Rockets and the Ozone Layer

Launch of space shuttle Discovery from cape Canaveral. (NASA)

Rockets and the Ozone Layer

Martin N. Ross and Paul F. Zittel

Rocket engine exhaust contains chemical compounds that react with ozone in the stratosphere. A new measurement program suggests that current space transportation activities only minimally affect Earth's protective ozone layer.

Protecting Earth's ozone layer remains an important environmental issue. Without this shielding layer, ultraviolet (UV) radiation would harm life on Earth. We hear alarming statistics on increasing incidences of skin cancer and other disorders that may be linked to a thinning of Earth's ozone layer. We know that the presence of chlorofluorocarbons (CFC)—chemicals used as solvents and refrigerants—and other industrial gases in the atmosphere is the major cause of ozone depletion. But what about exhaust from launch vehicles? Can the cumulative effect of emissions from rockets launched every three or four days from various launch sites around the globe significantly alter Earth's delicately balanced, natural sunscreen?

Space transportation, once dominated by government, has become an important part of our commercial economy, and the business of launching payloads into orbit is expected to nearly double in the next decade. Each time a rocket is launched, combustion products are emitted into the stratosphere. CFCs and other chemicals banned by international agreement are thought to have reduced the total amount of stratospheric ozone by about 4 percent. In comparison, recent predictions about the effect on the ozone layer of solid rocket motor (SRM) emissions suggest that they reduce the total amount of stratospheric ozone by only about 0.04 percent.

Even though emissions from liquid-fueled rocket engines were not included in these predictions, it is likely that rockets do not constitute a serious threat to global stratospheric ozone at the present time. Even so, further research and testing needs to be done on emissions from rockets of all sizes and fuel system combinations to more completely understand how space transportation activities are affecting the ozone layer today and to predict how they will affect it in the future.

Commercial airlines fly in the upper troposphere at an altitude of about 12 kilometers; the WB-57F aircraft typically flies at an altitude of 19 kilometers, below the peak in ozone concentration but well enough into the stratosphere that the chemical and mixing processes observed during RISO missions are representative of the ozone layer as a whole. Reactive gases that constitute a small part of the rocket exhaust consume ozone, which deactivates them. Represented by X, these are thought to be mainly the chlorine atom, nitric oxide, and the hydroxyl radical, depending on the rocket propellant. A variety of chemical processes can reactivate ozone-destroying molecules, so that each can destroy many ozone molecules before finally being removed from the stratosphere.

The Ozone Umbrella

Ozone, composed of three oxygen atoms, is the result of the action of UV radiation on oxygen molecules, composed of two oxygen atoms. In the upper regions of the atmosphere, UV light breaks apart oxygen molecules into two oxygen atoms, one of which then combines with a second oxygen molecule to form ozone. Born of UV light, ozone is also a powerful absorber of UV light, accounting for its protective role. Most of the ozone that protects Earth's surface is concentrated in the atmospheric region called the stratosphere, usually taken as the region between about 14 and 50 kilometers altitude. The term "ozone layer" refers to the portion of the stratosphere between about 15 and 30 kilometers altitude, where the bulk of the ozone is concentrated.

Compared with the mass of all the gas in the stratosphere, the mass of combustion emissions from even the largest rocket is miniscule, so it's easy to conclude that the effect of all rocket launches on the ozone layer must be inconsequential. The ozone layer, however, is maintained by a delicate balance of the production, transport, and destruction of ozone molecules. Relatively small amounts of sufficiently active chemical compounds can upset this balance and cause important changes in the amount and distribution of ozone. Rocket engines produce small amounts of such active compounds.

The three main phases of propulsion system emissions. Instruments carried by high-altitude aircraft can only measure the intense disturbances in the local phase. Stratospheric disturbances during the mesoscale and global phases are too slight to be observed directly and must be predicted using computer models of atmospheric chemistry and dynamics. These models, however, strictly depend upon key data from measurements obtained during the local phase.

