How do rocket emissions impact ozone and climate?

Rockets emit a variety of substances depending on their propellant. Some, like liquid hydrogen and oxygen (H₂/O₂) are very clean, emitting
mainly water (H₂O) and some nitric oxide (NO), which is produced by the heat of combustion. Others, like aluminum/ammonium perchlorate
(or "Solid Rocket Motors", SRMs) release hydrochloric acid (HCl) and alumina (Al₂O₃) particles. Rockets that use hydrazine (N₂H₄) and
nitrogen tetroxide (N₂O₄) (sometimes called "hypergolic", because these chemicals spontaneously ignite on contact) produce large quantities
of nitrogen oxides, which can further react with water vapor and sulfate in the atmosphere to form small particles containing nitric acid. Kerosene
rockets (essentially "aircraft fuel") produce CO₂ and black carbon ("soot"), which are climate-active gases (meaning that they absorb infrared or
visible light, heating the surrounding air).

There is a new type of propellant called "hydrid" that is being used by some private companies. Hybrids are a mixture of a liquid oxidizer,
nitrous oxide (N₂O), and a solid synthetic rubber (a butadiene) that, when burned in the oxygen-poor environment of the upper atmosphere
produce CO₂ and large amounts of soot (which is readily visible in photos of these rockets, because it is black or grey in color) and probably
large amounts of nitric oxides (although there are no measurements in these plumes to verify the presence of NOx). This photo gives a clear
view of the black carbon plume of one such rocket as it enters the stratosphere.



Climate

The main exhaust products that can contribute to climate change are H₂O, CO₂, soot, and alumina.

(1) There is a very large amount of H₂O naturally present in the atmosphere. Therefore, except high in the mesosphere, the contirbution from
rockets is thought to be very small - probably too small to make any significant difference that could be directly attributed to 'rockets.' However,
if temperatures are cold enough, the water vapor can create a contrail made of small ice crystals. These ice crystals will eventually evaporate
as the plume dilutes and mixes into background air. But if the lighting is just right, they can be very spectacular, as they can make a very colorful
rainbow that can be seen from the ground. Because the mesosphere is very thin, these mesospheric clouds are very easy to spot from the ground,
and there are literally thousands of photos. However, being thin, they have very little impact on climate. Although there are few studies of these
particular clouds, they likely warm the atmosphere (contrary to some proposals that they might actually help cool the planet because they are
bright). The problem is that the amount of infrared absorption by ice crystals is greater than the amount of sunlight they reflect, so in the 'net',
they warm the atmosphere.

(2) CO₂ is often cited as the 'most important manmade greenhouse gas.'  Even so, rockets don't contribute much to the background CO₂, which is
already quite large due to natural and man-made emissions. It is very likely that unless rocket activities increase by many orders of magnitude, CO₂
emissions will not contribute to any changes in climate that can be directly attributed to 'rockets.' However, it is important to note that CO₂ emitted
by rockets will have the same affect on Earth's temperatures as CO₂ emitted at the surface (to first order). This is because the CO₂ molecules in
both cases spend about the same amount of time in the atmosphere, eventually becoming indistinguishable from each other.

(3) Black carbon (often called 'soot,' although soot can refer to non-carbon products of combustion as well) is the exhaust product that is
of greatest concern, not only from rockets, but from vehicles in general (like diesel trucks, power plants, etc.). This is because the material is
very efficient in absorbing visible light (which is why it is black!), thereby heating the atmosphere when sunlit. It is estimated that black carbon
emitted by rockets is over 1 million times more efficient at heating the atmosphere than an equivalent amount of CO₂ by weight. Under most
combustion conditions (such as at earth's surface or in the troposphere where aircraft fly) there is so much oxygen that very little black carbon
is produced by combustion, so its contribution to climate change is smaller than that of CO₂ (although soot is still nearly one-fourth of the
global problem, and dominant in some regions like India and Asia).  In the stratosphere, where there is less oxygen due to the low pressures
at high altitudes, black carbon can become a large fraction of the combustion products (5% or more, by weight). Under these conditions, the
absorption of solar radiation by black carbon will dominate over the infrared absorption by CO₂ emitted in the same plume. Because the
lifetime of black carbon in the upper atmosphere is 5-10 years and longer, this black carbon has a disproportionate impact on Earth's climate
than the same amount of emissions of black carbon in the lower atmosphere, where processes like rain and dry deposition (the loss of
materials to surfaces) remove particles in less than a month (e.g., smoke from fires).

