Saturday, July 23, 2022

More Hydrogen Less Methane about the Same HO

 Recently a bunch of wanna be paid attentiontos have made a grab for the majic ring by selling their view that the global warming effect of hydrogen is gonna be major. As usual such rumors spread like lightning on Twitter and the like.

Uncle Eli is here to tell you that this is finger on the scale stuff. The kick off was "Global modelling studies of hydrogen and its isotopomers using STOCHEM-CRI: Likely radiative forcing consequences of a future hydrogen economy", Richard G. Derwent, David S. Stevenson, Steven R. Utembe, Michael E. Jenkin, Anwar H. Khan, Dudley E. Shallcross  International Journal of Hydrogen Energy Volume 45, 18 March 2020, Pages 9211-922

It goes like this, Derwent, et al. modelled the effect of injecting a pulse of 1.67 Tg of H2 into the atmosphere and find that it has a global warming potential of 5 on a 100 year basis. Their model shows that this is because the H2 reacts with HO radicals which are thus not available to react with methane, CH4, so the concentration of methane,  an important greenhouse gas will be higher. There is also a secondary effect where ozone, O3, is produced.

The TL:DR or as the bunnies call it today, a Tweet, is that 

H2 is introduced to take the place of  CH4 so  to make an honest calculation of what happens in a hydrogen economy, (finger off the scale) you not only have to pulse H emissions but also decrease CH4 emissions by a similar amount.

The simplified kinetics of HO in the atmosphere is an equilibrium between HO and HO2. HO is very reactive and once formed finds something, anything that is not totally oxidized like CO2, to react with. It only lasts for a second or so. HO2 is much less reactive, but still lasts for maybe ten times as long. There is a constant interchange between them and regeneration by other reactions.

Murray et al. have a nice schematic of HO atmospheric chemistry in the supplement to a PNAS article, which Eli will borrow with a couple of additions, adding the H2 reaction with OH and the termination reaction for HO2 by recombination

Sc is the reaction of HO with CO, which accounts for 40-50% of HO reactions in the atmosphere, methane accounts for about 15%, the rest being ozone and other hydrocarbons but on a rough basis Sc is the reaction of HO with any carbon containing molecule except the totally oxidized CO2 

SN and LN are the source and loss of NO and NO2  respectively. NO conversion of HO2  to HO is what maintains the equilibrium between them

The basic reactions are

CH4  + HO → CH3 + H2

Followed by a series of reactions which eventually lead to 

the reaction of CO and HO
CO + HO + O2 → CO2 + HO2  [R4]

For H2 oxidation

HO + H2  → H2O + H 
followed rapidly by
H + O2 + M → HO+ M

where M is anything that does not react, in the case of the atmosphere nitrogen mostly. It represents a pressure effect which confines the reactants.  These two reactions are often put together 

HO + H2 +  O2 → H2O + HO2  [RH]

In the troposphere, conversion of HO2  to HO is driven by reaction by nitric oxide

HO2 + NO → HO + NO2       [R5] 

and NO is regenerated by photodissociation, which also yields ozone. This reaction occurs whether independent of whether the was formed by oxidation of H2 or CH4 .

NO2 + hν  + O2 → NO + O3           [R6] 

These chains are terminated by

HO2  + HO2 → H2O2 + O2                  [R8] 
NO2 + HO + M−→ HNO+ M          [R9] 

Looking at the scheme for atmospheric oxidation of methane HO is consumed in many of them and only regenerated from the reaction of NO and HO2.  The reactions scheme for H2 is much simpler with only one HO radical consumed per oxidized  H2 molecule. So on the whole, per molecule, less HO reacts away.

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