Implement brown carbon with photolysis

[megan-he]

Name and Institution (Required)

Name: Megan He
Institution: Harvard University

Describe the update

This PR implements brown carbon (BrC) in GEOS-Chem following the optical property approach of Hammer et al. (2016), with the main purpose of investigating its effect on photolysis and global [OH]. Rather than introducing a new BrC tracer, we apply Hammer et al.'s effective imaginary refractive index values at BrC/POC = 1.0. From Hammer et al.: "The columns with effective k values for BrC/POC fraction of unity offer the convenience of representing the effective absorption by BrC without needing to assume an arbitrary BrC/POC fraction, or to introduce a separate BrC tracer. The effective k values for unity BrC/POC fraction can be thought of as the effective imaginary refractive index for an internal mixture of BrC and colorless POC."

BrC optical properties are read from the existing /n/holylfs06/LABS/jacob_lab/Everyone/GEOS-CHEM/gcgrid/gcdata/ExtData/CHEM_INPUTS/CLOUD_J/v2025-01/FJX_scat-aer.dat entries derived from Hammer et al., with density = 1.3 g/cm3. BrC is treated as hygroscopic. Many of the changes are related to aerosol indexing due to the additional aerosol species.

Expected changes

Gao et al. (2026) treats all organic aerosol originated from residential combustion sector as BrC. So in a 1-year 2024 test simulation, 100% of biofuel OC (CEDS, DICE-Africa) and biomass burning OC (GFED4) is treated as fresh BrC, which is consistent with the BrC/POC = 1.0 approach in Hammer et al. Note that applying Hammer et al.'s optical properties to biofuel BrC is likely a conservative lower bound on biofuel BrC absorption.

JO1D values and OH concentrations decrease most strongly in biomass burning and biofuel regions, but global mean differences are minimal (<0.5%). Difference plots between the 1-year 2024 test simulation (with 6-month spinup) and a reference simulation for Jan and July at the surface and at 500 hPa are attached below. (In the OH surface plots, I masked out where reference OH was very low/near zero, essentially NH in winter.)

Reference(s)

Hammer, M. S., Martin, R. V., van Donkelaar, A., Buchard, V., Torres, O., Ridley, D. A., and Spurr, R. J. D. (2016). Interpreting the ultraviolet aerosol index observed with the OMI satellite instrument to understand absorption by organic aerosols: implications for atmospheric oxidation and direct radiative effects, Atmos. Chem. Phys., 16, 2507–2523. https://doi.org/10.5194/acp-16-2507-2016

Gao, Y., Liu, M., Zhou, R., Cai, X., Zhang, H., Song, Y., et al. (2026). Significant radiative absorption of brown carbon aerosols from residential fuel combustion in developing regions. Geophysical Research Letters, 53, e2026GL121829. https://doi.org/10.1029/2026GL121829

[lizziel]

Thanks @megan-he. I'm adding a note here that this PR also requires merge of geoschem/Cloud-J#35.

[yantosca]

Thanks @lizziel and @megan-he. I think this can go into 14.9.0 (or 15.0.0, depending on whatever we call the next release).