OH...

So we closed last week on the subject of '^•OH' - atmospheric ^•OH unlike the molecule OH- is a radical molecule created in small quantities in the troposphere. Its concentrations are very low and it has a life of less than 1 second but it plays an incredibly important role in digesting methane and other atmospheric pollutants and GHGs it is exposed to. A summary of this courtesy of the University of Leeds' School of Chemistry is given below:

"OH is primarily produced by the photolysis of ozone followed by reaction with H₂O. It is the primary daytime oxidising species responsible for the removal of CO, CH₄ (and higher hydrocarbons), H₂, NO₂, H₂S, (CH₃)₂S, NH₃, the hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). The concentration of OH defines the oxidising capacity of the atmosphere and hence the ability to control levels of species that contribute to global warming, acid rain, and photochemical smog."

Clearly then it is quite valuable little molecule with an important function, especially for methane with "The primary sink [for CH4 being by] hydroxyl radicals (OH), mostly in the troposphere, which accounts for around 90% of the global CH4 sink. Additional oxidation sinks include methanotrophic bacteria in aerated soils27,28 (~4%)" (Kirschke et al., 2013).

 The following schematic is a simplified diagram showing the main sources and sinks of •OH in the troposphere (Fiore, 2014).  The key ingredients to •OH are: 03, UV light and H20.

Sources and sinks of atmospheric OH 

(Fiore, A. Nature 513)

All 3 are needed to produce OH and if one is lacking the rate of OH production is reduced accordingly. We read last week about suggestions that in times of large scale methane releases atmospheric OH may be inundated so to speak by the amount of methane present in air -  that the OH produced is unable to process the methane and break it down at a rate that permits its current classification of a GHG with a 100 year GWP of 34 that it is known to have today.

As OH is so short lived and only found in the upper atmosphere we can't look at any direct OH record to find this out. We have to study methane records, the flux of which through time can be used to infer whether OH was in such short supply in the atmosphere that methane could not be broken down at the rate it does at present which we consider to be normal.

There are though two significant limitations to deducing results from methane records in this manner:

1. It only really works when the duration and magnitude of methane release is known
2. It only really works when the methane record has a respectable resolution

The latter of these is touched upon in Bock et al., 2012 - we learn:

"Air bubbles trapped in polar ice provide an almost direct record of atmospheric methane over the last 800kyr... Before being trapped in bubbles, the air slowly diffuses in the firn, from the surface down to the close-off zone. The bubble enclosure also takes place progressively. Hence, fast variations of the atmospheric signal are partly smoothed out."

It's clear then the difficulties of drawing conclusions from past methane degassing events when even large emissions form smoothed atmospheric signals in our best long term records - bubbles in ice cores. This hasn't stopped scientists from looking to the future and speculating (with the aid of climate models) what methane releases could cause if they were to take place.  

Isaksen et al.'s 2011 is a paper focused specifically on the modelling of atmospheric chemistry feedbacks in response to methane emissions, it comes to several conclusions of note that mirror those in similar recent papers:

1. Assuming several hypothetical scenarios of CH4 release associated with permafrost thaw, shallow marine hydrate degassing, and submarine landslides, we find a strong positive feedback on RF through atmospheric chemistry 
2. In particular, the impact of CH4 is enhanced through increase of its lifetime, and of atmospheric abundances of ozone, stratospheric water vapor, and CO2 as a result of atmospheric chemical processes. 
3. "Additional studies linking CH4 emissions to the possibilities for large future warming in the Arctic are needed." 

To wrap this lengthy (but important) post up I would say despite recent increases in interest in the abundance of OH and whether it is becoming increasingly depleted the very scarcity, longevity and residence location of OH makes it a tricky one to pin down and study. As many scientists are saying it clearly should be the focus of further study in coming years, especially as polar regions (with significant frozen methane reservoirs) appear to be responding to GW at a rate far greater than in the mid latitudes.


References:

Bock, J., Martinerie, P., Witrant, E. and Chappellaz, J. (2012). Atmospheric impacts and ice core imprints of a methane pulse from clathrates. Earth and Planetary Science Letters, 349-350, pp.98-108.

Fiore, A. (2014). Atmospheric chemistry: No equatorial divide for a cleansing radical. Nature, 513(7517), pp.176-178.

Kirschke, S., Bousquet, P., Ciais, P., Saunois, M., Canadell, J., Dlugokencky, E., Bergamaschi, P., Bergmann, D., Blake, D., Bruhwiler, L., Cameron-Smith, P., Castaldi, S., Chevallier, F., Feng, L., Fraser, A., Heimann, M., Hodson, E., Houweling, S., Josse, B., Fraser, P., Krummel, P., Lamarque, J., Langenfelds, R., Le Quéré, C., Naik, V., O'Doherty, S., Palmer, P., Pison, I., Plummer, D., Poulter, B., Prinn, R., Rigby, M., Ringeval, B., Santini, M., Schmidt, M., Shindell, D., Simpson, I., Spahni, R., Steele, L., Strode, S., Sudo, K., Szopa, S., van der Werf, G., Voulgarakis, A., van Weele, M., Weiss, R., Williams, J. and Zeng, G. (2013). Three decades of global methane sources and sinks. Nature Geoscience, 6(10), pp.813-823.

Isaksen, I., Gauss, M., Myhre, G., Walter Anthony, K. and Ruppel, C. (2011). Strong atmospheric chemistry feedback to climate warming from Arctic methane emissions. Global Biogeochem. Cycles, 25(2), p.n/a-n/a.