Dimethyl sulfide (DMS), emitted from the oceans, is the most abundant biological source of sulfur to the marine atmosphere. Atmospheric DMS is oxidized to condensable products that form secondary aerosols that affect Earth’s radiative balance by scattering solar radiation and serving as cloud condensation nuclei. We report the atmospheric discovery of a previously unquantified DMS oxidation product, hydroperoxymethyl thioformate (HPMTF, HOOCH2SCHO), identified through global-scale airborne observations that demonstrate it to be a major reservoir of marine sulfur. Observationally constrained model results show that more than 30% of oceanic DMS emitted to the atmosphere forms HPMTF. Coincident particle measurements suggest a strong link between HPMTF concentration and new particle formation and growth. Analyses of these observations show that HPMTF chemistry must be included in atmospheric models to improve representation of key linkages between the biogeochemistry of the ocean, marine aerosol formation and growth, and their combined effects on climate.
A 6-week
study was conducted at the University of Colorado Art
Museum, during which volatile organic compounds (VOCs), carbon dioxide
(CO2), ozone (O3), nitric oxide (NO), nitrogen
dioxide (NO2), other trace gases, and submicron aerosol
were measured continuously. These measurements were then analyzed
using a box model to quantify the rates of major processes that transformed
the composition of the air. VOC emission factors were quantified for
museum occupants and their activities. The deposition of VOCs to surfaces
was quantified across a range of VOC saturation vapor concentrations
(C*) and Henry’s Law constants (H) and determined to be a major sink for VOCs with C* < 108 μg m–3 and H > 102 M atm–1. The reaction
rates of VOCs with O3, OH radicals, and nitrate (NO3) radicals were quantified, with unsaturated and saturated
VOCs having oxidation lifetimes of >5 and >15 h, making deposition
to surfaces and ventilation the dominant VOC sinks in the museum.
O3 loss rates were quantified inside a museum gallery,
where reactions with surfaces, NO, occupants, and NO2 accounted
for 62%, 31%, 5%, and 2% of the O3 sink. The measured concentrations
of acetic acid, formic acid, NO2, O3, particulate
matter, sulfur dioxide, and total VOCs were below the guidelines for
museums.
Biogenic sources contribute to cloud condensation nuclei (CCN) in the clean marine atmosphere, but few measurements exist to constrain climate model simulations of their importance. The chemical composition of individual atmospheric aerosol particles showed two types of sulfate-containing particles in clean marine air masses in addition to mass-based Estimated Salt particles. Both types of sulfate particles lack combustion tracers and correlate, for some conditions, to atmospheric or seawater dimethyl sulfide (DMS) concentrations, which means their source was largely biogenic. The first type is identified as New Sulfate because their large sulfate mass fraction (63% sulfate) and association with entrainment conditions means they could have formed by nucleation in the free troposphere. The second type is Added Sulfate particles (38% sulfate), because they are preexisting particles onto which additional sulfate condensed. New Sulfate particles accounted for 31% (7 cm−3) and 33% (36 cm−3) CCN at 0.1% supersaturation in late-autumn and late-spring, respectively, whereas sea spray provided 55% (13 cm−3) in late-autumn but only 4% (4 cm−3) in late-spring. Our results show a clear seasonal difference in the marine CCN budget, which illustrates how important phytoplankton-produced DMS emissions are for CCN in the North Atlantic.
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