Add quick fix for aerosol pH calculation when dry

Xuan Wang wrote:

   My quick fix is very simple, which only changes one number in the source
   code:

   In GeosCore/ isorropiaII_mod.F, line 685, change

            NITRTEMP        = 1e-30_fp
            State_Chm%pHSav(I,J,L) = -999e+0_fp
         ELSE

   To
            NITRTEMP        = 1e-30_fp
            State_Chm%pHSav(I,J,L) = 20e+0_fp
         ELSE

Becky Alexander wrote:

   Note that [this fix] is not ideal. We have a better way of doing it
   which requires some sort of counting diagnostic.  We have this working
   for .bpch only because in the version of the code that we are using
   (the one from Xuan's paper, v11-02d), not all of the netCDF diagnostics
   work.  So we are unable to test this fix for the netCDF diagnostics.

Signed-off-by: Melissa Sulprizio <mpayer@seas.harvard.edu>

GeosCore/isorropiaII_mod.F

@@ -678,13 +678,13 @@ SUBROUTINE DO_ISORROPIAII( am_I_Root,  Input_Opt,  State_Chm,
          ! in mol/m3 air --> convert to mol/L water
          IF ( AERLIQ(8) < 1e-18_fp ) THEN
             ! Aerosol is dry so HPLUSTEMP and PH_SAV are undefined
-            ! We force HPLUSTEMP to 1d20 and PH_SAV to -999e+0_fp.
-            ! (hotp, ccc, 12/18/09)
+            ! We force HPLUSTEMP to 1d20 (hotp, ccc, 12/18/09)
+            ! Force pHSav to 20e0 (X. Wang, 6/27/19)
             HPLUSTEMP       = 1e+20_fp
             SULFTEMP        = 1e-30_fp
             BISULTEMP       = 1e-30_fp
             NITRTEMP        = 1e-30_fp
-            State_Chm%pHSav(I,J,L) = -999e+0_fp
+            State_Chm%pHSav(I,J,L) = 20e+0_fp
          ELSE
             HPLUSTEMP       = AERLIQ(1) / AERLIQ(8) * 1e+3_fp/18e+0_fp
             SULFTEMP        = AERLIQ(5) / AERLIQ(8) * 1e+3_fp/18e+0_fp