When did unleaded gasoline become available for sale (as an option, not mandatory) in California? That is, suppose you were a driver a few decades ago, who wanted to drive without pumping lead into the air. In what year would it have become possible to do so?
First, to be clear, lead is an additive and doesn't occur naturally in gasoline. It was in 1921 that General Motors discovered that adding lead (in the form of lead tetraethyl or TEL) to gasoline is a cheap way to increase octane and prevent engine knocking. So it would have been possible before that.
Soon after, in 1925 and 1926, there was a temporary ban at the federal level lasting about a year. After that, adding lead was the standard practice. Federal restrictions on lead in gasoline were phased in gradually, starting in 1974 and completed by 1986. A key factor in moving away from lead was the introduction of catalytic converters, which would quickly be destroyed by leaded gasoline.
Most of this is described in detail in "The Secret History of Lead", an article published by The Nation in 2000. So far I have found no evidence that lead-free gasoline was commercially available in California or any other state between May 1926 and 1974.
EDIT: The article "The Rise and Fall of Organometallic Additives in Automotive Gasoline" tracks the unleaded offerings of American gasoline companies in detail. Although the following quotes would seem to contradict, they imply that some unleaded gasoline was always available, typically as a "premium" product: Sun and/or Amoco.
In 1936, Ethyl reported that 72% of US gasoline was leaded (Anonymous, 1936a); by 1940, this reached 88% and TEL was blended into all but one major brand (Anonymous, 1940b). Sun, the sole brand, used catalytic cracking (then known as the Houdry Process) to produce a high-octane unleaded grade (then dyed blue). Sun did not offer a leaded grade until 1950 (Johnson, 1983).
Indiana Standard had always offered a high-octane unleaded grade. This unleaded grade (known as Amoco-Gas and later Amoco Super Premium) was originally produced by the American Oil Company, who became affiliated with Indiana Standard in 1922 (Dedmon, 1984).
Otherwise, most major producers began offering an unleaded product in 1970 or shortly after.
I drove from my home town of Quincy, Illinois to St. Louis, Missouri occasionally in 1962 after beginning work at the Chevrolet Division of General Motors there. I recall a Shell Oil gasoline station along the route that advertised either "Low Lead" or No Lead" or "Lead Free" gasoline. I recall at the time thinking and discussing with friends why anyone would buy such gasoline as it was sold at a higher price than gasoline with lead additives. I owned a Mobil Oil gasoline station in Royal Oak, Michigan the last 6 months of 1969 and sold gasoline with lead as regular for $0.32.9 and premium for $0.36.9 a gallon. By the end of 1969, I was selling regular for $0.19.9 and premium for $0.23.9 a gallon. The newest thing on the market at that time addressing the environment was the requirement for all cars to include positive crankcase ventilation (PCV).
Around 1970 is when I first saw the low lead and unleaded gasolines go on sale.
This was pursuant to the Clean Air Act, which instituted and ramped up over time, restrictions on auto emissions. TEL would ruin catalytic converters that were due to be mandated by 1974, so the gasoline industry began ramping up production and distribution ahead of time.
Note in this wiki that while TEL production has almost ceased in the US and Europe, it is being produced in China for local fuel sales.