Objective: To develop an advanced primary ozone standard with improved
SI-traceability. This project is in
partnership with researchers at the Bureau International des Poids et Mesures
(BIPM) to assure the comparability and SI-traceability of global ozone
measurements.
Problem: The concentration of ozone
in the atmosphere remains a significant issue from both scientific and
political perspectives. Ozone, at
tropospheric levels, is a health concern and contributes to climate change as a
greenhouse gas, while stratospheric ozone protects earth from harmful UV
radiation. These concerns have led to the establishment of air quality
standards and international protocols to reduce the emissions of pollutants
that either contribute to tropospheric ozone formation or deplete stratospheric
ozone. Since 1983, NIST has provided
Standard Reference Photometers (SRPs) based on UV photometry to ten US
Environmental Protection Agency (EPA) facilities to provide an infrastructure
for the calibration and traceability of ozone measurements within the US. More recently, the international interest in
ozone measurements has prompted eleven national laboratories to acquire NIST
SRPs.
Approach: To support the growing
national and international need of improved ambient ozone measurements, NIST is
partnering with BIPM to develop an advanced primary ozone reference standard and
to share the dissemination of secondary or transfer standards. The basic
project plan includes 1) Transfer the current ozone “national reference
photometer” technology to BIPM.
2) Assess the performance of the current technology with respect to
current and future ozone measurement requirements. 3) Provide recommendations for potential improvements of the SRPs
and survey other candidate secondary standards. 4) Develop a primary
reference standard with capabilities of SI-traceability verification.
The
major challenge of this project is to incorporate a secondary method that can
provide a solid foundation for the primary reference standard at relevant ozone
concentrations (0 nmol/mol to 100 nmol/mol).
Additionally, any uncertainties due to ozone transport must be
eliminated. Several approaches are
being considered. Gas phase titration
with nitric oxide is an attractive method, since the ozone concentration can be
directly linked to primary gravimetric NO and NO2 standards. Success
of this project, independent of the verification method, demands
instrumentation with high sensitivity, accuracy, precision, and rapid time
response for measuring chemical species at atmospheric levels (nmol/mol and
lower). In fact, minimum detection sensitivities
better than 1 nmol/mol will be required to maintain uncertainties at the 1 %
level for the current ground level ozone concentrations. A number of in-situ techniques are being
considered, including time-gated laser-induced fluorescence detection, tunable
diode laser absorption spectroscopy, and cavity ring-down spectroscopy for
detection of relevant chemical species and possible contaminants in the carrier
gas.
Results and Future
Plans: Current efforts have
focussed on the construction of two new NIST SRPs which are to be transferred to
BIPM shortly and an initial performance evaluation of the current SRPs. During an electronics upgrade of eleven
instruments, a distinct 0.5 % slope calibration bias was noted for most of these
instruments compared to SRP 2, the current principal NIST reference photometer.
The calibration bias may be due to: sample memory effects in SRP 2,
fluctuations in both temperature and pressure measurements for a number of
instruments, variations in the zero offset dependent on the sample
configuration, and significant detector drifts. An ongoing effort is dedicated to identifying and eliminating
this bias. We are also currently surveying appropriate verification methods and
state-of-the-art instrumentation that can provide a solid foundation for the new
primary reference standard.
As
a collaborative project between NIST and BIPM, the advanced primary ozone
photometer will provide an internationally validated ozone standard that can be
used to anchor all ozone measurements through a defined traceability structure.
The accuracy of the NIST SRP would be checked for consistency with the NIST SRM
gas program and primary standards developed by BIPM.