Articles | Volume 10, issue 10
Research article
27 Oct 2017
Research article |  | 27 Oct 2017

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, and Enrique González Ferradás

Abstract. 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 µg m−3 nominal concentration benzene in air) were subjected to two temperature cycles (20/5/20 °C and 20/35/20 °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 % at 20 °C). When measuring a concentration of 0.5 µg m−3 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 µg m−3, biases in measurements of 18 and 21 % for each respective analyser were obtained when the relative humidity of the sample was 80 % at 20 °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 µg m−3), biases of 2.2 % can be expected when the absolute humidity is 8.6 g cm−3 – corresponding to a relative humidity of 50 % at 20 °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.

Short summary
Benzene is a well-known carcinogen which is present in ambient air. Its concentrations have to be continuosly monitored by law. To obtain reliable and comparable measurements, benzene analysers have to be calibrated with standards. These standards are usually prepared without humidity. However, real measurements can be influenced by it. In this work we have found that ambient humidity has an effect on measurements, which depends on both the absolute humidity and the benzene concentration.