MAX-DOAS measurements of tropospheric NO<sub>2</sub> and HCHO in Nanjing and the comparison to OMI observations

Abstract

<p><strong>Abstract.</strong> In this paper, we present long term observations of atmospheric nitrogen dioxide (NO<sub>2</sub>) and formaldehyde (HCHO) in Nanjing using a Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) instrument. Ground based MAX-DOAS measurements were performed from April 2013 to February 2017. The MAX-DOAS measurements of NO<sub>2</sub> and HCHO vertical column densities (VCDs) are used to validate OMI satellite observations over Nanjing. The comparison shows that the OMI observations of NO<sub>2</sub> correlate well with the MAX-DOAS data with Pearson correlation coefficient (<i>R</i>) of 0.91. However, OMI observations are on average a factor of 3 lower than the MAX-DOAS measurements. Replacing the a priori NO<sub>2</sub> profiles by the MAX-DOAS profiles in the OMI NO2 VCD retrieval would increase the OMI NO<sub>2</sub> VCDs by ~ 30 % with correlation nearly unchanged. The comparison result of MAX-DOAS and OMI observations of HCHO VCD shows a good agreement with <i>R</i> of 0.75 and the slope of the regression line is 0.99. We developed a new technique to assemble the source contribution map using backward trajectory analysis. The age weighted backward propagation approach is applied to the MAX-DOAS measurements of NO<sub>2</sub> and HCHO to reconstruct the spatial distribution of NO<sub>2</sub> and HCHO over the Yangtze River Delta during summer and winter time. The reconstructed NO<sub>2</sub> fields show a distinct agreement with OMI satellite observations. However, due to the short atmospheric lifetime of HCHO, the backward propagated HCHO data does not show a strong spatial correlation with the OMI HCHO observations. The result shows the MAX-DOAS measurements are sensitive to the air pollution transportation in the Yangtze River Delta, indicating the air quality in Nanjing is significantly influenced by regional transportation of air pollutants. The MAX-DOAS data are also used to evaluate the effectiveness of air pollution control measures implemented during the Youth Olympic Games 2014. The MAX-DOAS data show a significant reduction of ambient aerosol, NO<sub>2</sub> and HCHO (30 %–50 %) during the Youth Olympic Games. Our results provide a better understanding of the transportation and sources of pollutants in over the Yangtze River Delta as well as the effect of emission control measures during large international event, which are important for the future design of air pollution control policies.</p>

Ground-Based MAX-DOAS Observations of CHOCHO and HCHO in Beijing and Baoding, China

“…This finding is consistent with glyoxal retrieved over Beijing and Northern China from OMI satellite data [42,43]. The increase in HCHO during summer is considered to be due to the photochemical production of HCHO, biogenic emissions during growing season and the burning of biomass in spring and autumn [31].…”

“…Higher levels were observed in summer, whereas reduction in HCHO level was noted during the winter season. The increase in HCHO during summer is considered to be due to the photochemical production of HCHO, biogenic emissions during growing season and burning of biomass in spring and autumn [31]. Monthly cycles for CHOCHO and HCHO obtained from MAX-DOAS observations were compared with OMI satellite data.…”

Enhancing MAX-DOAS atmospheric state retrievals by multispectral polarimetry – studies using synthetic data

“…During the inversion, the model parameters are iteratively adapted to bring simulated and real measurements into closure, thereby approaching the real atmospheric state. In recent years, a multitude of MAX-DOAS inversion algorithms have been implemented that apply different approaches in terms of parameterisation, a priori constraints and optimisation schemes (Irie et al, 2008;Clémer et al, 2010;Wang et al, 2013;Yilmaz, 2012;Bösch et al, 2018;Chan et al, 2019;Vlemmix et al, 2011;Beirle et al, 2019).…”

Supplementary material to "Intercomparison of MAX-DOAS vertical profile retrieval algorithms: studies on field data from the CINDI-2 campaign"

“…The Jacobian is calculated numerically using the finite difference method, while the box air mass factors for trace gas profile retrievals are calculated using the Monte Carlo module of LibRadTran (MYSTIC). The M 3 retrieval used in the CINDI-2 campaign is a modified version of the algorithm described in detail in Chan et al (2019) with the iterative optimization of the a priori profile disabled. Aerosol extinction profiles are retrieved in the linear space.…”