Quantification of NOX Emissions Based on Car MAX-DOAS Measurements over Beijing and Impacts of Wind Field

We carried out 14 days of Car MAX-DOAS experiments on the 6th Ring Rd of Beijing in January, September and October, 2014. The tropospheric vertical column densities (VCD) of NO₂ are retrieved and used to estimate the emissions of NO_x. The offline LAPS-WRF-CMAQ model system is used to simulate wind fields by assimilation of observational data and calculate the NO₂ to NO_x concentration ratios. The NO_X emissions in Beijing for different seasons derived from Car MAX-DOAS measurements are compared with the multi-resolution emission inventory in China for 2012 (MEIC 2012), and impacts of wind field on estimated emissions and its uncertainties are also investigated. Results show that the NO₂ VCD is higher in January than other two months and it is typically larger at the southern parts of the 6th Ring Road than the northern parts of it. Wind field has obvious impacts on the spatial distribution of NO₂ VCD, and the mean NO₂ VCD with south wind at most sampling points along the 6th Ring Rd is higher than north wind. The journey-to-journey variation pattern of estimated NO_X emissions rates (E_NOX) is consistent with that of the NO₂ VCD, and E_NOX is mainly determined by the NO2 VCD. In addition, the journey-to-journey E_NOX in the same month is different and it is affected by wind speed, the ratio of NO₂ and NOx concentration and the decay rate of NO_X from the emission sources to measured positions under different meteorological condition. The E_NOX ranges between 6.46×10²⁵ and 50.05×10²⁵ molec s^-1. The averaged E_NOX during every journey in January, September and October are respectively 35.87×10²⁵, 20.34×10²⁵, 8.96×10²⁵ molec s^-1. The estimated E_NOX after removing the simulated error of wind speed and observed deviation of NO₂ VCD are found to be mostly closer to the MEIC 2012, but sometimes E_NOX is lower or higher and it indicates that the MEIC 2012 might be overestimate or underestimate the true emissions. The estimated E_NOX on January 27 and September 19 are obviously higher than other journeys in the same month because the mean NO₂ VCD and Leighton ratio during these two periods are larger, and corresponding wind speeds are smaller. Additionally, because south wind may affect the spatial distribution of mean NO₂ VCD in Beijing which is downwind of south-central regions of Hebei province with high source emission rates, the uncertainty of the estimated E_NOX with south wind will be increased.