Add logical to allow for the fixing the duplicate heterogeneous chemistry reactions in UKCA StratTrop and CRI-Strat2 schemes

[Luke Abraham (theabro)]

PR Summary

Sci/Tech Reviewer:
Code Reviewer: James Bruten (@james-bruten-mo)

Within UKCA, N2O5 hydrolysis

N2O5 + H2O --> HONO2 + HONO2

is actually covered by 2 separate reactions, H04 and B85. There is actually considerable overlap of these reactions in the troposphere, leading to overcounting.

A fix is to ensure, using the tropospheric mask, that only H04 works in the stratosphere and B85 only works in the troposphere.

This will be controlled with a temporary logical and changes are required to the UKCA (MetOffice/ukca#30) to account for this. This issue was originally identified by Dr Yao Ge (@yg460-cam) in Cambridge.

Initially the logical, l_fix_ukca_n2o5_h2o, will be hard-coded to .false. in the UKCA interface, as this fix is targetting UKESM2. Later changes will be made to allow for this logical to be passed via namelist.

• linked MetOffice/um#71
• linked Hygroscopicity MS11 ratio option ukca#31

• closes Add logical to allow for the fixing the duplicate heterogeneous chemistry reactions in UKCA StratTrop and CRI-Strat2 schemes #432

Code Quality Checklist

• I have performed a self-review of my own code
• My code follows the project's style guidelines
• Comments have been included that aid understanding and enhance the readability of the code
• My changes generate no new warnings
• All automated checks in the CI pipeline have completed successfully

Testing

• I have tested this change locally, using the LFRic Apps rose-stem suite
• If any tests fail (rose-stem or CI) the reason is understood and acceptable (e.g. kgo changes)
• I have added tests to cover new functionality as appropriate (e.g. system tests, unit tests, etc.)
• Any new tests have been assigned an appropriate amount of compute resource and have been allocated to an appropriate testing group (i.e. the developer tests are for jobs which use a small amount of compute resource and complete in a matter of minutes)

trac.log

Test Suite Results - lfric_apps - um14.1_hetchem_fixes_lfric_apps/run1

Suite Information

Item	Value
Suite Name	um14.1_hetchem_fixes_lfric_apps/run1
Suite User	luke.abraham
Workflow Start	2026-04-15T15:16:57
Groups Run	developer

Dependency	Reference	Main Like
casim	MetOffice/casim@2026.03.2	True
jules	MetOffice/jules@2026.03.2	True
lfric_apps	theabro/lfric_apps@um14.1_hetchem_fixes	False
lfric_core	MetOffice/lfric_core@2026.03.2	True
moci	MetOffice/moci@2026.03.2	True
SimSys_Scripts	MetOffice/SimSys_Scripts@2026.03.2	True
socrates	MetOffice/socrates@2026.03.2	True
socrates-spectral	MetOffice/socrates-spectral@2026.03.2	True
ukca	theabro/ukca@um14.1_hetchem_fixes	True

Task Information

✅ succeeded tasks - 1164

Security Considerations

• I have reviewed my changes for potential security issues
• Sensitive data is properly handled (if applicable)
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Performance Impact

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AI Assistance and Attribution

• Some of the content of this change has been produced with the assistance of Generative AI tool name (e.g., Met Office Github Copilot Enterprise, Github Copilot Personal, ChatGPT GPT-4, etc) and I have followed the Simulation Systems AI policy (including attribution labels)

Documentation

• Where appropriate I have updated documentation related to this change and confirmed that it builds correctly

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Sci/Tech Review

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• Sufficient testing has been completed

(Please alert the code reviewer via a tag when you have approved the SR)

Code Review

• All dependencies have been resolved
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• Documentation is complete and accurate
• Security considerations have been addressed
• Performance impact is acceptable