Improvements to a laser-induced fluorescence instrument for measuring SO2 –...

Rickly, P., L. Xu, J. D. Crounse, P. Wennberg, and A. Rollins (2021), Improvements to a laser-induced fluorescence instrument for measuring SO2 – impact on accuracy and precision, Atmos. Meas. Tech., 14, 2429-2439, doi:10.5194/amt-14-2429-2021.

This work describes key improvements made to the in situ laser-induced fluorescence instrument for measuring sulfur dioxide (SO2 ) that was originally described by Rollins et al. (2016). Here, we report measurements of the SO2 fluorescence emission spectrum. These measurements allow for the determination of the most appropriate bandpass filters to optimize the fluorescence signal, while reducing the instrumental background. Because many aromatic species fluoresce in the same spectral region as SO2 , fluorescence spectra were also measured for naphthalene and anisole to determine if ambient SO2 measurements could be biased in the presence of such species. Improvement in the laser system resulted in better tunability, and a significant reduction in the 216.9 nm laser linewidth. This increases the online/offline signal ratio which, in turn, improves the precision and specificity of the measurement. The effects of these improvements on the instrumental sensitivity were determined by analyzing the signal and background of the instrument, using varying optical bandpass filter ranges and cell pressures and calculating the resulting limit of detection. As a result, we report an improvement to the instrumental sensitivity by as much as 50 %. 1 Background Sulfur dioxide (SO2 ) is responsible for a number of health and environmental impacts. Through reaction with the hydroxyl radical (OH), SO2 produces sulfuric acid, which affects the pH of aqueous particles and leads to acid deposition. Sulfuric acid also condenses onto organic and black carbon particles, producing sulfate, which increases the aerosol hygroscopicity and influences the accumulation of aerosol liquid water (Fiedler et al., 2011; Carlton et al., 2020). Sulfuric acid is believed to be the most important source gas globally for homogeneous nucleation and growth of new aerosol particles, which may occur primarily in the tropical upper troposphere (Brock et al., 1995; Dunne et al., 2016; Williamson et al., 2019). SO2 and sulfate particles can be transported long distances, driving the production of haze pollution in areas downwind of SO2 emissions (Andreae et al., 1988). Both the direct radiative forcing from aerosol and the indirect forcing from aerosol–cloud interactions are important for climate. While both tend to produce an offset to greenhouse-gas-induced warming by reducing incoming shortwave radiation, the effect of aerosol–cloud interactions is complicated and produces large uncertainties in climate models (Finlayson-Pitts and Pitts, 2000; IPCC, 2021). Changing emissions distributions, coupled with an incomplete understanding of the chemistry and microphysics associated with sulfur and aerosol formation in the atmosphere, necessitates further studies which require precise and accurate measurements of SO2 throughout the troposphere and lower stratosphere.

Regulation of anthropogenic emissions has resulted in decreased atmospheric SO2 concentrations in the United States and Europe since the 1970s. However, during the early 21st century, emissions began increasing dramatically in Asia as a result of increased fossil fuel burning (Smith et al., 2011; Hoesly et al., 2018). The main source of SO2 to the tro-

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Tropospheric Composition Program (TCP)