Aerosol indirect radiative forcing (IRF), which characterizes how aerosols alter cloud formation and properties, is very sensitive to the preindustrial (PI) aerosol burden. Dimethyl sulfide (DMS), emitted from the ocean, is a dominant natural precursor of non-sea-salt sulfate in the PI and pristine present-day (PD) atmospheres. Here we revisit the atmospheric oxidation chemistry of DMS, particularly under pristine conditions, and its impact on aerosol IRF. Based on previous laboratory studies, we expand the simplified DMS oxidation scheme used in the Community Atmospheric Model version 6 with chemistry (CAM6-chem) to capture the OH-addition pathway and the H-abstraction pathway and the associated isomerization branch. These additional oxidation channels of DMS produce several stable intermediate compounds, e.g., methanesulfonic acid (MSA) and hydroperoxymethyl thioformate (HPMTF), delay the formation of sulfate, and,
hence, alter the spatial distribution of sulfate aerosol and radiative
impacts. The expanded scheme improves the agreement between modeled and
observed concentrations of DMS, MSA, HPMTF, and sulfate over most marine
regions, based on the NASA Atmospheric Tomography (ATom), the Aerosol and
Cloud Experiments in the Eastern North Atlantic (ACE-ENA), and the
Variability of the American Monsoon Systems (VAMOS)
Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx)
measurements. We find that the global HPMTF burden and the burden of sulfate produced from DMS oxidation are relatively insensitive to the
assumed isomerization rate, but the burden of HPMTF is very sensitive to a
potential additional cloud loss. We find that global sulfate burden under PI and PD emissions increase to 412 Gg S (
The IPCC AR5 (Fifth Assessment Report of the United Nations Intergovernmental Panel on Climate Change; Myhre et al., 2013) and the recent preliminary
release of AR6 (Sixth Assessment Report;
Marine dimethyl sulfide (DMS; CH
Global/regional models often simplify the DMS oxidation processes for the
sake of computational costs. For example, the Community Atmosphere Model
with chemistry (CAM-chem) includes only the oxidation of DMS by OH and
NO
The lifetimes of stable intermediates from DMS oxidation can be up to days. As a result, these intermediates can delay the formation of DMS-derived sulfate, affecting not only the spatial distribution of sulfate aerosols but also the effective sulfate yield from DMS as unreacted sulfate precursors may be subject to physical removal through wet or dry deposition. Thus, neglecting these intermediates could lead to misrepresentation of the spatial distribution of sulfate aerosol loading and limit our ability to accurately determine aerosol radiative forcing.
Here, we implement a more detailed multigenerational and multiphase
chemical mechanism to describe DMS oxidation within the Community Atmosphere
Model version 6 with chemistry (CAM6-chem; Emmons et al., 2020) – the
atmosphere component of the Community Earth System Model version 2.1
(CESM2.1; Danabasoglu et al., 2020). The expanded chemistry captures the formation of stable intermediates such as MSA and HPMTF alongside SO
CESM2.1 consists of model components that quantitatively describe the atmosphere, land, sea ice, land ice, rivers, and ocean (Danabasoglu et al., 2020). Fluxes and state variables are exchanged through a coupler to describe the co-evolution of these Earth system components. Here, we run with a coupled atmosphere (CAM6-chem) and land (Community Land Model – CLM5) model and use prescribed data for the remaining Earth system components. In particular, sea surface temperature (SST) and sea ice conditions (Hurrell et al., 2008), as well as the mixing ratios of greenhouse gases (Meinshausen et al., 2017), are all fixed to present-day conditions. Following similar practices in previous studies (Gettelman, 2015; Gettelman et al., 2019), this configuration aims to constrain the potential environmental feedbacks, such that the aerosol effects (on atmospheric composition, cloud, and radiation) are due to the change in emissions and chemistry only.
In this work, CAM6-chem is run in an online configuration with free dynamics
at 1.9
Configuration of key simulation cases in this study.
We perform four sets of simulations for the PD and PI atmospheric conditions with the standard and our modified chemical schemes. Details of the runs are tabulated in Table 1. Each run is performed for 10 years, with the first year as spin-up, and the averages over the latter 9 years are presented in our results.
