Intercomparison of NO₂ measurement techniques

Abstract

An intercomparison was made near Niwot Ridge, Colorado, of three different instruments for measuring NO2 at low concentrations in ambient air: (1) the photolysis/chemiluminescence (PC) instrument, (2) the tunable diode laser absorption spectrometer (TDLAS), and (3) the Luminox instrument. Calibrated mixtures of NO2 in air and NO2 with possible interferants (HNO3, peroxyacetyl nitrate (PAN), H2O2, n‐propyl nitrate, and O3) were provided in simultaneous tests. In addition, ambient air measurements were made using the three instruments. Blind procedures were followed in preparing all results. Several conclusions were reached concerning the performance of these instruments during this intercomparison: (1) For NO2 levels above 2 parts per billion by volume (ppbv), similar results were obtained for all instruments; (2) Below 2 ppbv, the expected interferences from ozone and PAN influenced the NO2 measurements made using the Luminox instruments. Those interferences were sufficiently consistent that they could be corrected for by using the measured values Of O3 and PAN down to about 0.3 ppbv NO2; (3) The ozone interference on the Luminox instruments was removed by an ozone scrubber placed in the sampled air stream of the Luminox instrument. However, this did not remove PAN. In addition, the scrubber appeared to remove about 50% of the NO2 as well; (4) Although no interferences were identified for the TDLAS technique, care must be taken in the data analysis near (or below) the detection limit for the instrument. At these levels the data reduction program provided with the TDLAS will tend to find background noise that is correlated with the reference NO2 spectrum and calculate levels of NO2 that are too high; (5) No interferences or artifacts were found for the final results reported by the PC technique. However, these results for ambient measurements were corrected by subtracting an artifact that averaged 5 parts per trillion by volume (pptv) and by calculating a correction for the effect of ambient ozone. This latter correction averaged 1.0% in magnitude.

Interferences in photolytic NO<sub>2</sub> measurements: explanation for an apparent missing oxidant?

“…2 or 3 whether the PAN decomposition occurs within the BLC exclusively or within the inlet of the system, as has been claimed previously (Fehsenfeld et al, 1987). Previous studies (Fehsenfeld et al, 1990;Ridley et al, 1988;Sadanaga et al, 2010) have also reported a small PAN interference with photolytic converters, while some have found the contribution to the NO 2 signal from PAN to be negligible (Ryerson et al, 2000). Others (Val Martin et al, 2008) acknowledge the possibility for interference and estimate a small (2 to 4 pptv) positive bias.…”

Preparing to Measure the Effects of the NOxSIP Call— Methods for Ambient Air Monitoring of NO, NO2, NOy, and Individual NOzSpecies

“…Subtraction of a similar plot of the NO loss expected from the gas-phase NO + O 3 reaction alone indicates the effect of wall reactions. 14 In most sensitive NO chemiluminescence monitors, a prereaction chamber is used during monitoring to allow reaction of NO with O 3 prior to the reaction cell, leaving a background signal that is subtracted on subsequent measurement of NO. An estimate of the limit of detection (LOD) for a signal-to-noise ratio (SNR) of 2 is 10 pptv for a 1-sec integration time and total uncertainty of ±10% for concentrations well above the detection limit.…”

“…Because the TDL provides near unambiguous detection of target gases, it has frequently been used as a reference method in field test comparisons. Examples of its use for NO 2 include a comparison with the LMA-3 luminolbased NO 2 monitor at a field monitoring site 46 and other pre-1991 research, 4 including an intercomparison of NO 2 measurement techniques by Fehsenfeld et al 14 Reid et al 47 indicate that this type of system can achieve sensitivities on the order of 100 pptv.…”

“…Interference equivalents of 0.0033 for O 3 and 0.25 for PAN also were noted. Fehsenfeld et al 14 describe a comparison of NO 2 monitors, one of which was the LMA-3, at a remote site near Niwot Ridge, CO. O 3 and PAN interferences in the LMA-3 were noted to be "sufficiently consistent that they could be corrected for by using the measured values of O 3 and PAN down to about 0.3 ppbv NO 2 ." Gaffney et al 41,42 have demonstrated the use of a gas chromatograph/ luminol-type detector for continual monitoring of NO 2 and for peroxyacyl nitrates using a luminol solution that responds well to all these compounds.…”

“…This, in turn, reduces interferences from thermally labile species such as PAN. To monitor NO levels with one detector, a shutter device 14 can be installed between the light source and the photolysis cell, or an alternate sample flow path that bypasses the cell can be used. The photolytic converter does not approach 100% efficiency, so the difference signal (NO x -NO) must be divided by a conversion factor (CF) to determine the level of NO 2 .…”