/EnvironmentalProtection Agency1
OH Sub No. 60
Note On Data Nom 11iSg)lfiLHEARING
ECEIVED
Gases such as NO ana NU^ are generally measuhd by scientific instruments as
volumes in units of parts per billion (ppb). However the Ambient Air Quality Standards
are generally expressed in terms of mass concentrations such as pglm3.
~
The sum of
such as:
NO
and NOz, when expressed in terms of volume can be added directly
However, the sum of NO and NO2 when expressed in terms of mass concentration
(pglrn'), will depend on the relative amounts of each gas. Because of this a convention
has been put in place that conservatively assumes that all gases are in the form of NO2,
which because it has a higher mass than NO, will give a higher mass concentration of
NO,.
f c Fo
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yr ns
ig pe
ht ct
ow ion
ne pu
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se
.
The second consideration in terms of converting data from ppb to pg/m3 is the standard
reference conditions for the mass concentration. Three common temperatures are used
as reference temperatures. IUPAC is standardised to 273K, the EU to 293K and the
WHO to 248K. Each standard condition --will derive a slightly different mass
concentrationwhen converting a gas from ppb to pg/m3.
The current data has been updated to ensure that the EU standard conditions
(normalised to 293K) are used consistently and that NOx (pg/m3)is expressed in terms
of NO2 throughout:
Co
n
se
nt
o
NO (cig/m3) = (NO ppb)*MW (NO = 30)I24.066
= 1.25*(NOppb)
NO2 ()1g/m3) =
(NO,ppb)"MW (NO2 = 46) 124.066
= 1.91*(NOzppb)
EPA Export 26-07-2013:01:16:08
I
se
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Concentration (pglm3)
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EPA Export 26-07-2013:01:16:08
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EPA Export 26-07-2013:01:16:08
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EPA Export 26-07-2013:01:16:08
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EPA Export 26-07-2013:01:16:08