Fundamental Units in Environmental Measurements
Mass/Mass units:
ppmm = parts per million by mass = mg of solute/kg of solvent
(e.g., 1 ppm = 1 mg of contaminant per kg of soil)
ppbm = parts per billion by mass = mg of solute/1000 kg of solvent
(e.g., 1 ppb = 1 mg of contaminant per 1000 kg of soil)
Mass/Volume units:
mg of solute/liter of solvent (mg/L) = ppmm = parts per million by mass (for aqueous systems only)
(e.g., 1 ppmm = 1 mg of contaminant per liter of water).
mg of solute/1000 liter of solvent(mg/L)= ppbm = parts per billion by mass (for aqueous systems only)
(e.g., 1 ppbm = 1 mg of contaminant per 1000 liter of water)
In most aqueous systems, ppmm is equivalent to mg/L. This is because the density of pure water is approximately 1 kg/L at 5oC. Care should be given when equating ppmm to mg/L where dissolved substance contributes significantly to mass of water and the density varies significantly from 1 kg/L, for example, seawaters, brines, some recycled streams etc.
Solids in water:
Total Solids: Matter remaining as residue upon evaporation at 103 -105 oC.
Total Solids can be further classified into Total Suspended Solids (TSS) (nonfilterable) and Total Dissolved Solids (TDS) (filterable solids - FS). The glass-fiber filter with a 2 µm opening is used to filter the well mixed sample. The filter is then dried at 103 -105 oC and the weight (mg) increase of the filter is divided by sample volume to get TSS as mg/L.
TSS impairs light penetration and may harm aquatic life. TSS also may act as nutrient source or oxygen depleting organic matter. TDS are less organic in composition and consist of dissolved cations and anions. Biological oxidation or coagulation is necessary to remove TDS.
TSS and TDS can be further subclassified on the basis of their volatility at 550 ± 50oC as volatile or fixed. i.e TSS classified as VSS or FSS and TDS classified as VDS or FDS. The volatile fraction comprises of organic content which oxidizes at this temperature. The nonvolatile fraction (fixed) comprises of inorganic (or mineral) content.
Settleable solids: Solids that settle at the bottom of the cone shaped container (Imhoff cone) in a 60 minute period.
Volume/Volume units:
(frequently used for gas concentrations)
ppmv = parts per million by volume
ppbv = parts per billion by volume
As per Ideal gas law, PV = nRT, therefore at given temperature and pressure, volume ratios equals the mole ratios. And for given temperature and volume, pressure ratios equals mole ratios.
ppmv = (Vi/Vtotal )*10^6 = (molesi/molestotal )*10^6 = (Pi/Ptotal )*10^6 (at constant T, V)
Vi, Pi, molesi = Volume fraction, partial pressure and mole fraction of component i respectively.
Conversion for air concentration: mg/m^3 = ppmv * MW * (P/RT) * 10^6
Concentration of Major gases in the atmosphere:
ppmm = parts per million by mass = mg of solute/kg of solvent
(e.g., 1 ppm = 1 mg of contaminant per kg of soil)
ppbm = parts per billion by mass = mg of solute/1000 kg of solvent
(e.g., 1 ppb = 1 mg of contaminant per 1000 kg of soil)
Mass/Volume units:
mg of solute/liter of solvent (mg/L) = ppmm = parts per million by mass (for aqueous systems only)
(e.g., 1 ppmm = 1 mg of contaminant per liter of water).
mg of solute/1000 liter of solvent(mg/L)= ppbm = parts per billion by mass (for aqueous systems only)
(e.g., 1 ppbm = 1 mg of contaminant per 1000 liter of water)
In most aqueous systems, ppmm is equivalent to mg/L. This is because the density of pure water is approximately 1 kg/L at 5oC. Care should be given when equating ppmm to mg/L where dissolved substance contributes significantly to mass of water and the density varies significantly from 1 kg/L, for example, seawaters, brines, some recycled streams etc.
Solids in water:
Total Solids: Matter remaining as residue upon evaporation at 103 -105 oC.
Total Solids can be further classified into Total Suspended Solids (TSS) (nonfilterable) and Total Dissolved Solids (TDS) (filterable solids - FS). The glass-fiber filter with a 2 µm opening is used to filter the well mixed sample. The filter is then dried at 103 -105 oC and the weight (mg) increase of the filter is divided by sample volume to get TSS as mg/L.
TSS impairs light penetration and may harm aquatic life. TSS also may act as nutrient source or oxygen depleting organic matter. TDS are less organic in composition and consist of dissolved cations and anions. Biological oxidation or coagulation is necessary to remove TDS.
TSS and TDS can be further subclassified on the basis of their volatility at 550 ± 50oC as volatile or fixed. i.e TSS classified as VSS or FSS and TDS classified as VDS or FDS. The volatile fraction comprises of organic content which oxidizes at this temperature. The nonvolatile fraction (fixed) comprises of inorganic (or mineral) content.
Settleable solids: Solids that settle at the bottom of the cone shaped container (Imhoff cone) in a 60 minute period.
Volume/Volume units:
(frequently used for gas concentrations)
ppmv = parts per million by volume
ppbv = parts per billion by volume
As per Ideal gas law, PV = nRT, therefore at given temperature and pressure, volume ratios equals the mole ratios. And for given temperature and volume, pressure ratios equals mole ratios.
ppmv = (Vi/Vtotal )*10^6 = (molesi/molestotal )*10^6 = (Pi/Ptotal )*10^6 (at constant T, V)
Vi, Pi, molesi = Volume fraction, partial pressure and mole fraction of component i respectively.
Conversion for air concentration: mg/m^3 = ppmv * MW * (P/RT) * 10^6
Concentration of Major gases in the atmosphere:
Gas |
(% volume or moles) |
ppmv |
Nitrogen (N2) | 78.1 | 781,000 |
Oxygen (O2) | 20.9 | 209,000 |
Argon (Ar) | 0.93 | 9,300 |
Carbon dioxide (CO2) | 0.038 | 379 |