Do pressure changes have an influence on ambient air chemiluminescence NOx measurements?

Users of automatic air pollution monitors are largely unaware of how certain parameters, like pressure, can affect readings. The present work examines the influence of inlet pressure changes on chemiluminescence NOx measurements. These changes have been grouped into two categories: (i) those due to changes in atmospheric pressure and (ii) those produced by any other reason (e.g., clogs in the inlet sampling line). Atmospheric pressure changes were simulated varying both the inlet and the outlet sample line pressures in the same way, whereas sample pressure changes were produced modifying only the inlet sample line pressure. Analyzer calibration was performed with zero air and 200 nmol mol−1 of NO at 101.3 kPa. The test pressures ranged from −10 to +10 kPa with respect to the calibration pressure. ANOVA and Dunnettʹs tests were performed to look for significant differences between measurements obtained under calibration conditions and those obtained under different pressure conditions. The differences in measurements were practically negligible for fixed analyzers subjected to usual changes in atmospheric pressure (<±5 kPa); however, differences could be as high as 7% if ±10 kPa pressure changes took place in the inlet sample line for causes included in the second group.