(4) In our recent paper in Geophysical Research Letters we estimate that the impact of black carbon emissions by space tourism rockets
will change Earth's climate in ways that will be large enough that those changes could be attributed to the rocket activities themselves.
Some regions of the globe will experience a slight cooling, due to shading of the surface by the black carbon layer (which is predicted to
develop mainly in an annulus around the globe at the latitude of the launch site), and some will warm due to a change in atmospheric
circulation. None of these predictions are controversial - that is, similar kinds of changes have been predicted by others for different
emissions (aircraft operations, greenhouse gas emissions, etc.). What is important about the study is that is shows that the changes that
can be attributed to rockets will be large enough that they could be detectable and, if so, they will be directly attributable to the launch
activities themselves. Some of the the changes might be considered beneficial, some harmful. That is mainly an ethics debate. What is clear
is that the formation of a layer of soot in the stratosphere is essentially a solar radiation management issue. If scientists were
to propose to release soot into the stratosphere, it would be called a 'geoengineering' experiment and they would be required to file an
environmental impact statement.

(5) Very little is known about alumina particles and their impacts on climate. Although we have sampled the particles directly in rocket plumes,
their optical properties are poorly understood. Some people have proposed using alumina as bright particles to reflect sunlight to space as a
way to cool the surface. Unfortunately, alumina is a potent infrared absorber, so it is even more likely that these particles will warm the stratosphere
like soot, thereby altering the circulation and having unknown, adverse effects that are difficult to forecast. It will be necessary to perform more
measurements in the plumes of rockets and in the laboratory to have a sufficient understanding of alumina particles to predict their impacts on
climate.

Ozone


(1) Emissions of nitric oxide (NO) can react directly with ozone in daylight and in darkness, whereas in sunlight, emissions of
hydrochloric acid produce a highly reactive form of chlorine called chlorine monoxide, or ClO, that reacts with ozone. This same
molecule, ClO, is responsible for the ozone hole over Antarctica. In the plumes of rockets that use solid perchlorate propellant, ozone is
completely destroyed in the narrow column of the plume.



(2) As the chemicals mix, the nitrogen oxides become less important, as they dilute in the background where there is already a lot of
naturally occuring oxides of nitrogen. However, the hydrochloric acid emissions and water vapor are present in much higher concentrations
than in background air, so they continue to alter the normal chemistry of the environment that is affected by the mixing plume. Particles
of alumina that are also emitted by the rocket accelerate ozone-destroying reactions, amplifying those chemical reactions. As ozone mixes back
into the plume from unaffected, surrounding air, that ozone is destroyed by the rocket emissions. The photo above shows our aircraft sampling
in the plume of the Space Shuttle 1 hour after launch.

(3) Eventually, after a few weeks, the chemicals in the plume mix into the background air, where they become nearly indistinguishable from that air
(although sensitive instruments can still detect some of the exotic materials, like alumina, that are produced by the rocket and that do not occur
naturally). At this point, the emissions from a single rocket have very little impact on the atmosphere - the ozone 'hole' produced by the rocket has
filled back in by mixing with ozone-rich air. But over time, the aggregate emissions of all rockets alter the natural abundances of chemicals in the
stratosphere, leading to a small change in the ozone. The important chemicals are chlorine, alumina particles, and water vapor, all of which
contribute to the destruction of a little bit of the ozone layer all around the planet. Current estimates are that less than 0.1% of stratospheric ozone
is destroyed by rockets, but this number will likely increase as the rocket industry, but private and public, continues to expand.