Evaluation of tropospheric SO₂ retrieved from MAX-DOAS measurements in Xianghe, China

Abstract. Ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements of sulfur dioxide (SO₂) have been performed at the Xianghe station (39.8° N, 117.0° E) located at ~ 50 km southeast of Beijing from March 2010 to February 2013. Tropospheric SO₂ vertical profiles and corresponding vertical column densities (VCDs), retrieved by applying the optimal estimation method to the MAX-DOAS observations, have been used to study the seasonal and diurnal cycles of SO₂, in combination with correlative measurements from in situ instruments, as well as meteorological data. A marked seasonality was observed in both SO₂ VCD and surface concentration, with a maximum in winter (February) and a minimum in summer (July). This can be explained by the larger emissions in winter due to the domestic heating and, in case of surface concentration, by more favorable meteorological conditions for the accumulation of SO₂ close to the ground during this period. Wind speed and direction are also found to be two key factors in controlling the level of the SO₂-related pollution at Xianghe. In the case of east or southwest wind, the SO₂ concentration does not change significantly with the wind speed, since the city of Tangshan and heavy polluting industries are located to the east and southwest of the station, respectively. In contrast, when wind comes from other directions, the stronger the wind, the less SO₂ is observed due to a more effective dispersion. Regarding the diurnal cycle, the SO₂ amount is larger in the early morning and late evening and lower at noon, in line with the diurnal variation of pollutant emissions and atmospheric stability. A strong correlation with correlation coefficients between 0.6 and 0.9 is also found between SO₂ and aerosols in winter, suggesting that anthropogenic SO₂, through the formation of sulfate aerosols, contributes significantly to the total aerosol content during this season. The observed diurnal cycles of MAX-DOAS SO₂ surface concentration are also in very good agreement (correlation coefficient close to 0.9) with those from collocated in situ data, indicating the good reliability and robustness of our retrieval.