STANDARD AEROSOL-TYPE EQUATIONS

Equations for calculating aerosol types, e.g. sulfate, organic, soil, from elemental concentrations.

The following table lists the standard formulae and assumptions applied to IMPROVE sampler measurements to derive the principal fine aerosol species, reconstructed fine mass, and coarse mass. The brackets indicate the mass concentration of the aerosol species or element.  For a detailed discussion on these aerosol type equations see Malm et al, 1994, Chapter 2 of the 2000 IMPROVE report and Chapter 2 of the 1996 IMPROVE report

ammonium sulfate (NH₄SO₄):  The sulfur on the Teflon filter is always present as sulfate (SO₄).  In most cases the sulfate is fully neutralized ammonium sulfate , which is 4.125 times the sulfur concentration.  The sulfate at Eastern sites during the summer is not always fully neutralized resulting in acidic aerosol.  If 100% of the sulfur were sulfuric acid, the correct sulfate mass would be 74% of the calculated (NH₄)₂SO₄. 

Organic Carbon:  Aerosol samples collected on quartz filters are analyzed at Desert Research Institute for carbon using the Thermal Optical Reflectance (TOR) combustion method.  The sample is heated in steps and the evolved CO₂ is measured at each step.  The first four steps take place in a pure helium atmosphere to prevent combustion.  The carbon released in these steps (OC1-OC4) is interpreted as being evaporated organics.  2% O₂ is introduced at 550 °C and more carbon is released.  During the pure-helium stage, some of the organic material has been charred (pyrolized), darkening the filter (decreasing its reflectivity). The filter starts to lighten when oxygen is introduced oxidizing the char.  The carbon that has been recorded in the oxygen stage when the filter returns to its original reflectivity is interpreted as pyrolized organics, (OP).  The carbon evolved after the filter has returned to its initial reflectance is interpreted as elemental (E1-E3).   For a full description see Chow et al., in Atmospheric Environment. 27A, 1185-1201, (1993).

 Carbon components as a function of temperature and added oxygen.

Organic Mass by Carbon (OMC): The organic mass is the sum of the low temperature organics and pyrolized organics multiplied by a factor of 1.4:   OMC=1.4 * (OC1+OC2+OC3+OC4+OP) where the factor 1.4 is used to adjust the organic carbon mass (OC) for other elements assumed to be associated with the organic carbon molecule.

light-absorbing carbon (LAC):  This is the sum of elemental carbon fractions.  The pyrolized fraction is subtracted.  Preliminary analyses indicate that some of the O4 fraction may absorb light, and that OP may overestimate the pyrolytic mass.

organic carbon by hydrogen (OCH):  The hydrogen on the Teflon filter is associated with sulfate, organics, nitrate, and water.  Since the PIXE analysis is done in vacuum, all water will volatilize.  Also it is assumed that no significant hydrogen from nitrate remains.  If it is further assumed that the sulfate is fully neutralized ammonium sulfate, the organic carbon concentration can be estimated by subtracting the hydrogen from sulfate and multiplying the difference by a constant representing the fraction of hydrogen:

OCH = 11*(H-0.24*S)

The sulfur factor, H/S ratio, for ammonium sulfate is 8/32 = 0.25.  The C/OM ratio is 11 and operationally defined by forcing OCH to equal OC.  Comparison of OCH to OC is used in data validation procedures and OCH is used to estimate organic mass when carbon is not explicitly measured.

The OCH calculation is invalid when (1) there is high nitrate relative to sulfate, as at sites near Los Angeles and San Francisco, and (2) the sulfur is not present as ammonium sulfate.  This latter includes sites with marine sulfur, and sites in the eastern United States with unneutralized sulfate. The main advantage of using OCH at valid sites is that its precision is better than that for OC during periods of low organic, e.g. winter in the West.  At sites in the East, OCH is often low because of unneutralized sulfate, and imprecise because of the high sulfate relative to organic..

light-absorbing carbon (LAC):  This is the sum of elemental carbon fractions.  The pyrolized fraction is subtracted.  Preliminary analyses indicate that some of the O4 fraction may absorb light, and that OP may overestimate the pyrolytic mass.

soil (SOIL):  This is a sum of the soil derived elements (Al, Si, K, Ca, Ti, Fe) along with their normal oxides (Al2O3, SiO2, CaO, K2O, FeO, Fe2O3, TiO2).  The variable does not depend on the type of soil, such as sediment, sandstone, or limestone.  One fine element, K, however, may partly derive from smoke as well as soil.  Smoke potassium is eliminated from the calculation using Fe as a surrogate.  This is discussed in nonsoil potassium below.

nonsoil potassium (KNON):  Fine potassium has two major sources, soil and smoke, with the smoke potassium on much smaller particles than the soil potassium.  The potassium in coarse particles will be solely produced from soil.  The soil potassium is estimated from the measured concentration of Fe and the ratio of K/Fe of 0.6 measured on coarse samples ( 2.5 to 15 µm) collected between 1982 and 1986.  This ratio depends on the soil composition and varies slightly from site to site.  If the ratio were slightly smaller (say 0.5), the KNON values will be negative when there is no smoke influence.  The residual potassium, K - 0.6*Fe, is then assumed to be produced by smoke.  The burning of most organic fuels will produce potassium vapor.  During transport, this vapor will transform into fine particles.  The KNON parameter is not a quantitative measure of the total smoke mass, since the ratio of nonsoil potassium to total smoke mass will vary widely, depending on the fuel type and the transport time.  However, the KNON parameter can be used as an indicator of a nonsoil contribution for samples with large KNON.  In some situations there may be some fine Fe from industrial sources which could cause occasional smoke episodes to be lost.