**SANDWICHed STN data**

The purpose of SANDWICH technique is to provide estimates of PM2.5
components as they might be measured by the PM2.5 Federal Reference
Method. These estimates can be different than the data provided directly
by the EPA speciation measurements. As described in Frank (2006), AIR
EXPLORER provides estimates for "adjusted nitrate" to reflect the lower
than measured amount retained on FRM filters and "derived water" which
is a mass enhancement of sulfate, ammonium and adjusted nitrate. Also
provided are the "inferred carbonaceous mass" which is denoted in the
parameter list by "organic and total carbon by material balance" and a
revised estimate of crustal which does not utilize measured Aluminum.
The details are provided in the data dictionary. The SANDWICH technique
stands for measured Sulfate, Adjusted Nitrate, Derived Water, Inferred
Carbonaceous mass Hybrid Material Balance Approach."

The SANDWICH parameters are provided in the data dictionary in three
groupings. The complete list of variables is provided in Note 1 below.
First, are daily values derived using the methods in Reference 1.
Because of measurement uncertainties in measured chemical components vs
measured mass, as well as the uncertainties of estimating retained
nitrates and water, negative values of inferred carbon are possible.
This can happen, for example, when measured mass is understated or when
individual chemical components are overstated. In such cases, the
inferred carbon may be significantly different than the mass directly
derived from measured carbon. To account for these discrepancies,
modified SANDWICH estimates are provided that do not allow the inferred
carbon to be lower than a specified amount. This approach is described
in Note 2. Accordingly, a second set of SANDWICH estimates are provided
together with an indicator variable, "SANDWICH adjustment indicator,"
which is set equal to one for those situations where modified SANDWICH
estimates are developed. Finally, to allow for simple presentations and
analyses of the SANDWICHed data presented as major components of PM2.5
mass, a third set of parameters are provided. This is also described at
the end of note 2.

Before SANDWICH was applied to the STN data, a minimum amount of
consistency checks were performed on the raw speciation measurements.
These are described in Note 3.

The parameter list also includes estimates for salt. As described in
reference 1, salt could be used in the material balance of PM2.5. The
SANDWICHed estimates in Air Explorer, however, do not consider salt at
this time.

**Reference 1.** Frank, N. H. Retained Nitrate, Hydrated Sulfates, and
Carbonaceous Mass

in Federal Reference Method Fine Particulate Matter for Six Eastern U.S.
Cities. J. Air & Waste Manage. Assoc**. 2006** 56:500--511.

**Note1: SANDWICH and related variables.**

![](media/image1.wmf)

Also available on AIR EXPLORER are PM2.5 FRM mass (denoted as "PM2.5"),
measured sulfate (SO4), measured ammonium (NH4), total carbonaceous mass
(TCM) calculated as sum of organic carbon mass and elemental carbon,
where organic Carbon mass (OCM) is estimated as blank-corrected organic
carbon multiplied by 1.4.

According to Reference 1,

PM2.5= SO4+NH4+NO3r+water+ OCMmb+EC+Cr'+0.5, and TCMmb =OCMmb+EC.

**Note 2 (SANDWICH adjustments to eliminate negative OCM_mb).**

A minimum allowable value of OCMmb is specified as the \"OCMmb floor.\"
It is based on an assumed minimum value for measured OC (denoted as
OC\') and alternate estimates for the STN sampler specific positive
quartz filter sampling artifacts also known as the blank correction, b.
The STN sampler specific values for b are described in Reference 1.

For high OC\' values (ie. OC\' \> 6b), set lower limit for OCMmb based
on 0.75\*OC\' (lowered value of measured OC), 2b (higher value for
blank) and OC to OCM multiplier (i.e. \"k\") of 1.4. For low OC\' values
(OC\' \<=6b), use even lower value for measured OC (=0.5\*OC\') and
lower value for b (=0.5b).

The breakpoint is when \[OC\' - 2\*b = \[0.5\*OC\' - 0.5\*b\], or
OC\'=3b.

The \"ocm_floor\" is set equal to zero if the calculated value above is
\< 0.

If the estimated OCMmb is less than the \"OCM floor,\" as derived above,
then the measured or estimated SANDWICHed raw data values are adjusted
by

the factor \[FRM mass)/CFMa, where

CFMa=\[so4 + no3r + nh4 + water+ ec +cr +Pa + OCM_floor\]

The list of affected values are: {SO4, NO3r, NH4, EC, Cr, Al, SI, Fe,
Ca, Ti and Pa, the passive FRM sampling artifact normally set equal to
0.5}

For these records, the "SANDWICH adj_indicator" is set equal to 1.

PM2.5= SO4ª+NH4ª+NO3rª+waterª+ OCMmbª+ECª+Crª+Paª , where Pa =0.5 unless
adjusted to account for increase in OCMmb.

Similarly, when water and nitrates are combined with SO4ª +NO3rª, then
the data can be represented by the third set of parameters, where

PM2.5= \[SANDWICH sulfate mass\] + \[SANDWICH nitrate mass\] +
\[SANDWICH TCMmb\] + \[SANDWICH Crustal Mass\] + \[Passive\]

**Note 3.**

Carbon by material balance and SANDWICH mass values were not developed
for measurement days in which FRM mass is greater than measured SO4 or
estimated crustal; or

when CFM \<0.3 \*FRM mass or CFM \>2\*FRM mass,

where CFM= \[measured so4\] + \[Adjusted nitrate\] +\[measured NH4\]
+\[Derived water\] + \[Total Carbonaceous Mass\] + \[Crustal Mass\] +
0.5 .

In this formula, Total Carbonaceous Mass is derived from measured carbon
and is the same value reported elsewhere on Air Explorer