The published literature debates the extent to which naturally occurring stratospheric ozone intrusions reach the surface and contribute to exceedances of the U.S. National Ambient Air Quality ...Standard (NAAQS) for ground‐level ozone (75 ppbv implemented in 2008). Analysis of ozonesondes, lidar, and surface measurements over the western U.S. from April to June 2010 show that a global high‐resolution (∼50 × 50 km2) chemistry‐climate model (GFDL AM3) captures the observed layered features and sharp ozone gradients of deep stratospheric intrusions, representing a major improvement over previous chemical transport models. Thirteen intrusions enhanced total daily maximum 8‐h average (MDA8) ozone to ∼70–86 ppbv at surface sites. With a stratospheric ozone tracer defined relative to a dynamically varying tropopause, we find that stratospheric intrusions can episodically increase surface MDA8 ozone by 20–40 ppbv (all model estimates are bias corrected), including on days when observed ozone exceeds the NAAQS threshold. These stratospheric intrusions elevated background ozone concentrations (estimated by turning off North American anthropogenic emissions in the model) to MDA8 values of 60–75 ppbv. At high‐elevation western U.S. sites, the 25th–75th percentile of the stratospheric contribution is 15–25 ppbv when observed MDA8 ozone is 60–70 ppbv, and increases to ∼17–40 ppbv for the 70–85 ppbv range. These estimates, up to 2–3 times greater than previously reported, indicate a major role for stratospheric intrusions in contributing to springtime high‐O3events over the high‐altitude western U.S., posing a challenge for staying below the ozone NAAQS threshold, particularly if a value in the 60–70 ppbv range were to be adopted.
Key Points
Stratospheric intrusions can episodically increase surface ozone by 20‐40 ppbv
These intrusion events can push ground‐level ozone over the health‐based limit
Global high‐res model, satellite and in situ observations yield process insights
Biogeochemical rate processes in the Southern Ocean have an important impact on the global environment. Here, we summarize an extensive set of published and new data that establishes the pattern of ...gross primary production and net community production over large areas of the Southern Ocean. We compare these rates with model estimates of dissolved iron that is added to surface waters by aerosols. This comparison shows that net community production, which is comparable to export production, is proportional to modeled input of soluble iron in aerosols. Our results strengthen the evidence that the addition of aerosol iron fertilizes export production in the Southern Ocean. The data also show that aerosol iron input particularly enhances gross primary production over the large area of the Southern Ocean downwind of dry continental areas.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Secondary organic aerosols (SOA) constitute a significant fraction of ambient aerosols, but their global source is only beginning to be understood. Substantial evidence has shown that oxidation of ...water‐soluble organic species in the liquid cloud leads to the formation of SOA. To evaluate this global source and explore its sensitivity to various assumptions concerning cloud properties, we simulate in‐cloud SOA (IC‐SOA) formation based on detailed multiphase chemistry incorporated into the newly developed Geophysical Fluid Dynamics Laboratory (GFDL) coupled chemistry‐climate model AM3. We find global IC‐SOA production is around 20–30 Tg·yr−1between 1999 and 2001. Depending on season and location, oxalic acid accounts for 40–90% of the total IC‐SOA source (particularly between 800 hPa–400 hPa), and glyoxylic acid and oligomers (formed by glyoxal and methylglyoxal in evaporating clouds) each contribute an additional 10–20%. Besides glyoxal and methylglyoxal (extensively studied by previous research), glycolaldehyde and acetic acid are among the most important precursors leading to the formation of IC‐SOA, particularly oxalic acid. Different implementations of cloud fraction or cloud lifetime in global climate models could potentially modify estimates of IC‐SOA mass production by 20–30%. Dense IC‐SOA production occurs in the tropical and midlatitude regions of the lower troposphere (surface to 500 hPa). In DJF, IC‐SOA production is concentrated over the western Amazon and southern Africa. In JJA, substantial IC‐SOA production occurs over southern China and boreal forest regions. This study confirms a significant in‐cloud source of SOA, which will directly and indirectly influence global radiation balance and regional climate.
Key Points
IC‐SOA is an important source of organic aerosols
Glycolaldehyde and acetic acid are important IC‐SOA precursors
IC‐SOA Prod. is sensitive to the implementations of cloud fraction and lifetime
Climate models incorporate a number of adjustable parameters in their cloud formulations. They arise from uncertainties in cloud processes. These parameters are tuned to achieve a desired radiation ...balance and to best reproduce the observed climate. A given radiation balance can be achieved by multiple combinations of parameters. We investigate the impact of cloud tuning in the CMIP5 GFDL CM3 coupled climate model by constructing two alternate configurations. They achieve the desired radiation balance using different, but plausible, combinations of parameters. The present‐day climate is nearly indistinguishable among all configurations. However, the magnitude of the aerosol indirect effects differs by as much as 1.2 Wm − 2, resulting in significantly different temperature evolution over the 20th century.
