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Once international agreements demand it, effective, enforceable greenhouse gas reduction will require in-depth information on the fluxes and transports of these and other atmospheric constituents. Concentrations of aerosols like black carbon, and gases like carbon dioxide (CO2), water vapor, ozone, and nitrous oxide (N2O) vary across the globe and by season. Until recently, a fine-grained picture of the concentrations and understanding of the dynamics of these atmospheric components did not exist. The 5-phase HIPPO (HIAPER Pole-to-Pole Observation) project provides this perspective, having generated the first detailed mapping – both vertically and across latitudes – of the global distribution of greenhouse gases, black carbon, and related chemical species in the atmosphere.
“With HIPPO, we now have whole slices of the global atmosphere that, in many cases, appear differently than we expected,” says Steven Wofsy, HIPPO principal investigator and atmospheric scientist at Harvard University.
Scientists expect that this detailed view will allow them to more realistically approximate the global atmosphere’s chemical distribution and improve understanding of many land-ocean-atmosphere interactions. In addition to feeding basic scientific understanding, HIPPO will provide a vital source of data useful for informing policy related to climate and climate change. Carbon dioxide levels, sources (areas where more carbon is released to the atmosphere than is taken up), and sinks (where carbon uptake is greater than release) are a significant focus for HIPPO scientists.
“In tracking carbon dioxide exchange, we’re particularly interested in the tropical forests, the northern forests, and the ocean around Antarctica,” says Britton Stephens, an atmospheric scientist at the National Center for Atmospheric Research (NCAR) and HIPPO co-investigator. “HIPPO provides such a broad perspective, giving us an opportunity to see the different regional influences on carbon dioxide distributions around much of the globe.”
HIPPO, supported by the National Science Foundation, the National Oceanic and Atmospheric Administration (NOAA), NASA, and a number of universities, collects detailed, high-accuracy measurements of atmospheric constituents. Launching its proof of concept in spring 2008, the first series of global flights began in January 2009, with subsequent flights occurring twice in 2010, with two more departing in 2011 (June and August).
The NSF/NCAR Gulfstream V (GV) flew researchers and precision instruments measuring more than 150 gases and atmospheric constituents, from nearly pole to pole across the Pacific Ocean, flying at altitudes varying between 1,000 and 45,000 feet above sea level, depending on the daily project objective. The first campaign, typical of the ones to follow, began in Boulder, Colorado, explored air over the Arctic, headed next to Christchurch, New Zealand, before flying over the Southern Ocean, with subsequent layovers in Tahiti, Easter Island, and Central America.
The big exhale: Carbon Dioxide
With three of the five missions complete, Stephens brings attention to what he calls the Northern Hemisphere’s “exhale.” HIPPO experimental design called for seasonal data collection to get a complete, year-round perspective on global atmospheric processes. In the first three missions, which occurred during Northern Hemisphere fall, winter, and early spring, the scientists noted significant changes in CO2 distribution and concentrations on each mission.
“By lining up the same slice of atmosphere in seasonal order over the course of the three missions, it’s possible to see build-up of carbon dioxide concentrations in the atmosphere over fall, winter and spring,” says Stephens. “A giant pool of CO2 grows in the Northern Hemisphere as photosynthesis slows, and fossil-fuel CO2 emissions and plant and soil respiration continue.”
Notably, in the most northerly regions of the Arctic, the researchers found rapid filling of the atmosphere with CO2 at high altitudes during winter and spring, likely moved by the warm conveyor belt (see callout box), which challenges existing perceptions of atmospheric processes. The last two HIPPO missions should help provide a clearer view on the all-season, big picture perspective on carbon dioxide dynamics. These missions will occur in June and September, when Northern Hemisphere CO2 concentrations will be at their lowest as vegetation growth and photosynthetic processes peak. Throughout this period, Stephens expects to see a massive inhalation of CO2 across the Northern Hemisphere.
Measuring CO2 at the variety of altitudes and latitudes gives scientists much tighter constraints – and therefore greater understanding – on the total amount of CO2 release (or uptake) for the hemisphere. Older estimates of hemispheric exchange, which relied on information collected at the surface, turn out to be off by about 30 percent, says Stephens. “Looking up through the boundary layer using imperfect atmospheric transport models has been like staring through foggy swim goggles – finally, HIPPO is giving us a clear view.”
