diff --git a/docs/source/dataanalysis/dataanalysis.rst b/docs/source/dataanalysis/dataanalysis.rst
index c36db2bd1..24b4cff7e 100644
--- a/docs/source/dataanalysis/dataanalysis.rst
+++ b/docs/source/dataanalysis/dataanalysis.rst
@@ -14,6 +14,27 @@ For data analysis of openPMD data, see examples of `many supported tools, Python
 
 See also `WarpX' documentation on openPMD <https://warpx.readthedocs.io/en/latest/dataanalysis/formats.html>`__.
 
+Additional Beam Attributes
+""""""""""""""""""""""""""
+
+We add the following additional attributes on the openPMD ``beam`` species at the monitor position.
+
+Reference particle:
+
+* ``beta_ref`` reference particle normalized velocity :math:`\beta = v/c`
+* ``gamma_ref`` reference particle Lorentz factor :math:`\gamma = 1/\sqrt{1-\beta^2}`
+* ``s_ref`` integrated orbit path length, in meters
+* ``x_ref`` horizontal position x, in meters
+* ``y_ref`` vertical position y, in meters
+* ``z_ref`` longitudinal position z, in meters
+* ``t_ref`` clock time * c in meters
+* ``px_ref`` momentum in x, normalized to proper velocity :math:`p_x = \gamma v_x`
+* ``py_ref`` momentum in y, normalized to proper velocity :math:`p_y = \gamma v_y`
+* ``pz_ref`` momentum in z, normalized to proper velocity :math:`p_z = \gamma v_z`
+* ``pt_ref`` energy deviation, normalized by rest energy
+* ``mass`` reference rest mass, in kg
+* ``charge`` reference charge, in C
+
 
 Reduced Beam Characteristics
 ----------------------------
diff --git a/src/particles/elements/diagnostics/openPMD.H b/src/particles/elements/diagnostics/openPMD.H
index 0dad78094..009929fa4 100644
--- a/src/particles/elements/diagnostics/openPMD.H
+++ b/src/particles/elements/diagnostics/openPMD.H
@@ -82,6 +82,8 @@ namespace detail
         BeamMonitor& operator= (BeamMonitor && other) = default;
 
         /** Prepare entering the element before starting push logic.
+         *
+         * And write reference particle.
          *
          * @param[in] pc particle container
          * @param[in] ref_part reference particle
@@ -105,7 +107,11 @@ namespace detail
             int step
         );
 
-        // TODO: move to private function
+        /** Write a tile of particles
+         *
+         * @param pti particle tile iterator
+         * @param ref_part reference particle
+         */
         void operator() (
             PinnedContainer::ParIterType & pti,
             RefPart const & ref_part
diff --git a/src/particles/elements/diagnostics/openPMD.cpp b/src/particles/elements/diagnostics/openPMD.cpp
index 96e8d8146..1456bb965 100644
--- a/src/particles/elements/diagnostics/openPMD.cpp
+++ b/src/particles/elements/diagnostics/openPMD.cpp
@@ -260,12 +260,22 @@ namespace detail
             }
         }
 
-        // beam mass
-        beam.setAttribute( "mass", ref_part.mass );
+        // reference particle information
         beam.setAttribute( "beta_ref", ref_part.beta() );
         beam.setAttribute( "gamma_ref", ref_part.gamma() );
+        beam.setAttribute( "s_ref", ref_part.s );
+        beam.setAttribute( "x_ref", ref_part.x );
+        beam.setAttribute( "y_ref", ref_part.y );
+        beam.setAttribute( "z_ref", ref_part.z );
+        beam.setAttribute( "t_ref", ref_part.t );
+        beam.setAttribute( "px_ref", ref_part.px );
+        beam.setAttribute( "py_ref", ref_part.py );
+        beam.setAttribute( "pz_ref", ref_part.pz );
+        beam.setAttribute( "pt_ref", ref_part.pt );
+        beam.setAttribute( "mass", ref_part.mass );
+        beam.setAttribute( "charge", ref_part.charge );
 
-        // openPMD coarse position
+        // openPMD coarse position: for global coordinates
         {
             beam["positionOffset"]["x"].resetDataset(d_fl);
             beam["positionOffset"]["x"].makeConstant(ref_part.x);
@@ -319,7 +329,7 @@ namespace detail
                           }, true);
         */
 
-        // prepare element access
+        // prepare element access & write reference particle
         this->prepare(pinned_pc, ref_part, step);
 
         // loop over refinement levels
@@ -331,10 +341,7 @@ namespace detail
             using ParIt = PinnedContainer::ParIterType;
             // note: openPMD-api is not thread-safe, so do not run OMP parallel here
             for (ParIt pti(pinned_pc, lev); pti.isValid(); ++pti) {
-                // push reference particle in global coordinates
-                this->operator()(ref_part);
-
-                // push beam particles relative to reference particle
+                // write beam particles relative to reference particle
                 this->operator()(pti, ref_part);
             } // end loop over all particle boxes
         } // end mesh-refinement level loop