Optical Properties of Individual Tar Balls in the Free Troposphere

Journal Article · Thu Oct 19 00:00:00 EDT 2023 · Environmental Science and Technology

DOI:https://doi.org/10.1021/acs.est.3c03498· OSTI ID:2205586

Mathai, Susan ^[1]; Veghte, Daniel ^[2]; Kovarik, Libor ^[3]; Mazzoleni, Claudio ^[4]; Tseng, Kuo-Pin ^[3]; Bucci, Silvia ^[5]; Capek, Tyler ^[4]; Cheng, Zezhen ^[3]; Marinoni, Angela ^[6]; China, Swarup ^[3]

---

1. Pacific Northwest National Laboratory (PNNL), Richland, WA (United States); Michigan Technological University, Houghton, MI (United States)
2. Pacific Northwest National Laboratory (PNNL), Richland, WA (United States); The Ohio State University, Columbus, OH (United States)
3. Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
4. Michigan Technological University, Houghton, MI (United States)
5. Italian National Research Council, Bologna (Italy). Institute of Atmospheric Sciences and Climate (ISAC); University of Vienna (Austria)
6. Italian National Research Council, Bologna (Italy). Institute of Atmospheric Sciences and Climate (ISAC)

Tar balls are brown carbonaceous particles, highly viscous, spherical, amorphous, and light absorbing. They are believed to form in biomass burning smoke plumes during transport in the troposphere. Tar balls are believed to have a significant impact on the Earth’s radiative balance, but due to poorly characterized optical properties, this impact is highly uncertain. Here, we investigate the chemical composition and optical properties of individual tar balls transported in the free troposphere to the Climate Observatory “Ottavio Vittori” on Mt. Cimone, Italy (2165 meters above sea level) using multi-modal micro-spectroscopy. Our results show that tar balls contributed 50% of carbonaceous particles by number in the size range from 0.25 to 1.8 µm. Of those tar balls, 16% were inhomogeneously mixed with other constituents. Using electron energy loss spectroscopy, we retrieved the complex refractive index from 200 to 1200 nm for both inhomogeneously and homogeneously mixed tar balls. We found no significant difference in the average refractive index of inhomogeneously and homogenously mixed tar balls (1.40 - 0.03i, and 1.36 - 0.03i at 550 nm, respectively). Furthermore, we estimated the top of the atmosphere radiative forcing using the Santa Barbara DISORT Atmospheric Radiative Transfer model (SBDART) and found that a layer of only tar balls with an optical depth of 0.1 above vegetation would exert a positive radiative forcing ranging from 2.8 Wm-2 (on a clear sky day) to 9.5 Wm-2 (when clouds are below the aerosol layer). In conclusion, understanding the optical properties of tar balls can help reduce uncertainties associated with the contribution of biomass-burning aerosol in current climate models.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities (SUF); USDOE Office of Science (SC), Biological and Environmental Research (BER)
• Grant/Contract Number: AC05-76RL01830; AC02-05CH11231
• OSTI ID: 2205586
• Report Number(s): PNNL-SA--185434
• Journal Information: Environmental Science and Technology, Journal Name: Environmental Science and Technology Journal Issue: 44 Vol. 57; ISSN 0013-936X
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
Tar balls
brown carbon
free troposphere
optical properties
radiative forcing