Calibration of CO and CO₂ Monitors Used in Periodic Inspection of Vehicles at Fixed Stations for Environmental Control

Abstract

Global efforts for environmental cleanliness through the control of gaseous emissions from vehicles are gaining momentum and attracting increasing attention. Calibration plays a crucial role in these efforts by ensuring the quantitative assessment of emissions for informed decisions on environmental treatments. This paper describes a method for the calibration of CO/CO₂ monitors used for periodic inspections of vehicles in cites. The calibration was performed in the selected ranges: 900 - 12,000 &#181;mol/mol for CO and 2000 - 20,000 &#181;mol/mol for CO₂. The traceability of the measurement results to the SI units was ensured by using certified reference materials from CO/N₂ and CO₂/N₂ primary gas mixtures. The method performance was evaluated by assessing its linearity, accuracy, precision, bias, and uncertainty of the calibration results. The calibration data exhibited a strong linear trend with R&#178; values close to 1, indicating an excellent fit between the measured values and the calibration lines. Precision, expressed as relative standard deviation <i>(%RSD),</i> ranged from 0.48 to 4.56% for CO and from 0.97 to 3.53% for CO₂, staying well below the 5% threshold for reporting results at a 95% confidence level. Accuracy measured as percent recovery, was consistently high (&#8805; 99.1%) for CO and ranged from 84.90% to 101.54% across the calibration range for CO₂. In addition, the method exhibited minimal bias for both CO and CO₂ calibrations and thus provided a reliable and accurate approach for calibrating CO/CO₂ monitors used in vehicle inspections. Thus, it ensures the effectiveness of exhaust emission control for better environment.