Units and Unit Conversions

    All measurements include both a number and the units associated with that measurement.  Ratios have ratios of units in place of simple units,  for example, miles/hour. The most widely accepted unit system employed in scientific measurements is the SI System of Units, from the French, Systeme International d' Unities.  The SI system uses a set of 7 independent base units from which all other units are derived by simple algebraic operations. Some derived units that are particularly useful have unique names, for example, volume is the special name for the units derived from cubing meters (m), or m³.  To identify particularly large or small numbers of units scientific notation can be used but it is more common to use decimal prefixes with SI units.
 

DIMENSIONS UNIT NAME ABBREVIATION EXAMPLE OF DERIVED UNIT (Special Name) mass kilogram kg density : kg m-3 length meter m force: m kg s-2 (Newton, N) time second s  electrical charge : s A (Coulomb) temperature kelvin K (oC) electric current ampere A potential difference : m2kg s-3 A-1 (Volt) amount of substance mole mol concentration : mol m-3 luminous intensity candela cd luminance : cd m-2

STANDARD PREFIXES USED WITH SI UNITS FACTOR PREFIX  SYMBOL 10-12 pico p 10-9 nano n 10-6 micro m 10-3 milli m 10-2 centi c 10-1 deci d 101 deca da 102 hecto h 103 kilo k 106 mega M 109 giga G

Unit Conversions

Comparing Volumetric and Gravimetric Unit

There are two sets of units most commonly used to describe air pollution concentration -- volumetric and gravimetric units.  The units are used to identify the amount of a particular pollutant gas either by volume or by mass, that might be extracted from a unit or air.   Volumetric units give the mixing ratio between the pollutant gas of interest and the original volume of air. Owing to fact that a fixed volume of any gas at a given pressure and temperature always contains the same number of gas molecules, regardless of the identity of the gas, this ratio also corresponds to the ratio of the number of pollutant gas molecules to the number of air molecules in the original gas volume.

This concentration can be expressed as parts of pollutant gas per million parts of air, or ppm (also ppmv to indicate a volume rather than a mass ratio.)  Concentration is also shown mmol/mol to comply directly with the SI base unit for amount of substance, the mole, (where a mole of any substance is defined 6.022 x 10²³ molecules of that substance).

Conversion between gravimetric and volumetric units relies on the application of the combined gas law, PV = nRT where P= pressure, V= volume, n= the amount of substance in moles and R is the universal gas constant.  At standard temperature and pressure, STP, (25o C, 760 mm Hg) 1 mole of any gas or mixture of gases occupies 0.0224 m-3, and 1 mole of a pure gas weighs M kg where M is the molar mass (molecular weight) of the gas. Abbreviations for Volumetric and Gravimetric Units Volumetric Units Gravimetric Units ppm 10-6 ul l-1 mmol mol-1 mg m-3  ppb 10-9 nl l-1 nmol mol-1 mg m-3 ppt 10-12 pl l-1 pmol mol-1 ng m-3

It is uncommon, although sometimes possible, to see volumetric concentration expressed as ml / l (microliters per liter).  Most data reports,(and consequently most models), still require input in ppb, or for very low concentrations of ppt (parts per trillion) as the units for volume-based concentration. Volumetric units of concentration are unaffected by changes in temperature and pressure.  As a result they are especially useful where large fluctuations in temperature and pressure during sampling are difficult to measure.

Gravimetric units delineate the mass ratio of a pollutant to a specific volume of air.  The units are mg m^-3.  Unlike volumetric units, gravimetric units can be used for solid or liquid particles as well as gaseous pollutants.  They are generally used when the sample is collected on a filter and so the concentration relates directly to the mass of contaminant collected for a measured volume of air sampled.
 

Standard Temperature and Pressure Corrections

For most air quality measurements, temperature and pressure are unlikely to be at standard conditions.  It will be necessary to correct gas volume measurements to correct to standard conditions in order to compare the gas sample collected to other samples that have also been collected at a variety of non-standard conditions.  The Ideal Gas Equation is used to perform these corrections.