DMS emissions from the ocean (
On average, the global annual total marine
SO
In all simulations, anthropogenic emissions are from the Community Emissions
Data System (CEDS; Hoesly et al., 2018), and biomass burning emissions are from the CMIP6 inventory (van Marle et al., 2017). Biogenic emissions are estimated online from CLM5, using the Model of Emissions of Gases and Aerosols from Nature (MEGAN) version 2.1 (Guenther et al., 2012). CAM6-chem assumes that 2.5 % by molar of sulfur emitted from the energy and industry sector is already in the form of primary sulfate aerosols (in accumulation mode). Volcanic emissions are fixed at the same level in both PI and PD simulations. Emissions from continuously outgassing volcanoes are constant (97.5 % as SO
The standard CAM6-chem contains three gas-phase DMS oxidation reactions
(Table 3; Barth et al., 2000; Emmons et al., 2010). These reactions simplify the DMS oxidation chemistry by treating only gas-phase reactions and producing SO
The three DMS oxidation reactions in the standard CAM6-chem.
A schematic summary of our expanded atmospheric chemistry of DMS
oxidation in CAM6-chem (Tables 4, 5, 6, and 7). Key relatively long-lived species (DMS, MSA, HPMTF, SO
To improve the representation of DMS oxidation in CAM6-chem, we add a suite
of new reactions that describe the chemical evolution from DMS to SO
The H-abstraction reactions of DMS with OH or Cl generate MSP, which then
either reacts with NO or RO
Summary of the MSA-producing branch of the H-abstraction pathway in the DMS chemistry implemented into CAM6-chem.
Summary of the isomerization branch of the H-abstraction pathway in the DMS chemistry implemented into CAM6-chem.
Table 6 summarizes the new gas-phase reactions in the OH-addition pathway of DMS oxidation. We update the gas-phase reactions in the model to consider the oxidation of DMS by not only OH and NO
Gas-phase DMS oxidation (OH-addition pathway) implemented into CAM6-chem in this study.
We also introduce new aqueous-phase reactions of the OH-addition pathway, as shown in Table 7.
Aqueous-phase DMS oxidation (OH-addition pathway) implemented into CAM6-chem in this study.
Following a similar treatment employed by the Community Multiscale Air
Quality (CMAQ) model version 5.1 (Fahey et al., 2017), we calculate, for each species involved in the new aqueous-phase reactions, the phase transfer equations for gas-aqueous partitioning, as follows:
Summary of parameters of DMS and its oxidation intermediates used in this study.
The global burden of DMS in [MOD_2000] is 50 Gg S. It is 38 % lower than the standard run, [STD_2000], but remains within the range of 9.6–140 Gg S from other studies (Faloona, 2009; Kloster et al., 2006).
Figure 2 shows that the reduction is mainly over the Southern Ocean and is attributable to faster chemical losses via DMS
Spatial distribution of the annual mean column concentration (micrograms of sulfur per meter squared; hereafter
Spatial distribution of the fractional DMS oxidation (percent) from
[MOD_2000] through DMS
Globally, chemical loss is the largest sink of DMS (
Oxidation by BrO is responsible for 11 % of the global DMS removal, which falls midway within the previously estimated range of 8 %–29 % (Boucher et al., 2003; Khan et al., 2016; Chen et al., 2018). Regionally, its importance can be up to 50 %–60 % over the high latitudes in the Southern Hemisphere, which is close to a previous box model experiment (Hoffmann et al., 2016).
DMS
Lastly, the Cl oxidation reactions via either the addition or abstraction channels contribute equally (0.3 % each, globally) to the chemical removal of DMS, which is consistent with the proposal of Atkinson et al. (2004). Our estimated values are much lower than the 4 % found in a global model study (Chen et al., 2018) and the 8 %–18 % from box model studies (von Glasow and Crutzen, 2004; Hoffmann et al., 2016).
We note that recent studies (e.g., X. Wang et al., 2021) have shown that large discrepancies in Cl and BrO are found within the same models and/or sets of measurements. Further investigation of how uncertainties in the representation of the halogen cycle feed back onto DMS chemistry is, hence, warranted.
