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Atmosphere 2018, 9(11), 432; https://doi.org/10.3390/atmos9110432

Response of Surface Ultraviolet and Visible Radiation to Stratospheric SO2 Injections

1
Atmospheric Chemistry, Observations, and Modeling Laboratory, National Center for Atmospheric Research, Boulder, CO 80307, USA
2
Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, WA 99352, USA
3
Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY 14853, USA
4
Climate and Global Dynamics Laboratory, National Center for Atmospheric Research, Boulder, CO 80307, USA
*
Author to whom correspondence should be addressed.
Received: 17 August 2018 / Revised: 30 October 2018 / Accepted: 2 November 2018 / Published: 7 November 2018
(This article belongs to the Special Issue Radiative Transfer in the Earth Atmosphere)
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Abstract

Climate modification by stratospheric SO2 injections, to form sulfate aerosols, may alter the spectral and angular distributions of the solar ultraviolet and visible radiation that reach the Earth’s surface, with potential consequences to environmental photobiology and photochemistry. We used modeling results from the CESM1(WACCM) stratospheric aerosol geoengineering large ensemble (GLENS) project, following the RCP8.5 emission scenario, and one geoengineering experiment with SO2 injections in the stratosphere, designed to keep surface temperatures at 2020 levels. Zonally and monthly averaged vertical profiles of O3, SO2, and sulfate aerosols, at 30 N and 70 N, served as input into a radiative transfer model, to compute biologically active irradiances for DNA damage (iDNA), UV index (UVI), photosynthetically active radiation (PAR), and two key tropospheric photodissociation coefficients (jO1D for O3 + hν (λ < 330 nm) → O(1D) + O2; and jNO2 for NO2 + hν (λ < 420 nm) → O(3P) + NO). We show that the geoengineering scenario is accompanied by substantial reductions in UV radiation. For example, comparing March 2080 to March 2020, iDNA decreased by 25% to 29% in the subtropics (30 N) and by 26% to 33% in the polar regions (70 N); UVI decreased by 19% to 20% at 30 N and 23% to 26% at 70 N; and jO1D decreased by 22% to 24% at 30 N and 35% to 40% at 70 N, with comparable contributions from sulfate scattering and stratospheric O3 recovery. Different responses were found for processes that depend on longer UV and visible wavelengths, as these are minimally affected by ozone; PAR and jNO2 were only slightly lower (9–12%) at 30 N, but much lower at 70 N (35–40%). Similar reductions were estimated for other months (June, September, and December). Large increases in the PAR diffuse-direct ratio occurred in agreement with previous studies. Absorption by SO2 gas had a small (~1%) effect on jO1D, iDNA, and UVI, and no effect on jNO2 and PAR. View Full-Text
Keywords: geoengineering; sulfate aerosols; stratosphere; stratospheric ozone; ultraviolet radiation; erythemal radiation; photolysis coefficients; photosynthetically active radiation (PAR); direct-diffuse ratio geoengineering; sulfate aerosols; stratosphere; stratospheric ozone; ultraviolet radiation; erythemal radiation; photolysis coefficients; photosynthetically active radiation (PAR); direct-diffuse ratio
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).

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Madronich, S.; Tilmes, S.; Kravitz, B.; MacMartin, D.G.; Richter, J.H. Response of Surface Ultraviolet and Visible Radiation to Stratospheric SO2 Injections. Atmosphere 2018, 9, 432.

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