diff --git a/Metadata-standard-names.md b/Metadata-standard-names.md
index a8b9f45..f9a9faa 100644
--- a/Metadata-standard-names.md
+++ b/Metadata-standard-names.md
@@ -455,10 +455,10 @@ Note that appending '_on_previous_timestep' to standard_names in this section yi
     * `real(kind=kind_phys)`: units = kg kg-1
 * `mass_fraction_of_dust_accumulation_aerosol_particles_in_air`: Mass fraction of accumulation mode dust aerosol particles
     * `real(kind=kind_phys)`: units = kg kg-1
-* `number_fraction_of_coarse_aerosol_particles_in_air`: Number fraction of coarse mode dust aerosol particles
-    * `real(kind=kind_phys)`: units = 1
-* `number_fraction_of_accumulation_aerosol_particles_in_air`: Number fraction of accumulation mode dust aerosol particles
-    * `real(kind=kind_phys)`: units = 1
+* `number_fraction_of_coarse_aerosol_particles_in_air`: Ratio of number concentration of coarse mode dust aerosol particles to the molecular concentration of air
+    * `real(kind=kind_phys)`: units = particles molecules-1
+* `number_fraction_of_accumulation_aerosol_particles_in_air`: Ratio of number concentration of accumulation mode dust aerosol particles to the molecular concentration of air
+    * `real(kind=kind_phys)`: units = particles molecules-1
 ## emissions
 Emissions variables, contributed for the Community Emissions Data System (CEDS)
 * `emissions_of_co_due_to_anthropogenic`: Carbon monoxide emissions from anthropogenic sources, total
diff --git a/Metadata-standard-names.yaml b/Metadata-standard-names.yaml
index d7f8f85..62c5459 100644
--- a/Metadata-standard-names.yaml
+++ b/Metadata-standard-names.yaml
@@ -1072,15 +1072,17 @@ sections:
     kind: kind_phys
     units: kg kg-1
   - name: number_fraction_of_coarse_aerosol_particles_in_air
-    long_name: Number fraction of coarse mode dust aerosol particles
+    long_name: Ratio of number concentration of coarse mode dust aerosol particles
+      to the molecular concentration of air
     type: real
     kind: kind_phys
-    units: 1
+    units: particles molecules-1
   - name: number_fraction_of_accumulation_aerosol_particles_in_air
-    long_name: Number fraction of accumulation mode dust aerosol particles
+    long_name: Ratio of number concentration of accumulation mode dust aerosol particles
+      to the molecular concentration of air
     type: real
     kind: kind_phys
-    units: 1
+    units: particles molecules-1
 - name: emissions
   comment: Emissions variables, contributed for the Community Emissions Data System
     (CEDS)
diff --git a/standard_names.xml b/standard_names.xml
index 5658550..e45beb1 100644
--- a/standard_names.xml
+++ b/standard_names.xml
@@ -797,12 +797,12 @@
       <type kind="kind_phys" units="kg kg-1">real</type>
     </standard_name>
     <standard_name name="number_fraction_of_coarse_aerosol_particles_in_air"
-                   long_name="Number fraction of coarse mode dust aerosol particles">
-      <type kind="kind_phys" units="1">real</type>
+                   long_name="Ratio of number concentration of coarse mode dust aerosol particles to the molecular concentration of air">
+      <type kind="kind_phys" units="particles molecules-1">real</type>
     </standard_name>
       <standard_name name="number_fraction_of_accumulation_aerosol_particles_in_air"
-                   long_name="Number fraction of accumulation mode dust aerosol particles">
-      <type kind="kind_phys" units="1">real</type>
+                   long_name="Ratio of number concentration of accumulation mode dust aerosol particles to the molecular concentration of air">
+      <type kind="kind_phys" units="particles molecules-1">real</type>
     </standard_name>
   </section>
   <section name="emissions"