The global atmospheric sulfur burden is increased by 41 Gg S (or 4.1 %)
from [STD_2000] to [MOD_2000] (Fig. 4 and Table S1). Approximately half (23 Gg S) of this increment is associated with the recovery of the missing sulfur associated with the OH-addition reaction in the standard chemistry (the second reaction in Table 3), which does not conserve sulfur. The remaining total sulfur burden increase is attributable to the extended
chemistry scheme. As discussed above, the DMS burden in [MOD_2000] is lower than [STD_2000] by 38 % due to faster oxidation. This oxidation produces intermediates with a wide range of lifetimes. The addition of intermediates with relatively long physical lifetimes (to dry and wet deposition only) of HPMTF (1300 d) and MSA (8.5 d) delays the formation of SO
Spatial distribution of annual mean column concentrations
(
The PD global annual mean burden for sulfate aerosol is 582 Gg S in [MOD_2000], with an interannual variability of 46 Gg S (standard deviation of annual means). It is comparable to the 580 Gg S in a previous CAM6-chem study (Tilmes et al., 2019) and is within the estimates (420–660 Gg S) from studies using other models (e.g., Heald et al., 2014; Chen et al., 2018). The new DMS chemistry has increased the global sulfate burden by 47 Gg S (or 8.8 %) from the baseline value of 535 Gg S in [STD_2000]. The statistically significant increases in sulfate resulting from the expanded chemistry are mostly found over the tropical and subtropical oceans in the Southern Hemisphere (Fig. 5). There is no strong seasonality in the additional sulfate produced from our expanded chemistry. We estimate that the sulfate burden attributable to DMS increases by 41 % from 126 Gg S in [STD_2000] to 178 Gg S in [MOD_2000]. Most of this increase in sulfate burden (72 %) comes from the expansion of the gas-phase chemistry with a minor additional contribution from the aqueous-phase chemistry. In a sensitivity test where the isomerization branch reaction (Table 5) is removed from [MOD_2000], the global DMS-derived sulfate burden is reduced by 2.0 % (relative to [MOD_2000]).
The spatial distribution of the product branching ratios of DMS oxidation is
shown in Fig. 6. In addition to depositional removal, HPMTF converts into SO
Branching ratio (percent) of the multiphase DMS oxidation pathways
in [MOD_2000], considering HPMTF, SO
MSA is a key intermediate generated from the OH-addition channel of the
multiphase DMS oxidation, especially over the remote marine atmosphere. Our
result shows that aqueous-phase MSA formation accounts for most of the MSA
production as commonly reported (von Glasow and Crutzen, 2004; Barnes et al., 2006; Zhu et al., 2006; Hoffmann et al., 2016; Chen et al., 2018). In [MOD_2000], the global MSA burden is 7.5 Gg S, which is smaller than the range of 13–40 Gg S from previous model studies (Pham et al., 1995; Chin et al., 1996, 2000; Cosme et al., 2002; Hezel et al., 2011; Chen et al., 2018). In [MOD_2000], most MSA is formed over the Southern Ocean (Fig. S3). The lifetime of MSA is 0.6 d globally, shorter than the 5–7 d previously proposed (Chin et al., 1996, 2000; Cosme et al., 2002; Hezel et al., 2011; Pham et al., 1995), likely because we include the aqueous-phase OH oxidation to sulfate, which is a significant loss process for MSA. This oxidation accounts for
Table 9 summarizes the key observational datasets used here to compare with our PD model simulations for their wide coverage of the remote marine atmosphere. The Variability of the American Monsoon Systems (VAMOS) Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) is an international field project that took place during October and November in 2008 over the southeastern Pacific off northern Chile and southern Peru (Wood
et al., 2011). VOCAL-REx consists of both ship-based and airborne measurements for lower-atmospheric DMS (MSA
Key observational datasets used in this study.
Most DMS resides in the lower troposphere (Fig. 7). Annual mean surface DMS from [MOD_2000] ranges from 40–300 ppt (parts per trillion) over much of the ocean but can exceed 320 ppt over the Southern Ocean and northeastern Pacific, which are regions with high DMS emissions. DMS concentrations of
Horizontal distribution of annual mean surface mixing ratio and zonal mean vertical distribution of DMS (both in parts per trillion – ppt) modeled by [MOD_2000].
Figure 8 summarizes the spatial difference between the observed DMS from the VOCALS-REx and ATom missions and the simulated DMS. The model captures the peaks over the tropical Pacific and the Southern oceans off the coast of South America, but aircraft measurements detect hotspots that are not simulated by the model (Fig. 8a). During VOCALS-REx the ship-based measurements (BROWN) recorded a range of near-surface DMS from 18 to 111 ppt, while the airborne measurements (C130) reveal a vertically
decreasing trend of DMS mixing ratios, from 33 ppt at
Measured (ATom) and modeled values of HPMTF are vertically binned.
