Investigation of vehicle cold start primary NO2 emissions inferred from ambient monitoring data in the UK and their implications for urban air quality.

Abstract Nitrogen oxides (NO and NO 2 , collectively NO X) derived from vehicle exhausts are critical pollutants with significant implications for urban air quality and human and environmental health. In this study, we investigate trends in measured ambient nitrogen dioxide (NO 2) and NO x mixing ratios at urban traffic - dominated monitoring sites in the UK for the period 2009–2016. We apply an oxidant analysis approach alongside a number of assumptions to the ambient data to determine trends in the inferred primary NO 2 /NO x ratio, and examine evidence for enhanced vehicle "cold start" effects upon these inferred emissions. Ambient NO 2 and NO X mixing ratios have experienced an overall decrease of 17.2% and 11.3% respectively for the locations considered over this time period. The inferred primary NO 2 /NO x ratio for the majority of the study locations is found to have fallen by 29% (from 0.175 to 0.125) as a monthly mean from 2009 to 2016, with a statistically significant median decrease of 0.32 percentage points per year. However, during cold weather (temperatures less than or equal to 5 OC), the inferred primary NO 2 /NO x ratio averaged across all locations, when compared with normal conditions (temperatures higher than 5 OC) increased from 0.062 (±0.004) to 0.102 (±0.001) (64.5% higher) and from 0.056 (±0.004) to 0.098 (±0.001) (75% higher) for cold morning and evening rush hours, with substantially greater increases at some sites. This "cold start" result suggests that the combination of recent vehicle driving history and ambient weather conditions, in conjunction with technological constraints on the operating temperature range of emission control systems in some vehicles, affects NO x emissions and hence has a detrimental impact upon air quality in urban environments. Increased cold start emissions imply an increased NO 2 - derived health burden from air pollution, under certain meteorological conditions, assessment of which should consider changes in vehicle use as a result of weather, and hence altered personal exposure. Graphical abstract Image 1 Highlights • A methodology is developed to assess enhanced "cold-start" primary NO 2 emissions from vehicles. • Ambient mixing ratios of NO 2 and NO x measured at UK urban traffic sites have fallen over the period from 2009 to 2016. • Inferred primary NO 2 /NO x ratio experiences a statistically significant decrease from 17.5% to 12.5% between 2009 and 2016. • Inferred cold-start primary NO 2 vehicle emissions are significantly higher than those found under warmer conditions. • Inferred cold-start primary NO 2 vehicle emissions have measurable impacts upon urban air quality in the UK. [ABSTRACT FROM AUTHOR]