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Abstract In this paper, we present a quantitative determination of the NO x emission in Hefei using mobile Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements. The measurements were carried out during winter time from December 2016 to February 2017. The measured NO 2 vertical column densities (VCDs) show a strong spatial gradient with higher values within the city center, indicating the majority of NO x emission sources are located at the city center. A significant "holiday effect" was found by comparing measurements taken during weekdays and weekend. The weekend reduction is more significant in the suburban (∼28%) compared to the city center (∼13%). Mobile measurements of NO 2 vertical column densities (VCDs) were analyzed together with meteorological data to determine the NO x emission by applying the loop-integral method. Detailed error analysis of the NO x emission shows the variation of wind field and large measurement gap dominated the total error of NO x emission calculation. The result shows the NO x emission in Hefei during winter time varies in a wide range from 10 × 1024 to 40 × 1024molecs−1 with an average of 18.44 × 1024molecs−1. Our estimation is about 43% lower than the number reported in the previous emission inventory in 2012. The reduction of NO x emission reflects the successful implementation of emission control measures in recent years. Our result also shows about 73% of the total NO x in the city were transported from outside of the city during winter. The impacts of transported NO x are especially large when air masses originated from heavily polluted regions, i.e. North China Plain and Yangtze River Delta. The result presented provides a better understanding of the impacts of local emissions and transportation of pollutants on the local air quality of Hefei. Highlights • NO x emission in Hefei was determined using mobile MAX-DOAS measurements in 2017. • NO x emission in Hefei varies from 10 to 40 × 1024 molec/s during winter. • Our estimation is ∼43% lower compared to previous emission inventory in 2012. • About 73% of NO x measured in the city were transported from outside. [ABSTRACT FROM AUTHOR]