The Future Space Launch Market and Stratospheric Ozone

Traffic within (and through) the stratosphere was perhaps the first recognized anthropogenic force for stratospheric perturbation. In the early 1970’s, a proposed fleet of 400 Supersonic Stratospheric Transports (SSTs) — or equivalently, High-Speed Civil Transports (HSCTs) — was severely scrutinized to address concerns which included the impact of SST HO$_\text{x}$ and NO$_\text{x}$ emissions on stratospheric ozone. Initial 1-D photochemical model estimates of steady state ozone loss arising from such a fleet frequently exceeded 50% or more and contributed to a moratorium on US development of SSTs.

Enhancements in stratospheric water and nitrogen oxides produce enhancements in the cycling of ozone via the general ozone-loss processing motif depicted in figure 1. In brief, precursor species Y may be activated to form active species X. Once formed, X reacts quickly with ozone to form XO. XO then reacts quickly with atomic oxygen to reform species X. The cycle produces a net loss of stratospheric ozone. Chain reaction termination steps are idiosyncratic to the physicochemical environment. In some cases, as depicted in figure 1,  X and XO will recombine to reform the long-lived precursor species.

Figure 1: general motif for ozone loss processing in the middle stratosphere. Klobas Thesis 2018

These initial model results featured exaggerated estimates of ozone loss for several reasons. The kinetic rates of the HO$_\text{x}$– and NO$_\text{x}$– mediated reactions were highly uncertain – and most importantly, a previously unknown source of atmospheric chlorine significantly reduced the efficiency by which atmospheric NO$_\text{x}$ might process ozone. Elevated concentrations of chlorine from chlorofluorocarbons served to lock away nitrogen dioxide (produced from SST emission of NO) and produce chlorine nitrate, per the following reactions.

$\text{NO} + \text{O}_3 \rightarrow \text{NO}_2 + \text{O}_2$

$\text{Cl} + \text{O}_3 \rightarrow \text{ClO} + \text{O}_2$

$\text{ClO} + \text{NO}_2 + \text{M} \rightarrow \text{ClONO}_2 + \text{M}^*$

Chlorine nitrate is a long-lived reservoir of both odd chlorine and odd nitrogen, and the discovery of this reaction essentially resolved the major threat to ozone previously predicted from high-altitude SSTs.

Space launch vehicles also perturb stratospheric inventories of trace gases. During the early 1970’s, and contemporaneous with the SST debates in congress, were initial studies on the implications of increased use of solid rocket motors (SRMs).  SRMs frequently employ ammonium perchlorate ($\text{NH}_4\text{ClO}_4$) oxidizers with alumina fuel, producing large local enhancements in chlorine and stratospheric surface area following a launch. The effects are quantifiable and a series of in situ rocket plume encounters in the late 1990’s revealed large, transient ozone holes spanning several hundred kilometers after a launch.

Contemporary commercial space launch vehicles tend to shy away from SRMs for many reasons including: low specific impulse, inability to prematurely terminate/restart burns, and environmental concerns. Liquid rocket motors (LRMs) are more gentle to the ozone layer than SRMs; however, they still produce trace gas emissions as they transit the stratosphere. HO$_\text{x}$ and NO$_\text{x}$ are the primary reactive families produced in a LRM plume, depending on the fuel source utilized. LO$_2$/RP-1, a kerosene fuel/liquid oxygen oxidizer, will emit water, nitrogen oxides, contributing to ozone loss processing. LO$_2$/LH$_2$ is cleaner burning,  but still produces large enhancements in HO$_\text{x}$.

This leads to an interesting scenario: as anthropogenic halogens decline throughout the century as a result of the highly-successful Montreal protocol (and subsequent amendments), the efficiency of the odd-nitrogen and odd-hydrogen catalytic cycles will increase. Also projected to increase is the rate of launches and the mass of material placed into orbit as the private space launch market grows exponentially.  Today’s launch of the Falcon Heavy by SpaceX is an impressive milestone in monitoring this trend.

The news surrounding this event reminded me of a paper from a while back — an econometric analysis of the space launch market and the impact its  expansion might have on the stratospheric ozone layer.

Ross et al. (2009) provide constraints on the mass which may be placed into orbit depending on future choices in fuel mix.  They provide the following figure (figure 2), which I reproduce for fair use academic purposes. In the figure, losses of ozone are presented as a function of payload rate, and fractional use of SRMs vs LRMs.

Figure 2, reproduced from Ross et al. (2009)

It should be noted that the projected losses are parameterized from model studies of 20th century atmospheres. Effects from the changing climate, such as stratospheric cooling, changes in the strength of the Brewer Dobson Circulation, and declining halogen burdens may significantly change the calculus. Unfortunately these studies have yet to be performed.

The SpaceX Falcon Heavy launch window opens in several hours and, if successful, will be a transformational moment in commercial spaceflight, opening up deep space to private venture. Tune in to the launch livestream here.