Fix EMEP failure due to units

[thorbjoernl]

Change Summary

Related issue number

closes #1624

Checklist

• [ ] Start with a draft-PR
• [ ] The PR title is a good summary of the changes
• [ ] PR is set to AeroTools and a tentative milestone
• [ ] Documentation reflects the changes where applicable
• [ ] Tests for the changes exist where applicable
• [ ] Tests pass locally
• [ ] Tests pass on CI
• [ ] At least 1 reviewer is selected
• [ ] Make PR ready to review

[codecov[bot]]

Codecov Report

:x: Patch coverage is 73.07692% with 7 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 77.97%. Comparing base (7bc3389) to head (f0b0506). :warning: Report is 528 commits behind head on main-dev.

Files with missing lines	Patch %	Lines
pyaerocom/ungriddeddata_structured.py	66.66%	7 Missing :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##           main-dev    #1626      +/-   ##
============================================
+ Coverage     77.94%   77.97%   +0.03%     
============================================
  Files           163      163              
  Lines         21774    21790      +16     
============================================
+ Hits          16971    16991      +20     
+ Misses         4803     4799       -4     

Flag	Coverage Δ
unittests	77.97% <73.07%> (+0.03%)	:arrow_up:

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