Decades-long monitoring campaign used to improve understanding of the atmosphere's cleansing capacity


A study published in Nature by Prabir Patra of the Department of Environmental Geochemical Cycle Research at the Japan Agency for Marine-Earth Science and Technology and colleagues from Scripps Institution of Oceanography, UC San Diego, and elsewhere has used the long-term record of methyl chloroform (CH3CCl3) to infer the relative atmospheric cleansing capacity of the Northern and Southern hemispheres, which has important implications for the understanding of air pollutants and air quality.

Molecules known as hydroxyl (OH) radicals cleanse Earth’s lower atmosphere of pollutants; however, they occur unevenly and sparsely, which makes meaningful direct measurements challenging. But Patra and colleagues were able to calculate the atmospheric OH gradient between the two hemispheres using data of methyl chloroform – an industrial solvent that reacts with OH – from the Advanced Global Atmospheric Gases Experiment (AGAGE) and NOAA networks.

Key to the finding was the testing and use of a state-of-the-art model of atmospheric chemistry and transport to simulate methyl chloroform concentrations under different OH distributions. Prior estimates of OH have indicated between 13 and 42 percent more OH in the Northern Hemisphere than in the Southern Hemisphere, yet the results of Patra and colleagues indicate little contrast.

The work implies that current estimates based on atmospheric measurements and models of the emissions of gases such as methane, carbon monoxide, and nitrogen oxides from countries in the Northern Hemisphere may be too large. In fact, the estimated global emissions of such gases should be revised downward since a Northern Hemisphere atmosphere with lower OH would remove them less quickly.

Study co-authors include Tim Arnold, Jens Mühle, and Ray Weiss of Scripps Institution of Oceanography and international collaborators from 13 other institutions.

The data used in the study comes from measurements of a suite of trace gases at AGAGE stations at Cape Grim, Australia and Mace Head, Ireland begun in the late 1970s.

“The work also highlights the importance of data from long-term monitoring campaigns such as AGAGE and their relevance for use with the latest modeling tools in continuing to advance atmospheric science,” said Weiss, a co-principal investigator of AGAGE.

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