Early in the past decade, Aerospace conducted research for the Air Force Space and Missile Systems Center (SMC) Environmental Management Branch on how SRM exhaust affects stratospheric ozone. These studies, which raised several environmental concerns, were limited to laboratory and modeling simulations of rocket-plume chemistry. By the middle of the decade, it had become obvious that a complete understanding of rocket-exhaust effects required moving beyond the theoretical investigations to actual measurements. In 1995, responding to the concerns raised by the earlier studies, SMC requested that Aerospace establish a practical, quick-to-implement program to collect actual data from SRM plumes in the stratosphere. The program, named Rocket Impacts on Stratospheric Ozone (RISO), and led by the Aerospace Environmental Systems Directorate, initially focused on the stratospheric impacts of the heavy-lift Titan IVA. RISO has subsequently been expanded to include responsibility for investigating the impact of all current U.S. Air Force launch vehicles. The Air Force Office of Scientific Research joined with SMC during the RISO planning phase and supported several investigators on the RISO team. The initial pioneering RISO plume measurement campaigns began in 1996.

Ozone-Destroying Radicals

Complicated chemical and physical processes, only partially understood by atmospheric scientists, affect both the amount and distribution of ozone in the stratosphere. In general, ozone is formed in the equatorial stratosphere at altitudes above 30 kilometers. Large-scale winds continuously transport the ozone to lower altitudes and toward Earth's poles to form a layer about 10 kilometers thick, centered at about 22 kilometers altitude. The concentration of ozone is determined by the rate of ozone transport into the layer versus the rate of ozone loss by reaction with ozone-destroying radicals such as the chlorine atom (Cl), nitric oxide (NO), and the hydroxyl radical (OH). Because each radical is able to regenerate after destroying an ozone molecule (called a catalytic cycle), radical molecules exert a major influence on ozone even at the small quantities found in the stratosphere. This means that small changes in stratospheric composition caused by industrial activity, including rocket exhaust, might cause relatively large changes in the ozone layer.

Most chlorine emerges from solid-propellant rocket motors as hydrogen chloride (HCl). Some of the HCl is converted into reactive chlorine atom (Cl) and molecule (Cl₂) by downstream chemical processes called "afterburning." Computer models are used to predict how much of the chlorine is in the reactive form as a function of distance away from the motor nozzle. Here, a model predicts that about one-third of the HCl leaving the nozzle is converted into Cl and Cl₂ in the plume of an Athena II rocket as it flies through the ozone layer.

The Composition of Rocket Emissions

Both solid and liquid rocket-propulsion systems emit a variety of gases and particles directly into the stratosphere. A large percentage of these emissions are inert chemicals such as carbon dioxide that do not directly affect ozone levels. Emissions of other gases, such as hydrogen chloride and water vapor, though not highly reactive, indirectly affect ozone levels by participating in chemical reactions that determine the concentrations of the ozone-destroying radicals in the global stratosphere. A small percentage of rocket- engine emissions, however, are highly reactive radical compounds that immediately attack and deplete ozone in the plume wake following launch. Aerosol emissions, such as alumina particles, carbon (soot) particles, and water droplets, can also act as reactive compounds when heterogeneous chemical reactions take place on the surface of these particles.

Rocket emissions have two distinct effects on ozone: short-term and long-term. Following launch, rapid chemical reactions between plume gases and particles and ambient air that has been drawn into the plume wake cause immediate changes in the composition of the local atmosphere. During this phase, which lasts for several hours, the concentrations of radicals in the plume can be thousands of times greater than the concentrations found in the undisturbed stratosphere, and the ozone loss is dramatic.

Long-term effects occur as gas and particulate emissions from individual launches become dispersed throughout the global stratosphere and accumulate over time. The concentrations of emitted compounds reach an approximate global steady state as exhaust from recent launches replaces exhaust removed from the stratosphere by natural atmospheric circulation.

