Understanding satellite measurements of Sulfur Dioxide

Since we began making regular measurements of the vertical structure of ozone in the atmosphere in Costa Rica (Selkirk et al, 2010), numerous ozone profiles have shown unusual notches below 5 km. We speculated that these were due to interference of the ozone measurement by sulfur dioxide emitted from Turrialba, an active volcano 35 km ENE of our launch site in San José.

To test this, we launched a dual ozone sonde payload (i.e., an instrument that measures both ozone and sulfur dioxide and is tied to a large balloon) on February 3, 2012, following the technique developed in Morris, et al. (2010). As shown in Fig. 1, we were able to detect a 15 ppbv layer of SO₂ just below 4 km as well as a lower layer about 500 m above the surface. While the lower layer is likely due to urban pollution, the trajectories in Fig. 2 show that the source of the upper layer was very likely to have been the volcano.

A sonde is a scientific instrument, attached to a balloon, that transmits scientific data back to a ground computer as it moves up through the atmosphere. (Photo credit NASA/JPL)

Figure 1. Profiles of ozone and sulfur dioxide mixing ratios and potential temperature, dual ozone sonde launch at San José, Costa Rica [9.94°N, 84.04°W] 1559 UT February 3, 2012. Thick lines, ascent profiles (a), thin lines, descent (d).

The total amount of SO₂ computed along the path of the balloon is 1.4 DU (DU = 2.69x10¹⁶ molecules per square centimeter). As shown in Fig. 3, this falls squarely in the range of 1-1.7 DU retrieved by the Aura/OMI instrument overflying just three hours later.

With the near-constant fumarolic activity at Turrialba, further profile measurements at San José will very likely provide an opportunity to improve the OMI retrievals of SO₂ (Yang et al., 2007).

(Above) Figure 2. HYSPLIT trajectories initiated at 12 UT, 3 February 2012 at 3.6, 3.8 and 4 km from the locus of Volcan Turrialba (triangle; elev. 3340 m) . Yellow oval at location of sonde ascent at 15 UT.

(Above) Figure 3. SO₂ column retrievals over Costa Rica from the Aura/OMI instrument at 1851 UT on February 3, 2012. Triangle and yellow oval as in Figure 2.

Technical Description of Image:
Figure 1: The dual ozone sonde payload consists of two electrochemical concentration cell (ECC) ozone sondes mounted in tandem, one with an SO₂ filter attached to the inlet. The reaction with SO₂ in the ECC cathode cell is the stoichiometric opposite of the O3 reaction. Thus, provided the SO₂ mixing ratio is lower than the ambient ozone and the two ozone sondes are well calibrated, the SO₂ mixing ratio can be derived by differencing the two sondes. Figure 2: Output from the NOAA Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) Model. The model was run in Normal mode with GDAS archive winds.
Figure 3: This is the 3-km OMI SO₂ product which assumes the center of mass of the SO₂ to be at 3 km. Columns amounts are given in Dobson units (DU) which is the number of molecules of gas that would be required to create a layer 0.01 millimeters thick at a temperature of 0 degrees Celsius and a pressure of 1 atmosphere.

Scientific significance:
SO₂ is both a significant pollutant in urban regions and an important trace gas emitted by volcanoes around the world with importance both to human health and to the natural environment. While satellite remote sensing measurements provide world-wide column coverage, retrievals are sensitive to SO₂'s vertical structure, hence the importance of in situ profiling. Furthermore, while volcanic plumes are episodic even in regions close to active volcanoes, near-surface layers of SO₂ in polluted urban environments may well be going undetected and in the process inducing unknown negative bias into single-sonde ozone profiles in those locations.

Relevance for future science and relationship to Decadal Survey:
Aura OMI will continue monitoring anthropogenic and natural volcanic SO₂ pollution sources from space to detect trends in this important pollutant and aerosol precursor to provide an overlap with recently launched Ozone Monitoring and Profiling suite (OMPS) on the US Next generation weather satellite Suomi-NPP. The Decadal Survey recommended the Geostationary Coastal and Air Pollution Events (GEO-CAPE) Tier 2 mission which is planned to launch after 2020. It will allow more frequent monitoring of anthropogenic NO2 and SO₂ pollution over North and Central America. A next generation OMI-like sensor (TROPOMI) is planned for launch by ESA in 2015 as a Sentinel-5 precursor mission; it will measure SO₂ with an increased ground resolution of 7 km by 7 km. All of these satellite measurements will benefit from in situ profiling of SO₂.

Data Source: Figure 1: The Tico-Ozonesonde balloonsonde project at the Universidad de Costa Rica is a NASA-supported collaboration among Dr. Jorge Andrés Diaz at UCR, Dr. Henry B. Selkirk at GESTAR/NASA GSFC, Dr. Holger Vömel of the Deutscher Wetterdienst, and Dr. Bryan Johnson at NOAA/ESRL. Prof. Gary Morris of Valparaiso University supervised the preparation of the dual ozonesonde payload and performed the SO₂ profile calculations. Figure 2: HY-SPLIT trajectories courtesy of NOAA/ARL. Figure 3: Operational lower tropospheric SO₂ column retrievals available from NASA DISC archive.

Reference:

Morris, G. A., W. D. Komhyr, J. Hirokawa, J. Flynn, B. Lefer, N. Krotkov, and Fong Ngan (2010), A Balloon Sounding Technique For Measuring SO₂ Plumes, Journal of Atmospheric and Oceanic Technology, 27, 1318-1330, doi:10.1175//2010JTECHA1436.1

Selkirk, H. B., H. Vömel, J. M. Valverde Canossa, L. Pfister, J. A. Diaz, W. Fernández, J. Amador, W. Stolz, and G. S. Peng (2010), Detailed structure of the tropical upper troposphere and lower stratosphere as revealed by balloon sonde observations of water vapor, ozone, temperature, and winds during the NASA TCSP and TC4 campaigns, Journal of Geophysical Research, 115, D00J19, doi:10.1029/2009JD013209.

Yang, K., N. A. Krotkov, A. J. Krueger, S. A. Carn, P. K. Bhartia, and P. F. Levelt (2007), Retrieval of large volcanic SO₂ columns from the Aura Ozone Monitoring Instrument: Comparison and limitations, Journal of Geophysical Research, 112, D24S43, doi:10.1029/ 2007JD008825.

03.13.2012