The future of airborne sulfur-containing particles in the absence of fossil fuel sulfur dioxide emissions

Abstract

Sulfuric acid (H2SO4), formed from oxidation of sulfur dioxide (SO2) emitted during fossil fuel combustion, is a major precursor of new airborne particles, which have well-documented detrimental effects on health, air quality, and climate. Another precursor is methanesulfonic acid (MSA), produced simultaneously with SO2 during the atmospheric oxidation of organosulfur compounds (OSCs), such as dimethyl sulfide. In the present work, a multidisciplinary approach is used to examine how contributions of H2SO4 and MSA to particle formation will change in a large coastal urban area as anthropogenic fossil fuel emissions of SO2 decline. The 3-dimensional University of California Irvine-California Institute of Technology airshed model is used to compare atmospheric concentrations of gas phase MSA, H2SO4, and SO2 under current emissions of fossil fuel-associated SO2 and a best-case futuristic scenario with zero fossil fuel sulfur emissions. Model additions include results from (i) quantum chemical calculations that clarify the previously uncertain gas phase mechanism of formation of MSA and (ii) a combination of published and experimental estimates of OSC emissions, such as those from marine, agricultural, and urban processes, which include pet waste and human breath. Results show that in the zero anthropogenic SO2 emissions case, particle formation potential from H2SO4 will drop by about two orders of magnitude compared with the current situation. However, particles will continue to be generated from the oxidation of natural and anthropogenic sources of OSCs, with contributions from MSA and H2SO4 of a similar order of magnitude. This could be particularly important in agricultural areas where there are significant sources of OSCs.

Keywords: atmosphere; fossil fuel; methanesulfonic acid; new particle formation; sulfuric acid.

Fig. 1.

Potential energy diagram for the reaction of CH₃S(O)(O)O^• radical with methane (CH₄) or formaldehyde (HCHO). See SI Appendix, section 3 for details of the theory applied.

Fig. 2.

Model-predicted gas phase H₂SO₄ and MSA concentrations (ppb) in the SoCAB at 8:00 h, 12:00 h, 16:00 h, and 20:00 h. (A) H₂SO₄ concentrations (ppb) with SO₂ and H₂SO₄ emissions representative of 2011–2013. (B) H₂SO₄ concentrations (ppb) with sulfur fossil fuel emissions, boundary conditions, and initial conditions for SO₂ and H₂SO₄ set to zero. (C) MSA concentrations (ppb) with sulfur fossil fuel emissions, boundary conditions, and initial conditions for SO₂ and H₂SO₄ set to zero. The domain-wide average gas phase MSA concentration is not significantly sensitive to the scenario chosen for sulfur fossil fuel emissions.

Fig. 3.

Model-predicted gas phase SO₂ and DMS concentrations (ppb) in the SoCAB at 08:00 h, 12:00 h, 16:00 h, and 20:00 h with urban, agriculture, and ocean OSCs emission sources included. (A) SO₂ concentrations (ppb) with SO₂ and H₂SO₄ emissions representative of 2011–2013. (B) SO₂ concentrations (ppb) with sulfur fossil fuel emissions, boundary conditions, and initial conditions for SO₂ and H₂SO₄ set to zero. (C) DMS concentrations (ppb) with sulfur fossil fuel emissions, boundary conditions, and initial conditions for SO₂ and H₂SO₄ set to zero.

Fig. 4.

Model-predicted gas phase DMS concentrations (ppb) at 08:00 h, 12:00 h, 16:00 h, and 20:00 h with sulfur fossil fuel emissions, boundary conditions, and initial conditions for SO₂ and H₂SO₄ set to zero. (A) Only urban OSC emission sources including DMS emissions from humans and DMS, DMDS, and MTO from pet waste. (B) Urban and agriculture OSC emission sources (same conditions as A plus DMS and DMDS emissions from Chino, California). (C) Urban, agriculture, and ocean OSC emission sources (same conditions as B plus DMS from ocean).

Fig. 5.

Plots of Na³⁵Cl⁺ ion versus MSA ion (CH₃SO₂⁺). (A) NaCl and MSA are correlated (overall r² = 0.41) for (light blue ⋆) August 2, 2012 (9:15–14:20), (dark blue ▿) August 27, 2012 (10:00–20:05), and (dark blue ▾) August 31, 2012 (10:20–16:50). (B) NaCl and MSA are uncorrelated (overall r² = 0.0023) for (orange ▴) July 28, 2010 (10:10)–July 29, 2010 (15:00), (green ▴) May 14, 2012 (10:35–13:55), and (red ▴) August 26, 2014 (13:50–14:05). Measurements were made in Irvine, California. Mass loading for NaCl is uncorrected for its low sensitivity in the AMS due to inefficient vaporization. Data points represent 1–2-min sampling times. Dashed lines are linear fits to all data points in each plot.