Key Points
We construct alternate model configurations by modifying cloud parametersThe present‐day climate is nearly indistinguishable between all configurationsThe warming over the 20th century is substantially impacted
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
This study evaluates the sensitivity of long‐range transport of black carbon (BC) from midlatitude and high‐latitude source regions to the Arctic to aging, dry deposition, and wet removal processes ...using the Geophysical Fluid Dynamics Laboratory (GFDL) coupled chemistry and climate model (AM3). We derive a simple parameterization for BC aging (i.e., coating with soluble materials) which allows the rate of aging to vary diurnally and seasonally. Slow aging during winter permits BC to remain largely hydrophobic throughout transport from midlatitude source regions to the Arctic. In addition, we apply surface‐dependent dry deposition velocities and reduce the wet removal efficiency of BC in ice clouds. The inclusion of the above parameterizations significantly improves simulated magnitude, seasonal cycle, and vertical profile of BC over the Arctic compared with those in the base model configuration. In particular, wintertime concentrations of BC in the Arctic are increased by a factor of 100 throughout the tropospheric column. On the basis of sensitivity tests involving each process, we find that the transport of BC to the Arctic is a synergistic process. A comprehensive understanding of microphysics and chemistry related to aging, dry and wet removal processes is thus essential to the simulation of BC concentrations over the Arctic.
Atmospheric deposition of mineral dust supplies much of the essential nutrient iron to the ocean. Presumably only the readily soluble fraction is available for biological uptake. Previous ocean ...models assumed this fraction was constant. Here the variable solubility of Fe in aerosols and precipitation is parameterized with a two‐step mechanism, the development of a sulfate coating followed by the dissolution of iron (hydr)oxide on the dust aerosols. The predicted soluble Fe fraction increases with transport time from the source region and with the corresponding decrease in dust concentration. The soluble fraction is ∼1 percent near sources, but often 10–40 percent farther away producing a significant increase in soluble Fe deposition in remote ocean regions. Our results may require more rapid biological and physicochemical scavenging of Fe than used in current ocean models. We further suggest that increasing SO2 emission alone could have caused significant Fe fertilization in the modern northern hemisphere oceans.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The authors describe carbon system formulation and simulation characteristics of two new global coupled carbon–climate Earth System Models (ESM), ESM2M and ESM2G. These models demonstrate good ...climate fidelity as described in part I of this study while incorporating explicit and consistent carbon dynamics. The two models differ almost exclusively in the physical ocean component; ESM2M uses the Modular Ocean Model version 4.1 with vertical pressure layers, whereas ESM2G uses generalized ocean layer dynamics with a bulk mixed layer and interior isopycnal layers. On land, both ESMs include a revised land model to simulate competitive vegetation distributions and functioning, including carbon cycling among vegetation, soil, and atmosphere. In the ocean, both models include new biogeochemical algorithms including phytoplankton functional group dynamics with flexible stoichiometry. Preindustrial simulations are spun up to give stable, realistic carbon cycle means and variability. Significant differences in simulation characteristics of these two models are described. Because of differences in oceanic ventilation rates, ESM2M has a stronger biological carbon pump but weaker northward implied atmospheric CO₂ transport than ESM2G. The major advantages of ESM2G over ESM2M are improved representation of surface chlorophyll in the Atlantic and Indian Oceans and thermocline nutrients and oxygen in the North Pacific. Improved tree mortality parameters in ESM2G produced more realistic carbon accumulation in vegetation pools. The major advantages of ESM2M over ESM2G are reduced nutrient and oxygen biases in the southern and tropical oceans.
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BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Using the Geophysical Fluid Dynamics Laboratory's (GFDL's) fully coupled chemistry‐climate (ocean/atmosphere/land/sea ice) model (CM3) with an explicit physical representation of aerosol indirect ...effects (cloud‐water droplet activation), we find that the dramatic emission reductions (35%–80%) in anthropogenic aerosols and their precursors projected by Representative Concentration Pathway (RCP) 4.5 result in ~1 °C of additional warming and ~0.1 mm day−1 of additional precipitation, both globally averaged, by the end of the 21st century. The impact of these reductions in aerosol emissions on simulated global mean surface temperature and precipitation becomes apparent by mid‐21st century. Furthermore, we find that the aerosol emission reductions cause precipitation to increase in East and South Asia by ~1.0 mm day−1 through the second half of the 21st century. Both the temperature and the precipitation responses simulated by CM3 are significantly stronger than the responses previously simulated by our earlier climate model (CM2.1) that only considered direct radiative forcing by aerosols. We conclude that the indirect effects of sulfate aerosol greatly enhance the impacts of aerosols on surface temperature in CM3; both direct and indirect effects from sulfate aerosols dominate the strong precipitation response, possibly with a small contribution from carbonaceous aerosols. Just as we found with the previous GFDL model, CM3 produces surface warming patterns that are uncorrelated with the spatial distribution of 21st century changes in aerosol loading. However, the largest precipitation increases in CM3 are colocated with the region of greatest aerosol decrease, in and downwind of Asia.
