HIPPO Takes to the Skies: Measuring Earth's Atmospheric Constituents
by Rachel Hauser
With efforts to mandate mitigation and management of greenhouse gas and black carbon emissions looming, estimating fluxes and transports of atmospheric constituents becomes critical. Effects of aerosols, and carbon dioxide, water vapor, ozone, and methane and other gases, vary across the global atmosphere in terms of distribution, emission rates, and residence time. Beyond meeting possible emissions-reduction mandates, establishing a better approximation of the global atmosphere’s chemical distribution will improve understanding of many land-ocean-atmosphere interactions, feeding both basic scientific understanding and providing a vital source of data useful for informing policy. The 5-phase HIPPO (HIAPER Pole-to-Pole Observation) project provides the first detailed mapping of the globe’s vertical and horizontal distribution of greenhouse gases, black carbon, and related species.
HIPPO, designed to collect detailed, high-accuracy measurements of atmospheric constituents, launched its proof of concept phase in partnership with the START-08 mission in spring 2008 (note to John: link to Start-08 2008 NAR story – I think this is in 2009 NAR). The first series of global HIPPO flights ran in January 2009. The scheduled flight paths took the NSF/NCAR Gulfstream V (GV), HIPPO researchers and the variety of equipment responsible for measuring more than 150 gases and atmospheric constituents from pole to pole across the Pacific Ocean. Among the places visited on that first trip: Anchorage, Alaska, Christchurch, New Zealand, the Southern Ocean, Tahiti, Easter Island, and Central America. Since the initial campaign, two additional series of flights have occurred – the first in October to November 2009, the second in late March and April 2010 – with two more campaigns in the offing (May-June 2011 and August- September 2011). The precision instrumentation allows HIPPO scientists to take atmospheric measurements from the top of the troposphere to the Earth's surface.
“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.
Two things in particular stand out, Wofsy continues. One is that the distribution of black carbon and other dark-colored aerosols is more widespread and uniform than anticipated, with more than expected accumulation occurring at high latitudes. Additionally, nitrous oxide (N2O) concentrations are higher in the mid- and upper tropical troposphere than on the surface, which, lacking instrumentation and measuring capabilities prior to HIPPO, scientists could not have known pre-HIPPO. Readings from project instrumentation indicate that the biggest N2O sources are over land, but reasons for the N2O profile inversion currently elude understanding. What is known is that those models currently doing a good job of replicating near-surface-level atmospheric dynamics do not reproduce these phenomena. Model improvement is among the major HIPPO drivers; leveraging HIPPO observations will assist in model refinement. However, in some cases, observations alone cannot solve real-world replication processes, only model improvement will help.
The major model challenge from the perspective of carbon dioxide, says Britton Stephens, an NCAR atmospheric scientist and HIPPO Mission co-lead, 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 large 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.
Increases 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 swathes of atmosphere, from the ground well into the atmosphere. HIPPO profiles extend through the troposphere, expanding existing observational data sets – and knowledge – beyond that allowed by current ground-based capabilities. With these data, researchers will be able to test the accuracy of existing atmospheric models to better discriminate among them to identify those that best represent observed processes. Moreover, these data will help with design of new models and data-assimilation systems able to take full advantage of such observations. In turn, improved models will push forward human understanding of the processes responsible for uptake of human-emitted CO2, during and between field campaigns – and beyond.
Sidebar: The big exhale
With three flights completed, Britton 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 the Northern Hemisphere’s fall and winter, as the GV and its crew and payload flew between the high Arctic to the equator, significant changes in CO2 distribution and concentrations were detected on each mission.
“By lining the same slice of atmosphere up in seasonal order over the course of the three flights, it’s possible to see build-up of carbon dioxide concentrations in the atmosphere over fall, winter and spring,” says Stephens. “A plume of CO2 grows in the Northern Hemisphere as photosynthesis slows while fossil-fuel CO2 emissions continue.”
The last two HIPPO missions will be run in early and late summer, at which point Stephens expects to see a massive inhalation of CO2 over the Northern Hemisphere beginning in May and June as plants and trees leaf out again, with carbon drawdown peaking in August and September.