Retrieving tropospheric NO2 vertical column densities around the city of Beijing and estimating NOx emissions based on car MAX-DOAS measurements

We carried out 19 city-circle-around car multi-axis differential optical absorption spectroscopy (MAX-DOAS) experiments on the 6th Ring Road of Beijing in January, September, and October 2014. The tropospheric vertical column densities (VCDs) of NO2 were retrieved from measured spectra by the MAX-DOAS technique and used to estimate the emissions of NOx ( NO + NO2) from urban Beijing during the experimental periods. The offline LAPS-WRF-CMAQ model system was used to simulate the wind fields by assimilation of observational data and calculate the NO2-to-NOx concentration ratios, both of which are also needed for the estimation of NOx emissions. The NOx emissions in urban Beijing for the different months derived from the car MAX-DOAS measurements in this study were compared to the multi-resolution emission inventory in China for 2012 (MEIC 2012). Our car MAX-DOAS measurements showed higher NO2 VCD in January than in the other two months. The wind field had obvious impacts on the spatial distribution of NO2 VCD, with the mean NO2 VCD along the 6th Ring Road typically being higher under the southerly wind than under the northerly wind. In addition to the seasonal difference, the journey-to-journey variations of estimated NOx emission rates (E-NOx) were large even within the same month, mainly due to uncertainties in the calculations of wind speed, the ratio of NO2 and NOx con- centration, and the decay rate of NOx from the emission sources to the measured positions under different meteorological conditions. The ranges of E-NOx during the heating and non-heating periods were 22.6 x 10(25) to 31.3 x 10(25) and 9.6 x 10(25) to 12.0 x 10(25) molec. s(-1), respectively. The average ENO, values in the heating and non-heating periods were 26.9 +/- 6.1 x 10(25) molec. s(-1) and 11.0 +/- 1.2 x 10(25) molec. s(-1), respectively. The uncertainty range of E-NOx, was 20 %-52 %. The monthly emission rates from MEIC 2012 are found to be lower than the estimated E-NOx, particularly in January. Our results provide important information and datasets for the validation of satellite products and also show how car MAX-DOAS measurements can be used effectively for dynamic monitoring and updating of the NOx emissions from megacities such as Beijing.