Key Points
Aerosol reductions (RCP4.5) cause 1K warming and +0.1 mm/day of precipitation.
Sulfate indirect effects greatly enhance aerosol impacts on surface temperature.
Aerosol reductions increase precipitation in Asia by 0.5–1.0 mm/day by 2100.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
This study examines the impact of projected changes (A1B “marker” scenario) in emissions of four short‐lived air pollutants (ozone, black carbon, organic carbon, and sulfate) on future climate. ...Through year 2030, simulated climate is only weakly dependent on the projected levels of short‐lived air pollutants, primarily the result of a near cancellation of their global net radiative forcing. However, by year 2100, the projected decrease in sulfate aerosol (driven by a 65% reduction in global sulfur dioxide emissions) and the projected increase in black carbon aerosol (driven by a 100% increase in its global emissions) contribute a significant portion of the simulated A1B surface air warming relative to the year 2000: 0.2°C (Southern Hemisphere), 0.4°C globally, 0.6°C (Northern Hemisphere), 1.5–3°C (wintertime Arctic), and 1.5–2°C (∼40% of the total) in the summertime United States. These projected changes are also responsible for a significant decrease in central United States late summer root zone soil water and precipitation. By year 2100, changes in short‐lived air pollutants produce a global average increase in radiative forcing of ∼1 W/m2; over east Asia it exceeds 5 W/m2. However, the resulting regional patterns of surface temperature warming do not follow the regional patterns of changes in short‐lived species emissions, tropospheric loadings, or radiative forcing (global pattern correlation coefficient of −0.172). Rather, the regional patterns of warming from short‐lived species are similar to the patterns for well‐mixed greenhouse gases (global pattern correlation coefficient of 0.8) with the strongest warming occurring over the summer continental United States, Mediterranean Sea, and southern Europe and over the winter Arctic.
Global distribution of carbon monoxide Holloway, Tracey; Levy, Hiram; Kasibhatla, Prasad
Journal of Geophysical Research, Washington, DC,
27 May 2000, Volume:
105, Issue:
D10
Journal Article
Peer reviewed
Open access
This study explores the evolution and distribution of carbon monoxide (CO) using the National Oceanic and Atmospheric Administration (NOAA) Geophysical Fluid Dynamics Laboratory three‐dimensional ...global chemical transport model (GFDL GCTM). The work aims to gain an improved understanding of the global carbon monoxide budget, specifically focusing on the contribution of each of the four source terms to the seasonal variability of CO. The sum of all CO sources in the model is 2.5 Pg CO/yr (1 Pg = 103 Tg), including fossil fuel use (300 Tg CO/yr), biomass burning (748 Tg CO/yr), oxidation of biogenic hydrocarbons (683 Tg CO/yr), and methane oxidation (760 Tg CO/yr). The main sink for CO is destruction by the hydroxyl radical, and we assume a hydroxyl distribution based on three‐dimensional monthly varying fields given by Spivakovsky et al. 1990, but we increase this field by 15% uniformly to agree with a methyl chloroform lifetime of 4.8 years Prinn et al, 1995. Our simulation produces a carbon monoxide field that agrees well with available measurements from the NOAA/Climate Monitoring and Diagnostics Laboratory global cooperative flask sampling network and from the Jungfraujoch observing station of the Swiss Federal Laboratories for Materials Testing and Research (EMPA) (93% of seasonal‐average data points agree within ±25%) and flight data from measurement campaigns of the NASA Global Tropospheric Experiment (79% of regional‐average data points agree within ±25%). For all 34 ground‐based measurement sites we have calculated the percentage contribution of each CO source term to the total model‐simulated distribution and examined how these contributions vary seasonally due to transport, changes in OH concentration, and seasonality of emission sources. CO from all four sources contributes to the total magnitude of CO in all regions. Seasonality, however, is usually governed by the transport and destruction by OH of CO emitted by fossil fuel and/or biomass burning. The sensitivity to the hydroxyl field varies spatially, with a 30% increase in OH yielding decreases in CO ranging from 4–23%, with lower sensitivities near emission regions where advection acts as a strong local sink. The lifetime of CO varies from 10 days over summer continental regions to well over a year at the winter poles, where we define lifetime as the turnover time in the troposphere due to reaction with OH.
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