Influence of sample temperature and environmental humidity on measurements of benzene in ambient air by transportable GC-PID

Cristina Romero-Trigueros, Marta Doval Miñarro, Esther González Duperón, Enrique González Ferradás
2017 Atmospheric Measurement Techniques  
<p><strong>Abstract.</strong> Calibration of in situ analysers of air pollutants is usually done with dry standards. In this paper, the influence of sample temperature and environmental humidity on benzene measurements by gas chromatography coupled with a photoionisation detector (GC-PID) is studied. Two reference gas mixtures (40 and 5<span class="thinspace"></span>µg<span class="thinspace"></span>m<sup>−3</sup> nominal concentration benzene in air) were subjected to two temperature cycles
more » ... perature cycles (20/5/20<span class="thinspace"></span>°C and 20/35/20<span class="thinspace"></span>°C) and measured with two identical GC-PIDs. The change in sample temperature did not produce any significant change in readings. Regarding ambient humidity, the chromatographs were calibrated for benzene with dry gases and subjected to measure reference standards with humidity (20 and 80<span class="thinspace"></span>% at 20<span class="thinspace"></span>°C). When measuring a concentration of 0.5<span class="thinspace"></span>µg<span class="thinspace"></span>m<sup>−3</sup> benzene in air, the levels of humidity tested did not produce any significant interference in measurements taken with any of the analysers. However, when measuring a concentration of 40<span class="thinspace"></span>µg<span class="thinspace"></span>m<sup>−3</sup>, biases in measurements of 18 and 21<span class="thinspace"></span>% for each respective analyser were obtained when the relative humidity of the sample was 80<span class="thinspace"></span>% at 20<span class="thinspace"></span>°C. Further tests were carried out to study the nature of this interference. Results show that humidity interference depends on both the amount fractions of water vapour and benzene. If benzene concentrations in an area are close to its annual limit value (5<span class="thinspace"></span>µg<span class="thinspace"></span>m<sup>−3</sup>), biases of 2.2<span class="thinspace"></span>% can be expected when the absolute humidity is 8.6<span class="thinspace"></span>g<span class="thinspace"></span>cm<sup>−3</sup> &amp;ndash; corresponding to a relative humidity of 50<span class="thinspace"></span>% at 20<span class="thinspace"></span>°C. This can be accounted for in the uncertainty budget of measurements with no need for corrections. If benzene concentrations are above the annual limit value, biases become higher. Thus, in these cases, actions should be taken to reduce the humidity interference, as an underestimation of benzene concentrations may cause a mismanagement of air quality in these situations.</p>
doi:10.5194/amt-10-4013-2017 fatcat:ux24u67lbfe4tpbvbp4r5jnvxu