The theory of reactive chlorine production in SRM plumes was proved during a WB-57F mission through the plume wake of a Titan IVA. The concentration of chlorine molecule (Cl₂) was measured as the aircraft flew through the eight-kilometer-wide plume 40 minutes after launch. Because Cl₂ is not present in the undisturbed stratosphere, all of the measured Cl₂ could be attributed to the Titan IVA SRMs. The data were obtained by J. Ballenthin and associates of the Air Force Research Laboratory, Hanscom Air Force Base.

Before the RISO field campaigns, relatively little was known with certainty about the highly reactive components of rocket-engine emissions or the intensity of ozone destruction in the plume wake. In 1995, managers and researchers from the Air Force and the National Aeronautics and Space Administration (NASA) met to review rocket emissions and identify critical knowledge needs. The meeting participants concluded that airborne measurements inside actual stratospheric rocket plumes should be a priority for further research, and the RISO program was designed with those conclusions in mind.

The Role of Chlorine Radicals

Researchers have long been aware that hydrogen chloride (HCl) is a component of SRM exhaust. It had been assumed that HCl, which is relatively unreactive, would contribute to ozone depletion globally over the long term by slightly increasing radical chlorine levels in the stratosphere but would not alter ozone levels in the plume-wake region immediately after launch. Atmospheric scientists began to wonder, however, if unreactive HCl could be converted into highly reactive chlorine radicals in plume combustion processes, resulting in an immediate and possibly deep ozone loss in and around SRM plume wakes. Such a short-term loss could conceivably influence the intensity of the sun's harmful UV light on the ground near launch sites.

To find an answer, Aerospace researchers modified existing computer models of secondary combustion in SRM plumes by incorporating a more complete representation of the chemistry of chlorine compounds. Secondary combustion, also called "afterburning," refers to the intense chemical processing that takes place in rocket plumes after the hot gases have left the engine nozzle until they cool to the temperature of the surrounding atmosphere. These new afterburning models predicted that a significant amount of HCl in SRM exhaust would indeed be converted into chlorine radical in the hot plume. Given the inevitable, and important, implication of deep ozone loss, the reactive chlorine emission index (EI) of SRMs needed to be verified.

Alumina particles emitted by SRMs come in three distinct sizes, or modes. (A human hair is 20 times the diameter of the large-mode particle.) Only particles in the small mode remain in the stratosphere long enough to mix throughout the atmosphere and possibly play a global role in stratospheric chemistry. RISO studies of these modes in Titan IVA and space shuttle plumes showed that less than 0.1% of the total mass of SRM alumina particles appears in the small mode, which is 10 to 100 times smaller than previous estimates indicated.

An EI provides a standardized way of expressing how much of a particular exhaust component is emitted into the atmosphere by a rocket engine. The EI is calculated by dividing the total mass of a particular component in the plume (in grams) by the total mass of propellant burned (in kilograms). For rocket engines, the EI refers to the exhaust plume composition after the secondary combustion process has occurred and the plume has expanded to match the pressure and temperature of the surrounding atmosphere. EIs serve as a convenient way to analyze plume data, provide input for computer models, and quickly compare the potential stratospheric impacts of different propulsion systems.

The true extent of the immediate stratospheric response to the putative reactive chlorine emissions was not well understood; the results of various models were not in agreement. For example, predictions of the duration of the short-term ozone loss in the wake of a Titan IV-class vehicle varied from a few minutes to a few hours from model to model. Without actual plume data, it was impossible to evaluate the accuracy of the various models, and the resulting uncertainty allowed the possibility that the actual ozone loss exceeded all predictions. The behavior of ozone in SRM plume wakes needed to be measured, and the plume-wake models needed to be evaluated.

A third uncertainty concerned the alumina particles in SRM exhaust. These tiny particles (most are less than one-thousandth of a millimeter in diameter) have the same chemical makeup as sapphire (Al₂O₃). Some laboratory measurements had suggested that heterogeneous chemical reactions on the surface of alumina particles might contribute to ozone loss by converting chlorine from inactive to active forms. The potential importance of this effect is critically determined by the exact sizes of the alumina grains in the exhaust. The largest grains fall out of the stratosphere within several days, and so their surfaces do not have time to promote significant chemistry in the global sense. The smallest grains may remain aloft for several years, however, possibly promoting ozone-harmful reactions throughout the stratosphere. To resolve this question, alumina particles in SRM plume wakes needed to be collected and the EI of the smallest of them measured.