The thick lines show the medians. Error bars and gray shadings indicate that the data ranged between corresponding upper and lower quantiles. The thin red line indicates the results from a sensitivity test with
Figure 9 compares the mean vertical profile of HPMTF mixing ratios observed during ATom against the model [MOD_2000]. Over the Pacific and Atlantic regions, HPMTF mixing ratios are largest at lower altitudes and decrease to
Medians of observed (ATom) and modeled concentration of MSA aerosol are vertically binned. The thick lines show the medians. Error bars and gray shadings indicate that the data ranged between the corresponding upper and lower quantiles.
Our simulation shows that the gas-phase MSA formation is small compared to
aqueous-phase formation, in line with previous work (Barnes
et al., 2006; von Glasow and Crutzen, 2004; Zhu et al., 2006; Hoffmann et
al., 2016; Chen et al., 2018; Hoffmann et al., 2021). Near the sea surface,
simulated gas-phase MSA is
Concentrations of the sulfate aerosol simulated with both [STD_2000] and [MOD_2000] generally agree well with measurements from ATom (Fig. S6). Our model also performs well at the surface when compared against VOCAL-REx and ACE-ENA but is biased high above 1 km, likely reflecting biases in anthropogenic sulfate exported from continental regions.
As seen under PD conditions, the formation of intermediates expands the
overall lifetime of sulfur-containing species in the PI atmosphere, thereby
increasing the natural sulfate aerosol background. A summary of the burdens
and lifetimes of the sulfur-containing species from the PI simulations is
given in Table S1. The DMS burden in the PI from [MOD_1850] is 84 % larger than its PD counterpart, due to slower oxidation which prolongs the atmospheric lifetime. Oxidation by OH via the H abstraction (38 % of total DMS oxidation in [MOD_1850]) and the OH-addition channels (27 %) are still the primary loss pathways of DMS (Fig. S7). DMS
Changes to particle-phase sulfate and MSA due to the expanded DMS chemistry, as described above, may alter both aerosol–radiation and aerosol–cloud interactions. Given that particulate MSA is not included in the current CAM6-chem aerosol scheme, to account for its radiative impacts, we assume MSA interacts with radiation like sulfate optically by implementing an artificial rapid conversion of MSA to sulfate. These two adjusted cases are aliased as [MOD_RE_1850] and [MOD_RE_2000], respectively. Details of this implementation are described in the Supplement.
Following the recommendation in Ghan (2013), we focus our analyses on the shortwave (SW) DRE. The PD global annual mean sulfate DRE modeled with [MOD_RE_2000] and [STD_2000] are
Contrasting the zonal means of
The rise in the sulfate burden from PI to PD, driven by anthropogenic emissions, occurs mainly over the land in the Northern Hemisphere; this impact is much larger than the increase in sulfate produced by the expanded DMS chemistry (Fig. 11). The zonal extrema of the PD sulfate burden, aerosol optical depth (AOD), and DRE are co-located around 30
The direct radiative forcing (DRF) is estimated by differencing the DRE
estimated with anthropogenic emissions at 1850 and 2000 levels. The DRF of
[MOD_RE] and [STD] attributed to sulfate and MSA aerosols are
calculated as
While the central estimate of the IRF of aerosols from the AR5, which
reflects constraints from selected satellite and general circulation model (GCM) analyses, is
Contrasting the zonal means of changes in
The global mean
The strengthening of the IRF is opposite in sign to the expected response to the increase in PI sulfate. Carslaw et al. (2013) describe how cloud albedo is much more strongly sensitive to CCN in the PI, suggesting that higher PI aerosol burden may decrease the cloud response to anthropogenic increases. Figure 13 illustrates the change in SW CRE and sulfate burden from multiple simulations from the PI to the PD eras. Counter to expectations, we find that the SW CRE is more sensitive to each unit increment of sulfate burden (steeper slope) when the PI aerosol burden is higher (in [MOD_RE]; shown in dark red) compared to our baseline simulations ([STD]; shown in blue). Figure S10 shows that the PI-to-PD changes in CCN and cloud properties are also more sensitive to the change in sulfate burden in [MOD_RE] than [STD]. Hence, each unit PI-to-PD increase in sulfate burden in [MOD_RE] appears to induce more numerous and smaller cloud droplets and enrich cloud water content, thus enhancing cloud albedo. This could contribute to the enlarged change in SW CRE (or the IRF) in [MOD_RE], even though its PI-to-PD sulfate burden increment is smaller than [STD].
