Fix duplicate heterogeneous chemistry reactions in UKCA StratTrop and CRI-Strat2

[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 is controlled by a temporary logical, l_fix_ukca_n2o5_h2o.

This issue was originally identified by Dr Yao Ge (@yg460-cam) in Cambridge, and she provided the original UKCA code changes at UM13.9.

het_rxn_diffs.pdf shows comparisons between two simulations performed by Yao at UM13.9, highlightning the improvement given by this change (top) compared to the default behaviour (bottom).

• linked MetOffice/um#71
• linked Add logical to allow for the fixing the duplicate heterogeneous chemistry reactions in UKCA StratTrop and CRI-Strat2 schemes lfric_apps#433

• closes Fix duplicate heterogeneous chemistry reactions in UKCA StratTrop and CRI-Strat2 #30

Code Quality Checklist

(Some checks are automatically carried out via the CI pipeline)

• I have performed a self-review of my own code
• My code follows the project's style guidelines
• Comments have been included that aid undertanding and enhance the
  readability of the code
• My changes generate no new warnings

Testing

• I have tested this change locally, using the UKCA rose-stem suite
• If shared files have been modified, I have run the UM and LFRic Apps rose
  stem suites
• If any tests fail (rose-stem or CI) the reason is understood and
  acceptable (eg. kgo changes)
• I have added tests to cover new functionality as appropriate (eg. system
  tests, unit tests, etc.)

Testing was performed in the UKCA short testing suite:

• Including the change but with the logical turned off (new diagnostics were output): u-dy021
• Including the change and with the logical turned ON (new diagnostics were output): u-dy079

This tests StratTrop and CRI-Strat2 with rigorous tests.

UMDP3 was checked with rose-stem.

trac.log

Test Suite Results - ukca - um14.1_hetchem_fixes/run1

Suite Information

Item	Value
Suite Name	um14.1_hetchem_fixes/run1
Suite User	luke.abraham
Workflow Start	2026-03-25T13:30:04
Groups Run	scripts

Dependency	Reference	Main Like
SimSys_Scripts	MetOffice/[email protected]	True
ukca	theabro/[email protected]_hetchem_fixes	False

Task Information

✅ succeeded tasks - 2

Security Considerations

• I have reviewed my changes for potential security issues
• Sensitive data is properly handled (if applicable)
• Authentication and authorisation are properly implemented (if applicable)

Performance Impact

• Performance of the code has been considered and, if applicable, suitable
  performance measurements have been conducted

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

Sci/Tech Review

• I understand this area of code and the changes being added
• The proposed changes correspond to the pull request description
• Documentation is sufficient (do documentation papers need updating)
• 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
• Related Issues have been properly linked and addressed
• CLA compliance has been confirmed
• Code quality standards have been met
• Tests are adequate and have passed
• Documentation is complete and accurate
• Security considerations have been addressed
• Performance impact is acceptable