Abstract
NOX (NOX = NO + NO2) emissions measurements in Beijing are of great significance because they can aid in understanding how NOX pollution develops in mega-cities throughout China. However, NOX emissions in mega-cities are difficult to measure due to changes in wind patterns and moving sources on roads during measurement. To obtain good spatial coverage on different ring roads in Beijing over a short amount of time, two mobile differential optical absorption spectroscopy (DOAS) instruments were used to measure NOX emission flux from April 18th to 26th, 2018. In addition, a wind profile radar provided simultaneous wind field measurements for altitudes between 50 m and 1 km for each ring road measurement. We first determined NOX emission flux of different ring roads using wind field averages from measured wind data. The results showed that the NOX emission flux of Beijing’s fifth ring road, which represented the urban part, varied from (19.29 ± 5.26) × 1024 molec./s to (36.46 ± 12.86) × 1024 molec./s. On April 20th, NOX emission flux for the third ring was slightly higher than the fourth ring because the two ring roads were measured at different time periods. We then analyzed the NOX emission flux error budget and error sensitivity. The main error source was the wind field uncertainty. For some measurements, the main emission flux error source was either wind speed uncertainty or wind direction uncertainty, but not both. As Beijing’s NOX emissions came from road vehicle exhaust, we found that emission flux error had a more diverse sensitivity to wind direction uncertainty, which improved our knowledge on this topic. The NOX emission flux error sensitivity study indicated that more accurate measurements of the wind field are crucial for effective NOX emission flux measurements in Chinese mega-cities. Obtaining actual time and high resolved wind measurements is an advantage for mega-cities’ NOX emission flux measurements. The emission flux errors caused by wind direction and wind speed uncertainties were clearly distinguished. Other sensitivity studies indicated that NOX/NO2 ratio uncertainty dominated flux errors when the NOX/NO2 ratio uncertainty was >0.4. Using two mobile-DOAS and wind profile radars to measure NOx emission flux improved the quality of the emission flux measuring results. This approach could be applied to many other mega-cities in China and in others countries.
Highlights
Using the statistical method outlined in Section 2.2.2, we first calculated the time averaged wind field to generate corresponding wind profile
The emission flux error sensitivity study indicated that the emission flux error, in some cases, was highly sensitive to wind speed uncertainty or wind direction uncertainty, rather than both
The emission flux error had diverse sensitivity to wind direction uncertainty, which is a novel addition to the existing literature on this topic
Summary
We calculated the time averaged wind speed. The average wind speed and its uncertainty were determined by Equations (2) and (3), respectively. The wind direction uncertainty of the measured wind data is σ(θzj)t = ln(ρ2zj) In this step, the wind speed uncertainty σwt and the wind direction uncertainty σθt introduced by time averaging were determined by Equation (7) according to the error combination law, which are: σwt =. The step was to average the wind field by the NOX concentration’s exponentially decreasing profiles This was an exponential weight function [29,31] and the weight at the height of zj is: Qzj =. The z0 we chose is a rough estimate and has a rather small influence on the average wind field [31] In this step, the uncertainty caused by the wind profile was introduced.
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