PI-to-PD changes in the SW CRE and sulfate burden of simulations in
this study. [STD] (blue) refers to the simulation with model default
chemistry. [MOD_RE] (dark red) denote the cases with our expanded DMS chemistry implemented with all gas-phase, aerosol-phase, and in-cloud reactions. [GAS_RE] (yellow), which only includes the expanded gas-phase reactions, is also shown. Arrows indicate changes from PI (tails) to PD (heads). Horizontal and vertical error bars span the 1
This sensitivity may also be related to the spatial distribution in the
change in DMS-derived sulfate burden. Figure 12 shows that the increase
in sulfate burden from PI to PD is stronger in the marine atmosphere, as
expected. In a sensitivity test, [GAS_RE], where only the
gas-phase reactions of the new DMS oxidation scheme are enabled but not the
aqueous-phase reactions (with the exception of aqueous-phase oxidation of
SO
We expand the chemical mechanism in CAM6-chem to better describe DMS oxidation in the atmosphere, determine the formation of the intermediate sulfur products, and estimate the aerosol radiative implications under the PI and PD periods.
Uncertainty in our estimate of sulfate response to the new DMS chemistry is
largely associated with estimated reaction rates. Some rate constants for
the multiphase reactions are obtained from a limited set of box model and
laboratory studies which have not been validated with field measurements.
For example, our rate constant for MS
This study included a relatively new chemical mechanism for the formation
and loss of HPMTF. The rate of isomerization of MSP (
In this study, we dramatically expand the DMS oxidation mechanism within an
Earth system model. Doing so increases the global sulfate burden by 8.8 %
in PD and 29 % in PI. While we anticipated that a larger PI burden of
sulfate would dampen the aerosol IRF, our simulations instead suggested that
the role of aqueous-phase chemistry, though modest in terms of the sulfate
burden, confounds this effect. In a simulation with only updated gas-phase
chemistry, the higher PI burden decreases the magnitude of the IRF, as
anticipated (
The modified codes of CESM2 developed in this study will be made available upon request to the corresponding authors (Ka Ming Fung at kamingfung@mit.edu and Colette L. Heald at heald@mit.edu).
Our simulation results are reproducible using the setup described. ACE-ENA data were obtained from the Atmospheric Radiation Measurement (ARM) User Facility, a U.S. DOE Office of Science user facility managed by the Biological and Environmental Research Program (
The supplement related to this article is available online at:
KMF, CLH, and JHK formulated the overarching research goals and aims. KMF and CLH designed the methodology. KMF implemented the new code into CAM6-chem, based on the standard model developed by SW, DSJ, AG, ZL, XL, and RAZ. KMF validated the model results against observational data provided by ECA, DRB, JLJ, PCJ, PRV, TSB, JES, and MZ. KMF analyzed the data and created the figures. KMF and CLH wrote the initial draft of this paper. All authors reviewed this paper.
The contact author has declared that neither they nor their co-authors have any competing interests.
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
We thank Rebecca Schwantes, Simone Tilmes, and Louisa Emmons, for their contribution to the modeling of this study. This material is based upon work supported by the National Center for Atmospheric Research (NCAR), which is a major facility sponsored by the National Science Foundation (NSF; grant no. 1852977). The high-performance computing was conducted on Cheyenne (
This research has been supported by the U.S. Department of Energy (DOE; grant no. DE-SC0018934); the U.S. DOE Office of Science, Office of Biological and Environmental Research (BER), and Earth and Environmental System Modeling (EESM) program as part of its Earth System Model Development (ESMD) activity; NASA (grant nos. 80NSSC18K0630, 80NSSC19K0124, and 80NSSC21K1451); Atmospheric Radiation Measurement (ARM) and the DOE's Atmospheric System Research, an Office of Science Biological and Environmental Research program.
This paper was edited by Anja Schmidt and reviewed by two anonymous referees.