The new oxidant has “significant capacity” to oxidise sulphur dioxide

Scientists have for the first time found unequivocal evidence of another oxidant playing a vital role in the formation of gaseous sulphuric acid in the atmosphere. Until now, it was thought that the combination of OH (hydroxyl radical) oxidant with sulphur dioxide (SO) was the dominant way by which gaseous sulphuric acid is formed in the atmosphere. The results are published on August 9, 2012 in Nature.

R.L. Mauldin from the University of Helsinki and other co-authors are yet to identify the oxidant with certainty, and have named it as ‘X.’ They found the new oxidant has “significant capacity” to oxidise sulphur dioxide.

The investigation into the presence of ‘X’ was conducted in the boreal forest region in Finland. The OH concentrations are not constant throughout the day. The maximum concentration is found at noon and much lower values during night.

Measurements taken in the field for a week showed that the concentration of ‘X’ also does not remain constant during the day. But its levels are much higher than the hydroxyl radical (OH) during evenings and nights. And the levels of gaseous sulphuric acid measured were also much higher than expected if only OH were present.

“Sulphuric acid originating from this non-OH source may contribute up to 50 per cent of the total HSO budget, demonstrating the important role of this HSO formation route,” they write.

Since the new oxidant was found to be particularly abundant during the evenings and nights, the scientists postulate that it must be related to “reaction of surface emission, such as naturally produced hydrocarbons, with ozone.”

To confirm their findings, the scientists carried out laboratory experiments where SO was exposed to mixtures of ozone and various alkenes, and the resulting atmospheric sulphuric acid levels were measured.

The experiments were repeated using an OH scavenger. To their surprise, sulphuric acid was still being formed, thus confirming the results obtained in the field.

The most efficient way of producing atmospheric sulphuric acid from a non-OH source could be from monoterpenes than for other alkenes, they note. Apparently, monoterpenes, including limonene and alpha pinene used in the experiment, are emitted by trees. And these two were found in abundance in the field.

They followed the next logical step of reconfirming its actual production by the trees. They cut branches from different trees and measured the amount of OH and ‘X’ produced.

The amount of OH produced was “minor in comparison to production of ‘X.’”

In conclusion, they state: “Our findings add to the already substantial significance of forests in the Earth system by introducing a previously unknown oxidant, probably an sCI, capable of oxidizing at least SO and possibly also other atmospheric trace gases relevant to atmospheric chemistry.”

“The technique used by them to measure OH is known as chemical ionization mass spectroscopy (CIMS), and it has been used in a range of environments. It is therefore surprising that the significance of background signals has not been recognized in previous studies,” notes a News piece in the same issue of the journal.

“The forested environment studied by the authors emits large quantities of alkene, and so provides ideal conditions for the formation of ‘X.’ Measurements of ‘X’ are now needed in other environments, to determine its global impact on the production of atmospheric sulphuric acid.”

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