SAI Impacts on Airplane Air Quality
Alvia Gaskill
1. Although focusing on a hypothetical hazard due to sulfuric acid/sulfuric acid aerosol in cabin air of commercial or other flights north of 60º latitude, the product of SAI programs both Arctic/Antarctic only and globally, the paper does not discuss how significant this would be to air travel or offer any solutions. I'll attempt to do so here, leaving it to others to firm up/clean up my estimates and conclusions.
2. According to the paper, exposure to H2SO4 would be limited to those flights, that is, regardless of where the SO2 is released, only in the Arctic would the H2SO4 levels be of possible concern with this as one example where tropical releases also result in similar levels in the Arctic due to the "bathtub drain effect" of the BDC: gmd-15-8221-2022.pdf
3. Of the (est.) nearly 40 million flights this year (>100,000/day) that would at some point reach 11-13Km, a very small percentage will travel above 60º N and almost none above 60º S. Yearly Number of Flights - Air Traffic Statistics Exact figures are hard to come by as the total varies throughout the year and many more planes will go above 60º N at some point than those on designated polar routes. I couldn't get the FAA database on this to produce results by latitude so based on other sources I am going to estimate the total of all such flights by other means.
By number of passengers flying through an airport at or above 60º N. This is a conservative estimate because much of the traffic will likely be to locations south of these and the time above 60º N limited to the area around the airport.
Altogether 19.8 million passengers flew through Finnish airports in 2024 | Statistics Finland
Fairbanks International Airport - Wikipedia
By flights per year:
4. With such a relatively small number of such flights and passengers, one option for eliminating the exposure risk would then be to no longer fly above 60º N, rerouting or canceling the routes. In addition to the long-haul destinations served, major cities near 60º N like Helsinki, Finland; Oslo, Norway and St. Petersburg, Russia would be directly impacted. There is already precedence for no-fly zones over the Russian Arctic due to the war with Ukraine and outside the Arctic, flights over Ukraine itself and other troubled regions (N. Korea, Somalia, etc.) have been suspended for years. Polar route - Wikipedia
5. This action could also apply to any other precursor or SAI program involving the Arctic (H2S, calcite, silica). Risks to passengers and flight crews of passenger and cargo aircraft and the equipment (air frames and engines) would be eliminated. However, flight crews and aircraft delivering the precursor (if planes are used—balloons are problematic for other reasons at any scale including tethered with attached hoses) would still be at risk. Engine damage due to ingested calcite or silica would also still apply.
6. With total flights expected to double in just a few years, it is doubtful the airlines, governments or the flying public would go along with this. Thus, a way to keep the H2SO4 out of the cabin air needs to be found. Avoiding air frame and engine damage is a separate issue and requires its own evaluation. Although the published work on this topic seem to assume there will be no damage, I think more objective evidence is required.
7. The current approach as outlined in the paper is to use a portion of air from the turbine compressor, the bleed air and after compressing and cooling to room temperature, pump it into the cabin. This air is then passed through a HEPA filter impregnated with activated carbon. The combination of these removes both particulates and VOCs. Around 40-50% is recycled, sometimes more. How clean is the air on planes? | National Geographic
8. You might ask, why not filter all the air before it enters the cabin? While this approach is being considered by aircraft design and operational engineers, the reason it is not done that way now is because by removing some of the compressor air, this reduces the air available for thrust and increases fuel usage. More than doubling the amount removed would further reduce oxygen available for thrust. With increasing numbers of complaints about fume odors from engines while the plane is on the ground, this redesign is being discussed within the industry. advanced-cabin-air-filtration.pdf
9. While the 787 takes all cabin air directly from ambient (it is filtered before entering the cabin), except for the 787, this approach has not been adopted for newer planes for some reason. Talking about it but not acting on it. It is also unclear if retrofits are possible on existing aircraft. Here’s How The Boeing 787 Dreamliner’s Cabin Air System Is Different Than Every Other Aircraft
10. Although the paper assumes all the H2SO4 in the recycled air will be removed by the HEPA filter (nominal cut point of 0.3 microns), no evidence to support this was provided. If the aerosol formed in the cabin from H2SO4 is larger than that, a theoretical case can be made. However, I found no examples of HEPA filters being used to remove any of the sulfur species of interest from ambient air (H2S, SO2, SO3, H2SO4, H2SO4 aerosol). The filters might be expected to remove calcite or silica, however.
11. The industry standard for removing SO2 in indoor air is a cartridge containing an acid gas sorbent. The same type of cartridge is recommended for H2SO4. Sulfuric Acid respiratory protection I couldn't find any data to support this, however. How this could be incorporated into an aircraft ECS is also unknown to me. I think there will be a problem with longevity, requiring frequent changeouts. HEPA filters in an ECS can last for nearly a year of flight time but that's largely because the planes fly though relatively clean air while at cruising altitude. Boeing 787 Cabin Air Filter HEPA Upper Recirc
12. I also think the California standard of 1 ug/m3 should be considered as it is the most protective available and flight crews of both commercial and precursor delivery aircraft would be subject to exposure times equivalent to that for the EPA NAAQS for SO2 which is 9 ug/m3 for an annual mean averaged over 3 years. NAAQS Table | US EPA SO2 and H2SO4 are not the same but in the absence of better data, the 1 ug/m3 standard is a good default choice.
13. One way to answer most of these questions is with a field test, not in the stratosphere but on the ground. The FAA has conducted studies of bleed air contaminants injected into a 747 cabin so presumably could do the same with H2SO4. Cabin Air Quality | Federal Aviation Administration
14. Pall Corp, the providers of ECS filters for most of the airlines should also be consulted. Cabin Air Filtration | Pall Corporation Due to the relatively small number of volcanic eruptions resulting in noticeable levels of sulfur species in the cabin air, there has been no need for such research. However, if SO2 is to be used in field studies or deployment such tests are warranted.
15. And although one would get the impression from the paper that all drawbacks to the use of SO2 are potentially equally disqualifying (effects on astronomy, whiteness of the sky, changes in monsoons), they are not. Sea level rise is an existential threat to human civilization. Harmful changes in monsoons need to be carefully considered. Effects on air travel or astronomical measurements however inconvenient are not existential threats. Authors and reviewers need to start making this distinction, including when giving interviews to the geoengineering hostile media. Journalists should also take note and not tabloidize every study.
In the Daily Mail article excerpted below, Alan* also contradicted his own paper when he said safe levels would have to be determined. The abstract said the modeled levels while concerning were safe. He needed to make clear in the interview there are different health guidelines based on different assumptions, not just base a conclusion on the one that resulted in unsafe air quality. Safe in Europe, but not in California. Controversial plan to dim the sun could choke airlines with clouds of 'hazardous' sulphuric acid, putting passengers and crews at risk | Daily Mail Online
*He or the reporter also confused latitude with altitude.
