A HIPPO Takes to the Skies to Taste Earth's Atmosphere

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.

Researchers know that concentrations of aerosols like black carbon and gases like carbon dioxide, water vapor, ozone and nitrous oxide 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.

Researchers across the globe launched the five-phase HIPPO (HIAPER Pole-to-Pole Observation) project to provide this perspective; it has 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," said Steven Wofsy, HIPPO principal investigator and atmospheric scientist at Harvard University.

What HIPPO will tell us

Scientists expect that this detailed view will allow them to more realistically approximate the global atmosphere's chemical distribution and improve understanding of how the land, ocean and atmosphere interact. 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," said 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 (NSF), the National Oceanic and Atmospheric Administration (NOAA), NASA and a number of universities, collects detailed, high-accuracy measurements of atmospheric constituents. A proof of concept was launched in spring 2008, and the first series of global flights began in January 2009, with subsequent flights occurring twice in 2010 and twice in 2011.

The plane that was used for HIPPO, a Gulfstream V, flew researchers and precision instruments measuring about 150 gases and atmospheric constituents, from nearly pole to pole across the Pacific Ocean, flying at altitudes varying between 500 and 47,000 feet above sea level, depending on the daily project objective. The first campaign--typical of the ones to follow--began in Boulder, Colo., explored the air over the Arctic; the moving lab 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 the last of the five missions recently completed, 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, occurring during Northern Hemisphere's fall, winter and early spring, the scientists noted significant changes in carbon dioxide (CO₂) distribution and concentrations.

"By lining up the same slice of atmosphere in seasonal order over the course of the first three missions, it's possible to see build-up of carbon dioxide concentrations in the atmosphere over fall, winter and spring," said Stephens. "A giant pool of CO₂ grows in the Northern Hemisphere as photosynthesis slows and as 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, which challenges existing perceptions of atmospheric processes.

The last two HIPPO missions helped provide a clearer view on the all-season, big picture perspective on CO₂ dynamics. The fourth mission occurred in June and July of 2011, and the fifth during August and September of 2011; during these periods, Northern Hemisphere CO₂ concentrations were at their lowest as vegetation growth and photosynthetic processes peaked. As expected, throughout this period, the researchers saw a massive inhalation of CO₂ across the Northern Hemisphere, as the growing plants breathed in the CO₂.

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, said 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," he said.

Other important atmospheric components: Black carbon and nitrous oxide

Other measurements are generating excitement from the three completed campaigns, Wofsy said. 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)--were often found to have elevated concentrations in the mid- and upper-tropical troposphere even over areas where no N₂O was detected at the surface; without the instrumentation and measuring capabilities of HIPPO, scientists could knot have known this. Details on some of the unexpected--and unpredictable--findings related to these atmospheric components are outlined below.

Black Carbon

Black carbon affects climate, doing so both directly (by absorbing solar radiation) and indirectly (by forming clouds that will either reflect or absorb radiation, depending on their characteristics and location). 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 plants in forest fires and various industrial processes.

Most black carbon remains in the atmosphere for only days to weeks, but it 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, said 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," said 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, 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," said Schwarz.

During the second and third HIPPO missions, which occurred in the Northern Hemisphere's 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 (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," said 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."

Nitrous Oxide

On HIPPO flights, the scientists frequently saw higher levels of N₂O at higher altitudes than at the surface. Not only is N₂O a powerful greenhouse gas, it is also an important ozone-depleting substance, whose importance will increase in the future. Consequently, the presence of N₂O at these levels is more than scientifically intriguing.  A better understanding of where N₂O is found and in what concentrations is important information to guide both scientists and decision makers.

Primary N₂O emissions come from soils and the ocean; a large human-generated component originates as a result of fertilizer use for agriculture. These anthropogenic emissions are a relatively new source, and have been increasing since the mid 1800s--from 260 parts per billion (ppb) to 320 ppb, said Eric Kort, who recently completed his doctorate with Wofsy at Harvard. While not the only driver of the N₂O-related research on HIPPO, questions about the rapid rise in human-generated N₂O concentrations in the atmosphere add urgency to the N₂O investigation.

To their surprise, the HIPPO investigators 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, explained 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.

"Lots of N₂O is lofted from tropical regions," said 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 one part per billion. As society moves toward using and producing biofuels, use of fertilizers will likely increase, which will, in turn, amplify N₂O emissions. At some point, N₂O could offset benefits from CO₂ reduction. Because of this, and because of N₂O's 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," said Kort.

Improving global models

Matching up observed and modeled N₂O data to better predict behavior of the atmospheric constituents is a significant reason HIPPO measurements were carried out. 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, said 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 said.

Increase in model resolution may improve these issues somewhat, but 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 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.

"The HIPPO program was supported by NSF grants AGS-0628575, AGS-0628519, AGS-0628388 and AGS-0628452 to Harvard University, University of California (San Diego), University Corporation for Atmospheric Research, University of Colorado/CIRES and by the NCAR. The NCAR is supported by the National Science Foundation. Participation by several instruments (SP-2, Ozone, UCATS, PANTHER, NWAS flasks), and weather forecasting, were supported by offices and programs of the National Oceanic and Atmospheric Administration: the Atmospheric Composition and Climate Programme, the Office of Oceanic and Atmospheric Research and the Environmental Research Laboratory. The AWAS flasks system was supported by NSF grants NSF AGS-0849086 and AGS-0959853 to the University of Miami. VCSEL was supported by NSF grant AGS-1036275 to Princeton University." (Phil. Trans. R. Soc. A, May 28, 2011).

-- Rachel Hauser, National Center for Atmospheric Research, rhauser@ucar.edu

This Behind the Scenes article was provided to LiveScience in partnership with the National Science Foundation.