diff --git a/geoschem.go b/geoschem.go
index 4bc98b589..d7060d663 100644
--- a/geoschem.go
+++ b/geoschem.go
@@ -400,6 +400,13 @@ func (gc *GEOSChem) readChemGroupAlt(varGroup map[string]float64) NextData {
 	return nextDataGroupAltNCF(gc.geosChem, geosChemFormat, varGroup, gc.ALT(), gc.start, gc.end, gc.chemRecordDeltaInterval, gc.chemFileDeltaInterval, readNCF, gc.msgChan)
 }
 
+func (gc *GEOSChem) readChemGroupSTP(varGroup map[string]float64) NextData {
+	if gc.noChemHour {
+		return nextDataGroupSTPNCF(gc.geosChem, geosChemFormat, varGroup, gc.start, gc.end, gc.chemRecordDeltaInterval, gc.chemFileDeltaInterval, readNCFNoHour, gc.msgChan)
+	}
+	return nextDataGroupSTPNCF(gc.geosChem, geosChemFormat, varGroup, gc.start, gc.end, gc.chemRecordDeltaInterval, gc.chemFileDeltaInterval, readNCF, gc.msgChan)
+}
+
 var geosLayerConvert = func(nz int) func(NextData) NextData {
 	const (
 		geosLayers          = 72
@@ -698,37 +705,37 @@ func (gc *GEOSChem) W() NextData {
 }
 
 // AVOC helps fulfill the Preprocessor interface.
-func (gc *GEOSChem) AVOC() NextData { return gc.readChemGroupAlt(gc.aVOC) }
+func (gc *GEOSChem) AVOC() NextData { return gc.readChemGroupSTP(gc.aVOC) }
 
 // BVOC helps fulfill the Preprocessor interface.
-func (gc *GEOSChem) BVOC() NextData { return gc.readChemGroupAlt(gc.bVOC) }
+func (gc *GEOSChem) BVOC() NextData { return gc.readChemGroupSTP(gc.bVOC) }
 
 // NOx helps fulfill the Preprocessor interface.
-func (gc *GEOSChem) NOx() NextData { return gc.readChemGroupAlt(gc.nox) }
+func (gc *GEOSChem) NOx() NextData { return gc.readChemGroupSTP(gc.nox) }
 
 // SOx helps fulfill the Preprocessor interface.
-func (gc *GEOSChem) SOx() NextData { return gc.readChemGroupAlt(gc.sox) }
+func (gc *GEOSChem) SOx() NextData { return gc.readChemGroupSTP(gc.sox) }
 
 // NH3 helps fulfill the Preprocessor interface.
-func (gc *GEOSChem) NH3() NextData { return gc.readChemGroupAlt(gc.nh3) }
+func (gc *GEOSChem) NH3() NextData { return gc.readChemGroupSTP(gc.nh3) }
 
 // ASOA helps fulfill the Preprocessor interface.
-func (gc *GEOSChem) ASOA() NextData { return gc.readChemGroupAlt(gc.aSOA) }
+func (gc *GEOSChem) ASOA() NextData { return gc.readChemGroupSTP(gc.aSOA) }
 
 // BSOA helps fulfill the Preprocessor interface.
-func (gc *GEOSChem) BSOA() NextData { return gc.readChemGroupAlt(gc.bSOA) }
+func (gc *GEOSChem) BSOA() NextData { return gc.readChemGroupSTP(gc.bSOA) }
 
 // PNO helps fulfill the Preprocessor interface.
-func (gc *GEOSChem) PNO() NextData { return gc.readChemGroupAlt(gc.pNO) }
+func (gc *GEOSChem) PNO() NextData { return gc.readChemGroupSTP(gc.pNO) }
 
 // PS helps fulfill the Preprocessor interface.
-func (gc *GEOSChem) PS() NextData { return gc.readChemGroupAlt(gc.pS) }
+func (gc *GEOSChem) PS() NextData { return gc.readChemGroupSTP(gc.pS) }
 
 // PNH helps fulfill the Preprocessor interface.
-func (gc *GEOSChem) PNH() NextData { return gc.readChemGroupAlt(gc.pNH) }
+func (gc *GEOSChem) PNH() NextData { return gc.readChemGroupSTP(gc.pNH) }
 
 // TotalPM25 helps fulfill the Preprocessor interface.
-func (gc *GEOSChem) TotalPM25() NextData { return gc.readChemGroupAlt(gc.totalPM25) }
+func (gc *GEOSChem) TotalPM25() NextData { return gc.readChemGroupSTP(gc.totalPM25) }
 
 // SurfaceHeatFlux helps fulfill the Preprocessor interface by returning
 // sensible heat flux from turbulence [W/m2].
diff --git a/preproc.go b/preproc.go
index c0b87ae17..352ddfe41 100644
--- a/preproc.go
+++ b/preproc.go
@@ -1133,6 +1133,24 @@ func nextDataGroupAltNCF(fileTemplate string, dateFormat string, varNames map[st
 	}
 }
 
+// nextDataGroupSTPNCF reads a group of variables using nextDataGroupNCF
+// and multiplies the result by air density STP (1.2754 kg/m3).
+func nextDataGroupSTPNCF(fileTemplate string, dateFormat string, varNames map[string]float64, start, end time.Time, recordDelta, fileDelta time.Duration, readFunc readNCFFunc, msgChan chan string) NextData {
+	f := nextDataGroupNCF(fileTemplate, dateFormat, varNames, start, end, recordDelta, fileDelta, readFunc, msgChan)
+	const rho = 1.2754 // kg/m3, air density at STP
+	return func() (*sparse.DenseArray, error) {
+		data, err := f()
+		if err != nil {
+			return nil, err
+		}
+		out := sparse.ZerosDense(data.Shape...)
+		for i, val := range data.Elements {
+			out.Elements[i] = val * rho
+		}
+		return out, nil
+	}
+}
+
 // ncfFromTemplate opens a NetCDF file from the given template, where
 // the [DATE] wildcard in the given fileTemplate is replaced by the given
 // date, formatted as the given dateFormat.