UCX chemistry mechanism

On this page, we provide information about the Universal tropospheric-stratospheric Chemistry eXtension (UCX) mechanism in GEOS-Chem. UCX combines both tropospheric and stratospheric reactions into a single chemistry mechanism. It was developed by Sebastian Eastham, Debra Weisenstein, and Steven Barrett at the Laboratory for Aviation and the Environment at the Massachusetts Institute of Technology.

Overview

Please see the following documents, which describe the UCX mechanism in more detail:

1. Eastham, S.D., Weisenstein, D.K., Barrett, S.R.H., Development and evaluation of the unified tropospheric–stratospheric chemistry extension (UCX) for the global chemistry-transport model GEOS-Chem, Atmos. Env., June 2014. (Article)
2. Technical Note: The GEOS-Chem Unified Tropospheric-Stratospheric Chemistry Extension (UCX): Interim Distribution, September 2013 (Article)

--Sebastian D. Eastham 13:08, 03 March 2014 (EDT)

Obtaining the source code

Provisional code

Steven Barrett wrote:

As a few people have asked for the code for the strat chem version of GEOS-Chem, we've posted it as a simple compressed file here as a temporary location so they can use it right away until it gets integrated into the main code. [Until then], we'll just keep track of any fixes/issues that come up until then.

--Bob Y. 13:44, 30 September 2013 (EDT)

Standard code

UCX has been added to GEOS-Chem v10-01c, which is currently undergoing benchmarking and validiation. It will be included in the GEOS-Chem v10-01 public release, slated for late 2014.

--Bob Y. 14:44, 25 April 2014 (EDT)

Previous issues that are now resolved

The following issues were discovered and corrected when UCX was added into the standard GEOS-Chem code (in version v10-01):

Error in Fast-JX for tropopsheric only simulation

The Fast-JX photolysis package defines 12 wavelength bins that are used for tropospheric chemistry simulations (i.e. without UCX), and 18 wavelength bins for stratospheric chemistry simulations (i.e. with UCX). An bug in the original Fast-JX code caused the cross-sections to be interpolated incorrectly for tropospheric-only chemistry simulations. A full description of this error may be found in this post on our Development of Fast-JX in GEOS-Chem wiki page.

--Bob Y. 14:48, 13 May 2014 (EDT)

Modify treatment of N2O5 + H2O reaction in calcrate.F

The reaction for N2O5 + H2O was found to be problematic in long-term UCX simulations where the H2O concentration (stored in the EDUCTCONC variable in file GeosCore/calcrate.F) was low and caused the solver to choke. Sebastian Eastham suggested that we include the following fix in the standard code.

Sebastian Eastham wrote:

The division is necessary to compensate for the change from N2O5 -> 2HNO3 to N2O5 + H2O -> 2HNO3 - it's still "as in base GEOS-Chem" though in the sense that it doesn't check to see if N2O5 or H2O is the limiting reactant (and possibly herein lies the problem!). It might be worth seeing if the treatment used for the other aerosol reactions (not including the check against parameter PSCMINLIFE) would help.

Following Sebastian's suggestion, we have made the following modification in this section of calcrate.F for better numerical stability:

                  ! If all these checks are passed, we set k = ki/[B].
                  ! Rxn 11 is first-order - ignore
                  IF (PSCIDX.eq.1) THEN
!------------------------------------------------------------------------------
! Prior to 5/12/14:
! Use same treatment used for other aerosol reactions (not including the
! check against PSCMINLIFE) (sde, mps, 5/12/14)
!                     ! N2O5 + H2O is an old reaction, allow to proceed
!                     ! as it did in base GEOS-Chem
!                     IF (IS_SAFE_DIV(ADJUSTEDRATE,EDUCTCONC)) THEN
!                        ADJUSTEDRATE = ADJUSTEDRATE/EDUCTCONC
!                     ELSE
!                        ADJUSTEDRATE = 0d0
!                     ENDIF
!------------------------------------------------------------------------------
                     ! Convert from 1st-order to 2nd-order
                     IF (IS_SAFE_DIV(EDUCTCONC,LIMITCONC)) THEN
                        ! Temporarily store [B]/(T*[A])
                        LIMITCONC = EDUCTCONC/(PSCMINLIFE*LIMITCONC)
                        IF (ADJUSTEDRATE.gt.LIMITCONC) THEN
                           ADJUSTEDRATE = LIMITCONC
                        ENDIF
                     ELSE
                        ADJUSTEDRATE = 0d0
                     ENDIF
                     IF ((EDUCTCONC.gt.1.d2).and.
     &                      (IS_SAFE_DIV(ADJUSTEDRATE,EDUCTCONC))) THEN
                        ADJUSTEDRATE = ADJUSTEDRATE/EDUCTCONC
                     ELSE
                        ADJUSTEDRATE = 0d0
                     ENDIF
   

--Bob Y. 14:56, 13 May 2014 (EDT)