Practical experience of 2 and 4-week diffusion tube exposure data has shown some indication that the aggregate of measurements from two 2 week exposures is not equivalent to the measurement from a single 4-week exposure. Observed discrepancies have generally been attributed to UV interference with the TEA absorbent. In order to assess the extent of this problem, data produced by 2 and 4-week diffusion tube exposure periods have been compared. Blacked out diffusion tubes were also deployed and the measurement data compared with normal tube exposures to further assess the potential effect of photodegradation of the TEA absorbent by UV and visible light during exposure.
Evidence of a small systematic difference in the performance of 2 and 4-week diffusion tube exposure periods is observed by comparison of the regression equations presented in Table 1. These data indicate that 2-week exposures overestimate chemiluminescent measurements by 1-4% more than the 4 week exposures, depending upon exposure type.
A comparison of the regression equations for normal and blacked out tube exposures is provided in Table 1. Additionally, the regression equations for tube exposures, split into summer (April-September) and winter (October-March) exposure periods, are shown in Table 2 below. These analyses do not show conclusive evidence of systematic differences in the behaviour of these exposure types. UV absorption by the TEA complex of both blacked out tubes and winter exposures will be expected to be much reduced in comparison to normal and summer tube exposures, and therefore the absence of any significant differences in tube performance indicates that photodegradation of the TEA complex is not a major factor influencing the performance of 2 and 4-week tube exposures. Scatter plots showing relationship between diffusion tube and chemiluminescent data for winter and summer periods are shown in Figures 8-13 and Figures 14-19.
Table 2: Comparison of regression equations from winter and summer tube exposures
| Regression Equation | Ratio of Gradients | ||
| Winter | Summer | winter:summer | |
| 2 week normal | y = 1.08x | y = 1.09x | 0.99 |
| 4 week normal | y = 1.10x | y = 1.06x | 1.04 |
| 2 week blacked out | y = 1.05x | y = 1.07x | 0.98 |
| 4 week blacked out | y = 1.03x | y = 1.02x | 1.01 |
| 2 week sheltered | y = 0.91x | y = 0.91x | 1.00 |
| 4 week sheltered | y = 0.90x | y = 0.89x | 1.01 |
The small systematic differences noted in this study may, in part, explain previously observed discrepancies in 2 and 4-week measurement data. However, inadequate correction for blank nitrite levels on exposed tubes will have an equally significant effect on the average NO2 concentration produced by aggregated 2 week exposures compared with a single 4 week exposure. The effect of blank levels is inversely proportional to time, and it is estimated that for a batch of 2 and 4 week exposed tubes with a calculated travel blank of 0.04 mg NO2, the aggregate of two consecutive 2-week samples will overestimate by approximately 1.26 ppb compared to a single 4-week exposure, if appropriate blank correction is not applied.
It is also interesting to note that 2-week diffusion tube exposures consistently showed marginally greater linear correlation with chemiluminescent analyser measurements, compared with 4 week diffusion tube exposures. This observation is indicated by the coefficients of regression (r) for these datasets shown in Table 1.
5.6 Estimates of Accuracy and Precision
Accuracy of Diffusion Tube Measurements
The accuracy of diffusion tube exposure types was assessed the slope of the least squares regression equation.. Significant systematic differences from the 1:1 line (at P=0.05) were identified in 5 out of 6 of the diffusion tube exposure types compared to the chemiluminescent monitor. The notable exception was the 4 week blacked out exposure. In all cases, however, overall systematic differences were £10% as shown in Table 3 below.
Table 3: Estimates of diffusion tube measurement accuracy
| Exposure Type | Systematic Difference vs. Chemiluminescent Monitor | Estimate of Average Uncertainity | |
| Individual Measurement | Annual Average | ||
| 2 week normal | 9% over read | ±37% | ±16% |
| 4 week normal | 8% over read | ±35% | ±17% |
| 2 week blackened out | 6% over read | ±38% | ±16% |
| 4 week blackened out | 2% over read | ±37% | ±18% |
| 2 week sheltered | 9% over read | ±36% | ±10% |
| 4 week sheltered | 10% over read | ±24% | ±10% |
Also provided in Table 3, is an estimate of the uncertainty associated with the over/under-read for both individual and annual average tube measurements. These uncertainties have been c calculated from the standard deviation in the relative percentage differences from of diffusion tube measurements from chemiluminescent measurements.
This analysis shows that, for a 4 week normal exposure period, diffusion tubes will, on average, over-read by 8%, relative to the chemiluminescent analyser. Furthermore, this relationship has an uncertainty of ±35% (at P=0.05) for individual diffusion tube measurements and ±17% (at P=0.05) for the annual average. Comparison of these estimates of overall uncertainty for diffusion tube measurements, with similar estimates for chemiluminescent analysers6 shows good agreement between the diffusion tube annual average and chemiluminescent analyser.
The relationship between annual average diffusion tube vs. chemiluminescent measurement data are presented in graphic form in Figures 1a-6a.
Precision of Diffusion Tube Measurements
Duplicate 4 week normal diffusion tubes were co-exposed at a limited number of sites throughout the validation study. The precision of measurements for this exposure type was investigated by calculation of the coefficient of variation (CoV) of each duplicate (13 duplicates in total). The average CoV for duplicate tube exposures was calculated as 10.3%, with an overall range of 0.7%-37.4%. Calculated estimates of precision are consistent with those previously made in previous studies7,8.
5.7 Implications of the Study for the Analysis of Diffusion Tube Measurements of NO2
This extensive validation study has demonstrated that the difference between long-term average NO2 concentrations determined by diffusion tube and chemiluminescent analyser measurements is within the estimated overall uncertainty of the chemiluminescent results. Hence, it is considered inappropriate to apply correction factors to diffusion tube measurement data. Where diffusion tube data needs to be processed to produce equivalent chemiluminescent measurement data, the results of this study can be used to derive possible conversion factors. However, it should be noted that all of the monitoring sites used for the comparison in this study were AUN urban background locations. In other area types (eg. industrial or kerbside) the comparison may need to be re-assessed.
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Study undertaken jointly by Stanger Science and Environment and the National Environmental Technology Centre.
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