On 20/09/16 02:42, Peter Fairbrother wrote:
On 19/09/16 03:20, Norman Yarvin wrote:
On Sun, Sep 18, 2016 at 04:35:20PM +0100, Peter Fairbrother wrote:
On 18/09/16 03:24, Norman Yarvin wrote:
Note that "trike" was commonly 1,1,1-trichloroethane, until it got
banned for depleting the ozone layer.
It isn't actually banned per se; just manufacturing it, using it for
most purposes, and selling it in most cases are banned.
That stuff was relatively safe;
indeed, it's safety (non-reactivity) was likely also why it lasted
long enough to reach the ozone layer.
Except that it doesn't really reach the ozone layer, at least not in
significant quantities - unfortunately it is still listed as a class 1
ozone depleting substance, even though it is now known not to be one.
More, it's atmospheric life is only about 6 years. Continued small-scale
use would have no detectable effect on the ozone layer, but this is not
allowed.
How do you have something with an atmospheric life of 6 years and yet
which doesn't make it to the ozone layer?
Well of course it does, but not in huge proportion, especially compared
with cfc12 and cfc11. And it doesn't stay there as long.
For that matter, the stuff is such a good solvent that prior to the
ban it had been made in quantities of (from memory) something like a
million pounds a year.
More like a billion pounds a year.
One estimate is 186,000 tons/annum in the early 1980s (American
Conference of Governmental Industrial Hygienists, 1992).
Total production over the years was about 8 million tons (vs about 11
million for CFC-12), but at worst there was only 1/5 as much trike as
CFC-12 in the stratosphere.
Nowadays CFC-12 is beginning to trend down, and trike in the
stratosphere is almost non-existent.
If the ban (or near-ban) were lifted, I'd
guess there would be more than just "small-scale" use. I'd certainly
run out and buy some.
I would too - but I wasn't suggesting that the general ban should be
lifted, only that a total ban is probably unnecessary.
Yes, we were making 100s of thousands of tons per year in the 1980's,
and that is not allowable - but the odd ton, or even kiloton, would not
have a long-lasting or noticeable effect on the ozone layer, and would
allow specialised niche operations where trike is about the only
possibility.
A similar effectively-total ban is the ban on lead in electronic solder.
Ban lead in petrol, in plumbing solder, yes - but I'd like to see some
evidence of harm caused by lead in electronic solder.
We all now know lead-free solder is not really satisfactory, but the ban
was made before that came to be well-known, as part of a general ban on
lead. Is it now time to revisit it?
I did hear some talk about lead leaching out of dumps of used electronic
equipment which had been shipped to Africa, but I don't know the details
- however, another way of preventing this potential harm would be not to
ship old electronics to Africa...
(I read at some point that the rather-quick recovery of the ozone
layer after the Montreal Protocol was adopted was attributed to the
phaseout of 1,1,1-trichloroethane in particular: refrigerants such as
R12, with their longer half-lives, could not have been responsible.
Of course such conclusions are always somewhat speculative in nature,
and you may have better and more recent information.)
No, I'd agree, at least in part. About 10% of the ozone depletion was
caused by trike, and it's pretty much all gone now.
However, apart from the antarctic hole, in general in mid latitudes the
amount of ozone never fell more than about 5-6 percent - we mostly
caught that one in the bud - and in mid latitudes about 1/3 of the fall
has since recovered. It is a bit hard to say for sure, as the natural
variation and annual variation are about the same in magnitude as the
fall, and equatorial regions are suffering a bit.
Or put another way - the present confidence is at least in part due to
the realisation that a lot of the numbers used for the Montreal Protocol
are now regarded as, well, a bit larger than the reality proved.
-- Peter Fairbrother
