Evaluating in-use vehicle emissions using air quality monitoring stations and on-road remote sensing systems

This study investigated real world in-use vehicle emissions using two regulatory techniques simultaneously, namely on-road remote sensing (RS) systems and air quality (AQ) monitoring stations, aiming to provide a full pollution profile from tailpipe to roadside and atmosphere. Two large AQ and RS datasets collected during 2012-2018 were analyzed. The effects of various emission control programmes on the trends of tailpipe emissions and air quality were evaluated. Correlations between tailpipe emissions and roadside and ambient air quality were also explored. The results showed a decreasing trend of NO₂ at both roadside and ambient AQ stations from 2013 to 2016, which was attributed to the intensive implementation of a series of vehicle emissions control programmes. Although NO₂ was decreasing, O₃ was generally increasing for all AQ stations. AQ data showed that O₃ had little correlation with either NO₂ or NO_x, but was mainly determined by NO₂/NO_x ratio. Roadside NO₂/NO_x ratio increased first and then decreased or stabilized after 2014, while ambient NO₂/NO_x ratio increased steadily. RS data showed that the overall NO decreased quickly during 2012-2015 and then decreased moderately after 2015. The decrease was mainly attributed to the effective NO reduction from LPG vehicles. However, diesel NO remained high and reduced relatively slowly during the study period. Gasoline vehicles were relatively clean compared with LPG and diesel vehicles. Finally, good correlations were demonstrated between NO measured by RS sites and NO_x measured by roadside AQ stations, indicating that vehicle emissions were the major contributor to roadside NO_x pollution. Ambient NO_x emissions could be affected by various sources, leading to different correlation levels between RS and ambient AQ results.