Franz Trautner's team measured atrazine, a now-banned herbicide that blocks plant photosynthesis and was frequently used in cornfields, in several samples collected in a single cloud above corn crops in the Vosges mountains, with concentrations ranging from 24 to 260 nanograms per liter (ng/l), well above the permitted limit for drinking water.

The originality of our latest study is based on the quantification of pesticides: -in several cloud water samples, collected in two different seasons; -with air masses of different origins (different seasons, different temperatures and different geographical origins) … -with an analysis of 446 pesticides (herbicides, fungicides, insecticides, biocides) and some degradation products; -with very low detection limits using certified methodologies (Cofrac).

We already knew that a significant number of pesticides were present in waterways. It might seem logical, then, that they would also be found in clouds. However, such research has not been conducted much before. How do you explain this? A. B.: Clouds represent the most difficult environmental matrix to capture and sample: they are not water from a lake or river that can be easily sampled with a bucket and in large quantities. Nor are they dust or gases, which are always present in the atmosphere and can be collected automatically on filters or in balloons.

Clouds are event-based: they are not always there! Of course, it is possible to sample them from an airplane, as some of my colleagues do . But this method is technically complex because it is absolutely essential to avoid any contamination of the sample by the aircraft's engines. In addition, clouds are made up of fine droplets (between 10 and 50 micrometres in diametre), which must be collected to have a sufficient liquid sample to carry out all the analyses. Currently, in France, only the Puy de Dôme station, which has a high cloud occurrence (40 per cent of the time), allows the study of clouds.

Our system is not automated, meaning an operator must be on-site for collector assembly, sample collection, disassembly, and processing. We use a cloud collector called a "boogie" and very strict protocols for cleaning and sample collection.

The amount of water in clouds varies from 0.3 to 1 g/m 3 of air, which means that a lot of clouds need to be sucked up to get a few millilitres. This is one of the limiting factors in our analysis: the volume of cloud collected.

Our collections rarely last more than two hours, because we have to take atmospheric dynamics into account. It is much easier to study the characteristics of a sample when the history of the air mass is simple, rather than when it results from the combination of several different air masses. However, the more time passes, the more complex the composition of a cloud becomes, because the compounds carried in the different air masses, influenced by different sources (for example, marine and anthropogenic) mix in the same liquid sample. To avoid this, we limit the collection time to two hours, therefore our samples are small volumes and the amount of analysis we can carry out is limited.

But, to sum up, the innovative nature of our study is that we, cloud hunters, have the chance to work on a very little explored environmental matrix where everything remains to be discovered.

You were, I understand, the first to be surprised by the results observed.

A. B.: Frankly, for the sake of our beautiful green and blue planet, we were hoping not to find pesticides in the clouds! The first surprise was the detection of these compounds in all the samples analyzed, even the unsuspected ones, those with an air mass that travelled at altitude and over the Atlantic Ocean, therefore a priori at a distance from the land where pesticides are spread.

We therefore carried out several checks, including a cross-check with the aerosol measurements presented by Ludovic Mayer of Masaryk University (Czech Republic) and his colleagues , and our concentrations proved plausible. The observed concentrations remain low, however, in the order of nano to microgrammes per litre.

After discussions, we decided to calculate the total mass of pesticides potentially present in the clouds that fly over mainland France. To do this, we decided to formulate an important hypothesis, namely that the concentration measured in the Puy-de-Dôme clouds is representative of the low-altitude clouds present over the entire French territory. This is debatable, certainly, but probably not so far from the truth: the reviewers of our publication have never questioned this hypothesis. We thus estimated that there could be between 6.4 and 139 tons of pesticides present in the clouds above France.

So, you should know that clouds contain a lot of water, on the order of a billion tons, but, personally and naively, I didn't think I would find tons of pesticides! It is this estimate that triggered the most reactions and that caused the most talk, both positive and critical, but I believe that the essential thing, beyond the figures, is the collective awareness of the pollution that we bring into the environment.

How does your work contribute to our understanding of the circulation of pesticides in the environment? A. B.: From my perspective, this article shows that we have come full circle: pesticides are found in river water, lakes, groundwater, rain, and now clouds. The atmosphere is extremely dynamic and transports these compounds, even if they are at low concentrations, to the most remote places on Earth, and ultimately, some isolated places, such as the polar regions, which should not be directly impacted by pesticide pollution, are ultimately exposed by these long-distance transports.

But the atmosphere, and clouds in particular, are also a chemical reactor capable of transforming these molecules: the sun's rays trigger photochemical reactions that can degrade these compounds. This is why we sometimes find transformation products in our samples and not the original pesticide. It is therefore also important to understand how these molecules degrade in the environment.

In what directions will you continue your research following this publication? A. B.: Several colleagues have expressed keen interest in the results presented in this article. As a chemist, I feel that six samples collected at a single site are not enough to represent environmental variability. I therefore believe that this study should be extended to a larger number of samples, and if possible, taken from several sites! Despite this, I remain hopeful of finding many pesticide-free samples in the next clouds we collect… (The Conversation) RD RD