diff --git a/docs/src/ReactionCatalog.md b/docs/src/ReactionCatalog.md
index 5976de3..f24b72c 100644
--- a/docs/src/ReactionCatalog.md
+++ b/docs/src/ReactionCatalog.md
@@ -13,6 +13,7 @@ CurrentModule = PALEOboxes.Reservoirs
 ```
 ```@docs
 ReactionReservoirScalar
+ReactionReservoirAtm
 ReactionReservoir
 ReactionReservoirWellMixed
 ReactionReservoirConst
diff --git a/src/reactioncatalog/Reservoirs.jl b/src/reactioncatalog/Reservoirs.jl
index 436c9ea..ec1483e 100644
--- a/src/reactioncatalog/Reservoirs.jl
+++ b/src/reactioncatalog/Reservoirs.jl
@@ -121,6 +121,109 @@ function PB.register_methods!(rj::ReactionReservoirScalar)
     return nothing
 end
 
+
+"""
+    ReactionReservoirAtm
+
+A single scalar atmosphere reservoir (state variable).
+
+Creates a state Variable `R` (units mol, with attribute `vfunction=VF_StateExplicit`) and 
+(if parameter `const=false`) corresponding source-sink flux `R_sms` (units mol yr-1, with attribute `vfunction=VF_Deriv`).
+
+Two associated Variables are also created:
+- `pRatm`, partial pressure in units of bar or atm.  This is calculated by dividing `R` (mol) by
+  parameter `moles1atm` (which has a default value for global modern Earth atmosphere).
+- `pRPAL`, value normalized eg to a present-day modern Earth value. Calculated by dividing `R` (mol) by attribute `R:norm_value`,
+  which should be set in the .yaml file to a representative value.
+
+The local name prefix `R` should then be renamed using `variable_links:` in the configuration file.
+
+# Parameters
+$(PARS)
+
+# Methods and Variables for default Parameters
+$(METHODS_DO)
+"""
+Base.@kwdef mutable struct ReactionReservoirAtm{P} <: PB.AbstractReaction
+    base::PB.ReactionBase
+    
+    pars::P = PB.ParametersTuple(
+        PB.ParType(PB.AbstractData, "field_data", PB.ScalarData,
+            allowed_values=PB.IsotopeTypes,
+            description="disable / enable isotopes and specify isotope type"),
+        PB.ParBool("const", false,
+            description="true to provide constant value with no _sms Variable"),
+        PB.ParDouble("moles1atm", PB.Constants.k_moles1atm, units="mol",
+            description="moles for a pressure of 1 Earth atm.  Default value $(PB.Constants.k_moles1atm) for global modern Earth atmosphere")
+    )
+
+    norm_value::Float64 = NaN
+end
+
+function PB.register_methods!(rj::ReactionReservoirAtm)
+ 
+    @info "register_methods! ReactionReservoirAtm $(PB.fullname(rj)) field_data=$(rj.pars.field_data[])"
+    PB.setfrozen!(rj.pars.field_data)
+
+    if rj.pars.const[]
+        R = PB.VarPropScalar(      "R", "mol", "scalar constant reservoir", attributes=(:field_data=>rj.pars.field_data[],))
+        R_sms = PB.VarTarget(     "R_sms", "mol yr-1", "constant reservoir source-sinks", attributes=(:field_data=>rj.pars.field_data[],))
+    else
+        R = PB.VarStateExplicitScalar("R", "mol", "scalar reservoir", attributes=(:field_data=>rj.pars.field_data[],))
+        R_sms = PB.VarDerivScalar(     "R_sms", "mol yr-1", "scalar reservoir source-sinks", attributes=(:field_data=>rj.pars.field_data[],))
+    end
+    PB.setfrozen!(rj.pars.const)
+
+    # sms variable not used by us, but must appear in a method to be linked and created
+    PB.add_method_do_nothing!(rj, [R_sms])
+
+    vars = [
+        R,
+        PB.VarPropScalar(         "pRnorm", "", "scalar atmosphere reservoir normalized eg to present-day value"),
+        PB.VarPropScalar(         "pRatm", "atm", "scalar atmosphere reservoir partial pressure"),
+    ]
+       
+    if rj.pars.field_data[] <: PB.AbstractIsotopeScalar
+        push!(vars, 
+            PB.VarPropScalar(     "R_delta", "per mil", "scalar atmosphere reservoir isotope delta"))
+    end
+
+    # callback function to store Variable norm during setup
+    function setup_callback(m, attribute_value, v, vdata)
+        v.localname == "R" || error("setup_callback unexpected Variable $(PB.fullname(v))")
+        if attribute_value == :norm_value
+            m.reaction.norm_value = PB.value_ad(PB.get_total(vdata[]))
+        end
+        return nothing
+    end
+    # set filterfn to force setup even if R is constant, not a state Variable
+    PB.add_method_setup_initialvalue_vars_default!(rj, [R], filterfn = v->true, setup_callback=setup_callback)  
+
+    PB.add_method_do!(
+        rj,
+        do_reservoir_atm,
+        (PB.VarList_namedtuple(vars), ),
+    )
+
+    PB.add_method_initialize_zero_vars_default!(rj)
+    
+    return nothing
+end
+
+function do_reservoir_atm(m::PB.AbstractReactionMethod, pars, (vars, ), cr::PB.AbstractCellRange, deltat)
+    rj = m.reaction
+
+    vars.pRnorm[]  = PB.get_total(vars.R[])/rj.norm_value
+    vars.pRatm[]  = PB.get_total(vars.R[])/pars.moles1atm[]
+
+    if hasfield(typeof(vars), :R_delta)
+        vars.R_delta[] = PB.get_delta(vars.R[])
+    end
+
+    return nothing
+end
+
+
 """
     ReactionReservoir,  ReactionReservoirTotal