Abstract
Abstract. 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 (ENOx) were large even within the same month, mainly due to uncertainties in the calculations of wind speed, the ratio of NO2 and NOx concentration, and the decay rate of NOx from the emission sources to the measured positions under different meteorological conditions. The ranges of ENOx during the heating and non-heating periods were 22.6×1025 to 31.3×1025 and 9.6×1025 to 12.0×1025 molec. s−1, respectively. The average ENOx values in the heating and non-heating periods were 26.9±6.1×1025 molec. s−1 and 11.0±1.2×1025 molec. s−1, respectively. The uncertainty range of ENOx was 20 %–52 %. The monthly emission rates from MEIC 2012 are found to be lower than the estimated ENOx, 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.
Highlights
Over the past decade, serious haze events have occurred frequently in fall and winter in Beijing due to massive anthropogenic emissions from the combustion of fossil fuels and other sources (He et al, 2013; Zhang et al, 2013)
It was shown that the temporal variations in simulated wind speed at the four stations were consistent with the observations from the perspective of time serial of wind speed, but the simulations were higher than the observations due to impacts of the complex topography and limited observation data assimilated to the Local Analysis and Prediction System (LAPS)-Weather Research and Forecasting (WRF) model
The NOx emission rates in urban Beijing were estimated using the measured NO2 vertical column densities (VCDs) together with the refined wind fields, NO2-to-NOx ratios, and NO2 lifetimes simulated by the LAPS-WRF-Community Multiscale Air Quality Modeling System (CMAQ) model system, and the results were compared to the emission rates from the MEIC 2012
Summary
Serious haze events have occurred frequently in fall and winter in Beijing due to massive anthropogenic emissions from the combustion of fossil fuels and other sources (He et al, 2013; Zhang et al, 2013). Multi-axis differential optical absorption spectroscopy (MAX-DOAS) is a ground-based remote-sensing technique developed during the last 2 decades It makes use of the scattered sunlight measured from the horizontal through zenithpointing directions to retrieve the VCD and vertical profiles of trace gases and aerosols with relatively high sensitivity in the lower atmosphere (Hönninger et al, 2004; Wagner et al, 2004; Platt and Stutz, 2008). Compared to ground-based observations at a fixed site, car MAX-DOAS measurements can provide information on the horizontal spatial distribution of pollutants, which is important for explaining the urban/regional representativeness of satellite observations and validating the NO2 VCDs and NOx emission estimates from the new, high pixel resolution measurements by the TROPOMI instrument on Sentinel-5P over megacities such as Beijing.
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