In-Situ Plume Experiments

At its inception, RISO conducted three independent data-collection experiments. Two of these, both completed in 1998, used remote-sensing devices based at Cape Canaveral Air Force Station. First, a network of sensors measured the influence of stratospheric plumes on the intensity of harmful solar UV light on the ground near the launch site. Second, a multiple-wavelength lidar (light detection and ranging) system successfully illuminated plumes with laser beams to measure the optical properties of plumes over Cape Canaveral and provide insight into how plume exhaust mixes into the stratospheric background air. These two efforts conclusively demonstrated that even though radicals in rocket exhaust cause immediate loss of UV-absorbing ozone in individual plumes, rocket plumes disperse in a way that makes it highly unlikely that the intensity of UV light on the ground near launch sites would measurably increase following launches of even the largest rockets.

RISO's main focus, however, has been to develop a detailed understanding of rocket emission chemistry by directly measuring the composition of stratospheric air inside plume wakes during the critical time from several minutes to several hours after launch. RISO chose the NASA WB-57F aircraft to carry instruments into lower-stratospheric rocket plumes at an altitude of about 19 kilometers. During a typical mission, the WB-57F enters a plume about five minutes after launch and then executes figure-8 maneuvers around the launch-vehicle trajectory, encountering the plume wake about every 10 minutes for up to two hours after launch. The aircraft travels at about 200 meters per second and spends between 2 and 60 seconds in the plume during each encounter measuring composition.

Beginning in 1996, a variety of exhaust plumes were sampled by the instrumented WB-57F aircraft, including the space shuttle, Titan IVA, Delta II, Atlas IIAS, and Athena II. Three instruments carried during the 1996 missions proved the scientific value of the RISO concept. The instruments have steadily improved since the first missions. Seventeen state-of-the-art instruments carried during the 1999 missions collected a wide variety of gas and particulate data that will allow a more comprehensive characterization of plume-wake chemistry.

Highlights from Early Missions

Exhaust plume of a Titan IVA launched from Vandenberg Air Force Base, as seen from the WB-57F cockpit just before flying thorough the plume. Only two minutes old in this view, the plume displays a pattern of breakup from windshear and mixing characteristic of all stratospheric plumes. The California coast and San Miguel Island are visible. (USAF)

On April 14, 1996, the WB-57F carried into a Titan IVA plume a Neutral Mass Spectrometer developed by the Air Force Research Laboratory at Hanscom Air Force Base. Analysis of data from the spectrometer unambiguously demonstrated that the plume contained significant amounts of reactive chlorine molecule, a gas not found in the natural stratosphere. The RISO team concluded that the estimated chlorine molecule EI of the Titan IVA SRMs was generally consistent with predictions based on the Aerospace computer models of chlorine afterburning chemistry.

RISO WB-57F missions have carried up to four instruments to observe the extent and duration of immediate ozone loss in the plume wake. Data from each plume encounter allow investigators to quantify how much ozone is destroyed in the plume over time. Measurements from Titan IVA and space shuttle plumes show that the amount of ozone destruction does not increase without limit. RISO researchers have shown that ozone loss slows about one hour after launch, suggesting that the most ozone-destructive emissions have been deactivated by reactions with various gases in the surrounding air. This important observation has eased concerns that the short-term ozone loss in rocket plumes might be much greater than in the model predictions.

