Impact of rocket launches on chemical and aerosol composition of the atmosphere

We present the outcome of the European Space Agency (ESA) project ATILA (Atmospheric Impact of Launchers). Attention was paid to the impact of the exhaust products emitted by rocket propulsion systems on the atmosphere at various altitudes. In particular, the effects on stratospheric ozone, global aerosol distribution and loading, and the resulting changes in radiative features of the atmosphere were investigated. The study was conducted in 3 upscaling steps: • formation of hot gas and the nozzle-exit conditions (scale: tens / hundreds of metres, tailored engineering models, parameterizations), • early and intermediate evolution of the rocket plume (scale: kilometres, computational fluid dynamics (CFD), intermediate diffusion models), • impact on the Earth’s atmosphere (scale: 10 km to globe, chemical transport model (CTM)). The CFD computations were used for a few selected cases, thus forming the reference points for interpolation and extrapolation. The altitude of 18.7 km was chosen as a reference, owing to the measurement data available for the Athena II test case. Further, the altitudes 30 km, 42 km and 50 km were selected as reference altitudes for the CFD computations. Other computations were performed for the altitude range of 10-60 km in steps of 5 km. The data coming from the diffusion model in 5 km increments was linearly interpolated before being fed into the CTM. The trajectories of the rockets were calculated in the beginning using the in-house trajectory tool Tosca of DLR-SART. At larger scales, the effect of a single launch can possibly be noticed only at local-to-regional level. Corresponding CTM computations were performed at 10 km resolution for the ESA launch pad in Kourou. Results for all 19 ESA launches during 2007-2009 showed complicated shapes of the regional plume dispersion but next to no effect already a hundred km away from the launch pad. Comparisons with satellite data, such as MODIS AOD, confirmed that very little traces of rocket launches can be observed. Indeed, the modelled aerosol optical depths due to the rocket aerosols are at least an order of magnitude lower than the regional background. The impact on the stratospheric ozone also proved limited and altitude-dependent. The largest effect was predicted below and above the main ozone layer but inside the main layer the reduction was very small. Arguably the main reason for the very limited effect of an individual launch is that all main exhaust products exist in the air in non-negligible quantities, so that the rocket impact ends when the plume is sufficiently diluted. An exception are aerosols, more specifically, aluminum oxide and, possibly, black carbon. Assessment of their impact starts from understanding their fate in the atmosphere and possibilities of accumulation somewhere over the globe (amounts released by individual launches are again minuscule). Consideration of this possibility went beyond the ATILA description of work and has been co-supported by the ASTREX project of the Academy of Finland. To perform the long-term assessment, the SILAM chemistry transport model was run over 34 years with an hourly time step and 1.44 degree horizontal resolution over the globe covering the altitude ranges from the surface up to 60 km with 60 stacked layers. Out of the whole period of space launches 1957-2012 with 5301 launches at 1228 sites (225 ESA launches, 1970-2012), the 34-year period, 1980-2013, was taken as the one covering over 99% of the total emission. The spectrum of the released particles was represented with seven bins for aluminum oxide and four bins for soot. The bulk budget of the long-term simulations showed that indeed the aerosols from the rockets tend to stay long in the atmosphere. By present day, their abundance has stabilized, owing to a dynamic equilibrium between the new launches and removal processes. Spatial distribution of the aerosols strongly depends on the location of the launch site. There is a substantial inhomogeneity in the aerosol distribution, which shows the clear tendency of the masses to accumulate in polar regions at the altitude of 20-30 km. This tendency is worrisome because there the concentrations may eventually become sufficient to influence the heterogenic processes related to the ozone destruction cycle. The aerosol surface area concentration almost reaches 0.01 mg2 cm-3, which constitutes up to 5-10% of the background levels in these regions. Importantly, particles from equatorial launches of ESA tend to stay longer and distribute more equally between the northern and southern polar regions than those emitted at higher latitudes in the Northern Hemisphere.