"As molecular cluster ions are constantly present in the air in low concentrations, our experiment has identified a new atmospheric absorber of infra-red, and measured its contribution to the radiative balance," Karen Aplin of the University of Oxford told environmentalresearchweb.

Aplin and colleague Mike Lockwood from the University of Reading used a specially designed infrared sensor – an atmospheric thermopile radiometer – alongside a small cosmic-ray detector containing vertically stacked Geiger counters. The radiometer was tuned to the nine-micron wavelength at which earlier lab tests had indicated molecular cluster ions absorb.

"Cosmic rays ionize the air so we knew that when our detector fired there had been a shower of high-energy particles creating molecular cluster ions in the troposphere above the radiometer," said Aplin. "We used the detector firing as a 'trigger' to look for a response in the infrared radiation."

Cosmic rays – high-energy particles like protons and atomic nuclei from outside the solar system – typically create an "air shower" of secondary subatomic particles such as muons and electrons. In turn, these secondary particles can ionize molecules present in the atmosphere to create atmospheric molecular cluster ions.

"Laboratory chemistry experiments have measured infrared absorption from cluster ions, in particular a cluster ion known as the 'protonated hydrate' H2O + (H3O)n," said Aplin. "This species has previously been found to be one of those naturally present in the atmosphere, which, with the laboratory findings, led us to anticipate that it might have an infrared effect in the atmosphere too."

The team found that molecular cluster ions absorbed an average of 7 mWm–2 in the 9.15-μm absorption band. The integrated atmospheric energy change for each event was 2 Jm–2, an amplification factor of 1012 compared to the estimated energy density of a typical air shower. "This absorption is expected to occur continuously and globally, but calculations suggest that it has only a small effect on climate," the scientists wrote in Environmental Research Letters.

The team's lab experiments, as well as work by others, indicate that molecular cluster ions may affect several other absorption bands, said Aplin. "If this is found to be the case, the total atmospheric infrared absorption from molecular cluster ions may be greater than our estimates in the paper suggest, and might need to be allowed for in, for example, satellite cloud retrievals," she added. "In the longer term, it may be possible to separate out the infrared 'signatures' of individual atmospheric molecular cluster-ion species to identify them, providing a new tool for studying other planetary atmospheres as well as our own."

Aplin said she will now be looking for this effect at other locations and in other absorption bands using a range of different detection techniques. "There is also more work to be done on understanding the relationship between atmospheric molecular cluster-ion concentrations and infrared absorption."

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