A well-known atmospheric phenomenon, the “warm conveyor belt,” develops with the generation of storms in the North Pacific. These storms transport air – suspended in these air packets are pollutants such as black carbon – northward. HIPPO researchers could clearly identify this conveyor belt, which was marked by a filament of black carbon that began in a polluted region near the Earth’s surface (see the figure) around 40-degrees latitude North and was lofted in altitude until reaching heights of 6 km at 60 degrees N.
Other important atmospheric components: Black carbon and nitrous oxide
Other measurements are generating excitement from the three completed campaigns, notes Wofsy. HIPPO observations show a more widespread, uniform distribution of black carbon than anticipated, with greater than expected abundances occurring at high latitudes in the Northern Hemisphere. Additionally, concentrations of nitrous oxide (N2O), the third most important long-lived anthropogenic greenhouse gas (the other two being CO2 and methane), are higher than expected in the mid- and upper tropical troposphere than on the surface; lacking instrumentation and measuring capabilities prior to HIPPO, scientists could not have known this. Details on some of the unexpected – and unpredictable – findings related to these atmospheric components are outlined below.
Black carbon affects climate, doing so both directly – by absorbing solar radiation – and indirectly – by acting as condensation nuclei, which form clouds that will either reflect or absorb radiation, depending on cloud characteristics and location in the atmosphere. Black carbon deposited on snow or ice also enhances melt leading the Earth’s surface to absorb more sunlight. These dark aerosols have a variety of sources, coming from diesel fuel or coal combustion, burning of plants (e.g., forest fires), and various industrial processes.
Most black carbon remains in the atmosphere for only days to weeks but can still have a dramatic impact on global warming. HIPPO’s pole-to-pole measurements of black carbon may assist policy makers in developing strategies for reducing its climate change impact. Among other things, the HIPPO measurements have provided new knowledge on the life cycle of a black carbon particle as it travels from source (emission) to sink (removal) in the atmosphere. Used together with global aerosol models, HIPPO’s pole-to-pole measurements of black carbon captured in different seasons can be used to refine our knowledge of how black carbon aerosols affect climate, says Ryan Spackman, an atmospheric chemist in NOAA’s Earth System Research Laboratory.
Prior to HIPPO, a limited number of airborne measurements of black carbon were conducted. Of the studies available, all lack HIPPO’s combination of vertical and latitudinal detail. Since global aerosol models vary widely in projected black carbon concentrations, HIPPO data will prove invaluable for many aspects of climate research. Because most black carbon emissions occur at the surface, typically the amount of black carbon in the atmosphere decreases with altitude. In the Southern Hemisphere, which has fewer pollution sources than the Northern Hemisphere, however, this is not the case.
“In our first flights near the southern Pole, we saw the amount of black carbon in the atmosphere increasing with altitude,” says Joshua Schwarz, a physicist working in NOAA’s Earth System Research Laboratory. "This indicates that the black carbon was transported to the region from far away, with rain-out occurring at lower altitudes. This conclusion offers insights on the interplay of transport and removal mechanisms that can help in validation of global model results."
HIPPO covers a wide range of latitudes over a short time, reducing the likelihood that the scientists would miss transport of black carbon across the Pacific. This perspective helped them unravel the nuances of transport dynamics from removal processes, which strengthened the impact of their results.
In the first HIPPO mission, which occurred during Northern Hemisphere winter, the black carbon team analyzed pole-to-pole distributions of black carbon mass loadings, in the process learning that global aerosol models often overestimate black carbon in the atmosphere. “For black carbon, these observations have helped us to more easily separate the impacts of errors in modeling removal and errors in modeling transport and emissions,” explains Schwarz.
In the second and third HIPPO missions, which occurred in Northern Hemisphere fall and spring, the scientists observed large-scale black carbon pollution events associated with the intercontinental transport of vast amounts of pollution from Asia. Investigators observed elevated pollution at almost all altitudes in the Arctic, but especially at higher altitudes, where one might expect the air to be relatively clear and clean. The scientists discovered that pollutants can be easily transported to the Arctic as thin sheets of air in almost any season.
Another surprise waiting for the scientists was the seasonality of the plumes of black carbon-laden pollution at mid-latitudes (i.e., between Hawaii and Alaska). During springtime, the scientists identified pollution contributions from two predominant sources – human-made pollution from Asia and biomass burning from Southeast Asia.
