Interference of Sulphur Dioxide on Balloon-borneElectrochemical Concentration Cell Ozone Sensors overthe Mexico City Metropolitan Area

Abstract

An abnormal decrease in ozonesonde sensor signal occurred during air-pollution study campaigns in November 2011 and March 2012 in Mexico City Metropolitan Area (MCMA). Sharp drops in sensor signal around 5 km above sea level and above were observed in November 2011, and a reduction of signal over a broad range of altitude was observed in the convective boundary layer in March 2012. Circumstantial evidence indicated that SO2 gas interfered with the electrochemical concentration cell (ECC) ozone sensors in the ozonesonde and that this interference was the cause of the reduced sensor signal output. The sharp drops in November 2011 were attributed to the SO2 plume from Popocatépetl volcano southeast of MCMA. Experiments on the response of the ECC sensor to representative atmospheric trace gases showed that only SO2 could cause the observed abrupt drops in sensor signal. The vertical profile of the plume reproduced by a Lagrangian particle diffusion simulation supported this finding. A nearground reduction in the sensor signal in March 2012 was attributed to an SO2 plume from the Tula industrial complex north-west of MCMA. Before and at the time of ozonesonde launch, intermittent high SO2 concentrations were recorded at ground-level monitoring stations north of MCMA. The difference between the O3 concentration measured by the ozonesonde and that recorded by a UV-based O3 monitor was consistent with the SO2 concentration recorded by a UV-based monitor on the ground. The vertical profiles of the plumes estimated by Lagrangian particle diffusion simulation agreed fairly well with the observed profile. Statistical analysis of the wind field in MCMA revealed that the effect Popocatépetl was most likely to have occurred from June to October, whereas the effect of the industries north of MCMA, including the Tula complex, was predicted to occur throughout the year.

Keywords

• electrochemical concentration cell
• mexico city
• ozone
• ozonesonde
• sulphur dioxide