Quantification of NOX Emissions Based on Car MAX-DOAS Measurements over Beijing and Impacts of Wind Field
Authors: Cheng Xinghong 
Published: March 2020

&lt;p&gt;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 NO&lt;sub&gt;2&lt;/sub&gt; are retrieved and used to estimate the emissions of NO&lt;sub&gt;x&lt;/sub&gt;. The offline LAPS-WRF-CMAQ model system is used to simulate wind fields by assimilation of observational data and calculate the NO&lt;sub&gt;2&lt;/sub&gt; to NO&lt;sub&gt;x&lt;/sub&gt; concentration ratios. The NO&lt;sub&gt;X&lt;/sub&gt; 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 NO&lt;sub&gt;2&lt;/sub&gt; 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 NO&lt;sub&gt;2&lt;/sub&gt; VCD, and the mean NO&lt;sub&gt;2&lt;/sub&gt; 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&lt;sub&gt;X&lt;/sub&gt; emissions rates (E&lt;sub&gt;NOX&lt;/sub&gt;) is consistent with that of the NO&lt;sub&gt;2&lt;/sub&gt; VCD, and E&lt;sub&gt;NOX &lt;/sub&gt;is mainly determined by the NO2 VCD. In addition, the journey-to-journey E&lt;sub&gt;NOX&lt;/sub&gt; in the same month is different and it is affected by wind speed, the ratio of NO&lt;sub&gt;2&lt;/sub&gt; and NOx concentration and the decay rate of NO&lt;sub&gt;X&lt;/sub&gt; from the emission sources to measured positions under different meteorological condition. The E&lt;sub&gt;NOX&lt;/sub&gt; ranges between 6.46&amp;#215;10&lt;sup&gt;25&lt;/sup&gt; and 50.05&amp;#215;10&lt;sup&gt;25&lt;/sup&gt; molec s&lt;sup&gt;-1&lt;/sup&gt;. The averaged E&lt;sub&gt;NOX&lt;/sub&gt; during every journey in January, September and October are respectively 35.87&amp;#215;10&lt;sup&gt;25&lt;/sup&gt;, 20.34&amp;#215;10&lt;sup&gt;25&lt;/sup&gt;, 8.96&amp;#215;10&lt;sup&gt;25&lt;/sup&gt; molec s&lt;sup&gt;-1&lt;/sup&gt;. The estimated E&lt;sub&gt;NOX&lt;/sub&gt; after removing the simulated error of wind speed and observed deviation of NO&lt;sub&gt;2&lt;/sub&gt; VCD are found to be mostly closer to the MEIC 2012, but sometimes E&lt;sub&gt;NOX &lt;/sub&gt;is lower or higher and it indicates that the MEIC 2012 might be overestimate or underestimate the true emissions. The estimated E&lt;sub&gt;NOX&lt;/sub&gt; on January 27 and September 19 are obviously higher than other journeys in the same month because the mean NO&lt;sub&gt;2&lt;/sub&gt; 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 NO&lt;sub&gt;2&lt;/sub&gt; VCD in Beijing which is downwind of south-central&amp;#160;regions of Hebei province with high source emission rates, the uncertainty of the estimated E&lt;sub&gt;NOX&lt;/sub&gt; with south wind will be increased.&lt;/p&gt;