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Hydrofluorocarbons (HFCs) HFCs are man-made chemicals, many of which have been developed as alternatives to ozone-depleting substances (ODS) for industrial, commercial, and consumer products.

The global warming potentials of HFCs range from 140 (HFC-152a) to 11,700 (HFC-23).

The atmospheric lifetime for HFCs varies from just over a year for HFC-152a to 260 years for HFC-23.

Most of the commercially used HFCs have atmospheric lifetimes less than 15 years; e.g., HFC-134a, which i sused in automobile air conditioning and refrigeration, has an atmospheric life of 14 years.

Perfluorocarbons (PFCs) Primary aluminum production and semiconductor manufacture are the largest known man-made sources of two perfluorocarbons – CF4 (tetrafluoromethane) and C2F6 (hexafluoroethane).

The GWP of CF4 and C2F6 emissions is equivalent to approximately 6,500 and 9,200 tonnes, respectively. PFCs are also relatively minor substitutes for ozone-depleting substances (ODSs).

PFCs have extremely stable molecular structures and are largely immune to the chemical processes in the lower atmosphere that break down most atmospheric pollutants. Not until the PFCs reach the mesosphere, about 60 kilometers above Earth, do very high-energy ultraviolet rays from the sun destroy them. This removal mechanism is extremely slow and as a result PFCs accumulate in the atmosphere and remain there for several thousand years.

The estimated atmospheric lifetimes for CF4 and C2F6 are 50,000 and 10,000 years respectively.

Sulfur Hexafluoride (SF6) The global warming potential of SF6 is 23,900, making it the most potent greenhouse gas the IPCC has evaluated.

SF6 is used for insulation and current interruption in electric power transmission and distribution equipment, in the magnesium industry to protext molten magnesium from oxidation and potentially violent burning, in semiconductor manufacturing to create circuitry patterns on silicon wafers, and as a tracer gas for leak detection.

Like the other high GWP gases, there are very few sinks for SF6, so all man-made sources contribute directly to its accumulation in the atmosphere. Measurements of SF6 show that its global average concentration has increased by about 7% per year during the 1980s and 1990s, from less 1 ppt in 1980 to almost 4 ppt in the late 1990’s (IPCC, 2001).

HFC-134a has an atmospheric lifetime of about 14 years and its abundance is expected to continue to rise in line with its increasing use as a refrigerant around the world.

The only significant emissions of HFCs before 1990 were of the chemical HFC-23

Between 1978 and 1995, HFC-23 concentrations have increased from 3 to 10 parts per trillion (ppt), and continue to rise. Since 1990, when it was almost undetectable, global average concentrations of HFC-134a have risen significantly to almost 10 ppt (parts per trillion).

Its green-building standards are the toughest in the nation.

PORTLAND, OREGON population 533,492

San Francisco proves its worthiness with progressive purchasing policies (including phasing out toxic products and those from sweatshops)

$100 million invested in solar power

and an innovative study of the potential for generating renewable energy from the waves off its shores.

The city's acclaimed recycling program also contributes to its top-notch culinary reputation by sending compost made of food scraps to the region's famed vineyards and farms.

SAN FRANCISCO population 744,230

Mayor Greg Nickels (D) brought Seattle into the national spotlight when he launched the U.S. Mayors Climate Protection Agreement to reduce global-warming pollution nationwide.

The city government is retrofitting its heavy-duty diesel vehicles with devices that will cut particulate pollution in half

By investing in renewable energy and efficiency programs to offset its contributions to global warming, the city-owned utility has become the first in the country to reduce its net greenhouse-gas emissions to zero.