Airborne organic particles play a critical role in Earth’s climate1, public health2, air quality3, and hydrological and carbon cycles4. However, sources and formation mechanisms for semi-solid and solid organic particles5 are poorly understood and typically neglected in atmospheric models6. Laboratory evidence suggests that fine particles can be formed from impaction of mineral surfaces by droplets7. Here, we use chemical imaging of particles collected following rain events in the Southern Great Plains, Oklahoma, USA and after experimental irrigation to show that raindrop impaction of soils generates solid organic particles. We find that after rain events, sub-micrometre solid particles, with a chemical composition consistent with soil organic matter, contributed up to 60% of atmospheric particles. Our irrigation experiments indicate that intensive water impaction is sufficient to cause ejection of airborne soil organic particles from the soil surface. Chemical imaging and micro-spectroscopy analysis of particle physico-chemical properties suggest that these particles may have important impacts on cloud formation and efficiently absorb solar radiation. We suggest that raindrop-induced formation of solid organic particles from soils may be a widespread phenomenon in ecosystems such as agricultural systems and grasslands where soils are exposed to strong, episodic precipitation events8.
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We are grateful to K. Teske and P. Dowell for assistance in sample collection at the Southern Great Plains site. The Pacific Northwest National Laboratory (PNNL) group acknowledges support from the Chemical Imaging Initiative of the Laboratory Directed Research and Development Program at PNNL. The Lawrence Berkeley National Laboratory (LBNL) group acknowledges support from the US Department of Energy’s Atmospheric System Research Program, an Office of Science, Office of Biological and Environmental Research (OBER). The CCSEM/EDX, TEM and helium ion microscopy analyses were performed at Environmental Molecular Sciences Laboratory, a National Scientific User Facility sponsored by OBER at PNNL. PNNL is operated by the US Department of Energy by Battelle Memorial Institute under contract DE-AC06-76RL0. STXM/NEXAFS analysis at beamlines 5.3.2 and 11.0.2 of the Advanced Light Source at LBNL is supported by the Director, Office of Science, Office of Basic Energy Sciences of the US Department of Energy under Contract No. DE-AC02- 05CH11231. Beamline 11.0.2 also acknowledges support from the Office of Basic Energy Sciences Division of Chemical Sciences, Geosciences, and Biosciences by the Condensed Phase and Interfacial Molecular Sciences Program of the US Department of Energy. Soil NEXAFS spectra were acquired at the Canadian Light Source, which is supported by the Canada Foundation for Innovation, Natural Sciences and Engineering Research Council of Canada, the University of Saskatchewan, the Government of Saskatchewan, Western Economic Diversification Canada, the National Research Council Canada, and the Canadian Institutes of Health Research. We acknowledge use of the routine operation data from the Atmospheric Radiation Measurement Climate Research Facility at the Southern Great Plains site of OBER (http://www.archive.arm.gov). We acknowledge use of the NOAA Air Resources Laboratory for the provision of the HYSPLIT transport and dispersion model and READY website (http://www.ready.noaa.gov) used in this publication.