This date in science: On this date, Sweden announced it would ban aerosol sprays containing chlorofluorocarbons CFCs as the propelling agent. It was the first country in the world to do so. Sweden was the first to act on this evidence, which came before before the discovery of the Antarctic ozone hole.
Ozone is formed in the stratosphere when oxygen molecules photodissociate after intaking ultraviolet photons. This converts a single O 2 into two atomic oxygen radicals.
The atomic oxygen radicals then combine with separate O 2 molecules to create two O 3 molecules.
These ozone molecules absorb ultraviolet UV light, following which ozone splits into a molecule of O 2 and an oxygen atom. The oxygen atom then joins up with an oxygen molecule to regenerate ozone. This is a continuing process that terminates when an oxygen atom recombines with an ozone molecule to make two O 2 molecules.
The dot is a notation to indicate that each species has an unpaired electron and is thus extremely reactive. These elements are found in stable organic compounds, especially chlorofluorocarbonswhich can travel to the stratosphere without being destroyed in the troposphere due to their low reactivity.
Once in the stratosphere, the Cl and Br atoms are released from the parent compounds by the action of ultraviolet light, e. Cl and Br atoms destroy ozone molecules through a variety of catalytic cycles.
In the simplest example of such a cycle,  a chlorine atom reacts with an ozone molecule O 3taking an oxygen atom to form chlorine monoxide ClO and leaving an oxygen molecule O 2. The ClO can react with a second molecule of ozone, releasing the chlorine atom and yielding two molecules of oxygen.
The chemical shorthand for these gas-phase reactions is: More complicated mechanisms have also been discovered that lead to ozone destruction in the lower stratosphere.
The ozone cycle Global monthly average total ozone amount Lowest value of ozone measured by TOMS each year in the ozone hole A single chlorine atom would continuously destroy ozone thus a catalyst for up to two years the time scale for transport back down to the troposphere were it not for reactions that remove them from this cycle by forming reservoir species such as hydrogen chloride HCl and chlorine nitrate ClONO 2.
Bromine is even more efficient than chlorine at destroying ozone on a per atom basis, but there is much less bromine in the atmosphere at present. Both chlorine and bromine contribute significantly to overall ozone depletion.
Laboratory studies have also shown that fluorine and iodine atoms participate in analogous catalytic cycles. A single chlorine atom is able to react with an average ofozone molecules before it is removed from the catalytic cycle.
This is normally expressed in Dobson units ; abbreviated as "DU". The most prominent decrease in ozone has been in the lower stratosphere. Marked decreases in column ozone in the Antarctic spring and early summer compared to the early s and before have been observed using instruments such as the Total Ozone Mapping Spectrometer TOMS.
Antarctic total column ozone in September and October have continued to be 40—50 percent lower than pre-ozone-hole values since the s.
It is expected to recover around The greatest Arctic declines are in the winter and spring, reaching up to 30 percent when the stratosphere is coldest. Reactions that take place on polar stratospheric clouds PSCs play an important role in enhancing ozone depletion.
This is why ozone holes first formed, and are deeper, over Antarctica. Early models failed to take PSCs into account and predicted a gradual global depletion, which is why the sudden Antarctic ozone hole was such a surprise to many scientists.
Total column ozone declined below pre values between and for mid-latitudes. In the northern mid-latitudes, it then increased from the minimum value by about two percent from to as regulations took effect and the amount of chlorine in the stratosphere decreased. There are no significant trends in the tropics, largely because halogen-containing compounds have not had time to break down and release chlorine and bromine atoms at tropical latitudes.
Pinotubo in the Philippines.Aerosol cans no longer contain ozone-depleting substances called CFCs, or chlorofluorocarbons. Photograph by Mark Thiessen Healing the Ozone Layer: Chemist Says Treaty Is Working. Aerosol spray is a type of dispensing system which creates an aerosol mist of liquid particles.
This is used with a can or bottle that contains a payload and propellant under pressure. When the container's valve is opened, the payload is forced out of a small hole and emerges as an aerosol or mist.
The Environmental Protection Agency (EPA) on Friday informed the public of a ban on the importation of ozone depleting chemical substances into the country from January 1, After a report by the U.S. National Academy of Sciences concluded that credible scientific evidence supported the ozone depletion hypothesis, a few countries, including the United States, Canada, Sweden, and Norway, moved to eliminate the use of CFC's in aerosol spray cans.
Use of Ozone-Depleting Substances. Essential-use products are exempt from the ban by FDA on the use of CFCs and other ODS propellants in FDA-regulated products and from the ban by the Environmental Protection Agency (EPA) on the use of ODSs in pressurized dispensers.
or food contained in an aerosol product or other pressurized dispenser. Final and Proposed Regulations for n-Propyl Bromide (nPB) Through its Significant New Alternatives Policy (SNAP) Program, the U.S.
Environmental Protection Agency (EPA) is allowing or proposing to allow the use of n-propyl bromide (nPB) as an alternative to ozone-depleting substances.