“The black carbon mass loadings in pollution plumes in the remote Pacific were comparable with what we have observed in large American cities,” says Spackman. “Even more surprising, we discovered that this pollution extended over the entire depth of the troposphere – from near the surface of the ocean to 28,000-feet altitude.”
On each HIPPO flight the scientists frequently saw higher levels of N2O at altitude than occurred at the surface. Not only is N2O a powerful greenhouse gas, it may be the most important stratospheric ozone-depleting substance in the atmosphere. Consequently, more than simply being scientifically intriguing, a better understanding of where it is found and in what concentrations is important information for both scientists and climate-policy and other decision makers.
Primary N2O emissions come from soils and the ocean, however a large human-generated component originates as a result of fertilizer use for agriculture. A relatively new source, these anthropogenic emissions have increased since the mid 1800s – from 260 parts per billion (ppb) to 320 ppb, says Eric Kort, who recently completed his PhD at Harvard, where he worked with Wofsy. While not the only driver of the N2O-related research on HIPPO, the rapid rise in human-generated N2O concentrations in the atmosphere adds urgency to the N2O investigation.
To the surprise of HIPPO investigators, they often found elevated concentrations of N2O high in the atmosphere – even over areas where ground-based monitors did not indicate presence of the gas at the surface. The higher-than-expected levels of N2O at altitude indicate more dynamics at work than previously appreciated, explains Kort. Some analysis shows that large-scale convective activity (i.e., storms) and a lot of rainfall, which might result in increased microbial activity, might have a hand in achieving this reality. Convection wafts N2O up into the atmosphere, where the wind catches it, pushing the gas further upward and mixing it at higher altitudes.
“Lots of N2O is lofted from tropical regions,” says Kort. “HIPPO sensors show increased emissions in the tropics, but we don’t know if this occurs naturally, coming from tropical soil sources, or if other processes or perturbations, such as increased use of fertilizers upwind from the forests, causes this.”
Again, lacking direct observations, models of these dynamics historically have played a large role in gaining better predictions of likely N2O behavior. While some models accurately anticipated near-surface N2O abundances, none predicted the persistent elevated levels seen at altitude in the tropics.
Achieving better modeling results will be particularly important in the case of atmospheric N2O, which has increased year after year at a rate approaching 1 part per billion. As society moves toward using and producing biofuels, use of fertilizers will likely increase, which in turn will amplify N2O emissions. At some point, N2O could offset benefits from CO2 reduction. Because of this, and because of its importance as a greenhouse gas, scientists and policy makers want to have a well-honed awareness on the transport, fluxes, and removal processes affecting N2O.
“Nitrous oxide emissions are certainly something we need to be concerned about in terms of future international regulatory treaties because such non-CO2 emissions will be important. Currently, our knowledge of these emissions is far more limited than is the case for CO2,” says Kort.
Improving global models
Matching up observed and modeled N2O data to better predict behavior of the atmospheric constituents is a significant HIPPO raison d’être. The complexity, time and expense of missions like HIPPO make modeling an important way to extend use of the HIPPO data, and develop models that better replicate observed atmospheric characteristics.
Alone, neither observations nor models can fully resolve real-world processes. But improved observations that then feed into models can provide revealing new insights on climate dynamics. The major model challenge from the perspective of CO2, says Stephens, is representations of atmospheric mixing. Often the models used have grid structures that are coarser than the fine-scale processes responsible for mixing.
“So, if mixing happens due to convective cells or transport up and over a cold air mass, for example, the transport models used to track CO2 in the atmosphere do not represent these dynamics well,” Stephens explains.
Increase in model resolution may improve these issues somewhat, however it does not get around the need for robust observations that capture the characteristics of broad swaths of atmosphere, from the ground to high altitudes. HIPPO profiles extend through the troposphere, expanding existing observational data sets – and knowledge – beyond that allowed by current ground-based capabilities.
Using HIPPO data, researchers will be able to test the accuracy of existing atmospheric models to better identify those that most accurately represent observed processes. Moreover, these observations will also aid the design of more innovative models and data-assimilation systems – models and systems able to take full advantage of HIPPO observations. Such improvements will push forward understanding of the processes responsible for uptake of human-emitted CO2 during and between field campaigns – and beyond.