Surprisingly, data obtained from within the plumes of several different rockets show that launch vehicles with greatly differing SRM emission rates cause about the same amount of ozone loss between 30 and 60 minutes after launch. Ozone loss in the plumes of Delta II and Atlas IIAS rockets was about the same as the loss in the plumes of the much larger Titan IVA and space shuttle. Existing plume-wake models that include only SRM chlorine gas emissions have not predicted this result; why this discrepancy exists is not yet known. It may be that SRM emissions interact with the stratosphere in a fashion not yet accounted for in plume models, or perhaps the liquid-oxygen/kerosene core engines of the Atlas IIAS and Delta II produce reactive gases that act alone or with SRM emissions to cause some additional ozone loss. Further data collection and measurement of the actual radical EIs for the various systems, and the development of detailed models of plume-wake chemistry, are needed to solve this puzzle. Interagency ACCENT Program

The WB-57F high-altitude aircraft, operated by NASA's Johnson Space Center, carries large scientific payloads well into the stratosphere. The WB-57F has served the RISO program since 1996, flying though 11 rocket-exhaust plumes. The two-person aircrew includes a dedicated science officer who monitors instrument status and initiates plume data collections. (NASA)

Thiokol Propulsion, the company that manufactures the SRMs used on the space shuttle and Atlas IIAS, and Alliant Techsystems, the company that produces the Delta II SRM, joined RISO in 1998 as contributing partners. In 1999, RISO joined forces with NASA, the National Oceanic and Atmospheric Administration, and the National Center for Atmospheric Research as part of the Atmospheric Chemistry of Combustion Emissions Near the Tropopause (ACCENT) mission, a multiagency-sponsored effort to study the effects of aircraft and rocket-engine exhaust on the upper troposphere and lower stratosphere.

ACCENT brings together RISO and ongoing efforts of the NASA Atmospheric Effects of Aviation Program (AEAP). The ACCENT partnership grew out of the realization that a common set of atmospheric measurements from shared payloads on the WB-57F aircraft could serve the interests of both the Air Force and NASA programs. The ACCENT payload included as many as 17 instruments, significantly enhancing the RISO/ACCENT science team's ability to understand plume-wake chemistry and characterize rocket-engine emissions. Three successful plume-wake flights took place during the 1999 ACCENT deployments. The data collected will allow several important unresolved problems to be addressed, including estimating the nitric oxide radical EI of SRMs and the soot EI of liquid-oxygen/kerosene engines and measuring the intensity of chemical reactions that take place on the surface of alumina particles in the plume.

Looking Ahead

RISO represents but one component of ongoing Aerospace activities to provide the Air Force with cutting-edge research and technical guidance on a wide range of environmental issues—from solvent chemistry to toxic ground clouds and ozone depletion. For its part, the RISO team allows SMC to claim world-class scientific expertise with regard to the influence that rocket emissions have on Earth's ozone layer. The advances coming out of RISO are making the Air Force and the entire space-launch community confident that ozone loss from both individual and collective launches does not constitute a significant environmental hazard. RISO has proved that a low-cost program of ongoing plume-wake intercepts using appropriate instrumentation can help resolve the scientific problems surrounding the issue. RISO has also shown how joining forces with other agencies and industry increases the scientific return on investment for all interested parties.

Ozone concentration measured across the plume wakes of four different launch vehicles, all obtained about one hour after the different launches. Red represents data obtained while the WB-57F aircraft was inside the exhaust plume. The total amounts of chlorine emitted by the Titan IVA, Atlas IIAS, space shuttle, and Delta II SRMs at the altitude of these measurements are about 2, 0.2, 4, and 0.3 tons per kilometer of altitude. Despite such large differences in chlorine emission rates among the four rocket types, the ozone losses in the plumes are comparable. The data were obtained by J. Benbrook and W. Sheldon of the University of Houston.

The data and conclusions from the RISO program reinforce a presumption that rocket emissions do not seriously threaten the ozone layer at the present time. However, as the space transportation industry grows, as new launch systems are introduced, and as the ozone layer recovers from past damage caused by now-banned substances, the effect of rocket emissions on stratospheric ozone is likely to become a more visible issue. The space transportation community should continue to support scientific research efforts to fully understand the impact of rocket-propulsion systems on the composition of Earth's natural umbrella, the ozone layer.