^^^ I imagine they chose the largest airports first because they serve the biggest metro areas, which also tend to have the worst air pollution. Or it might just be that there are only 13 airports in California that have enough traffic volume and large enough parking areas to need
shuttles. CARB is also requiring ports, which also serve those metro areas, to clean up both the ship's own emissions by requiring them to use shore-side power while docked (and cleaner fuel while offshore), the MHE in the ports (dem/val projects underway now), the trucks ditto, and I imagine eventually the trains serving those ports. And it's not just 1,000 vehicles; as the article mentioned they're also working on regs for the airport MHE, i.e. all the baggage/fuel/food/tow etc. tractors and trucks; the shuttles are just one step.
I've been unable to find a list of the 13 airports involved, but the list here: https://en.wikipedia.org/wiki/List_of_t ... California
which includes Tijuana, probably has all but one of them.
CARB has also been providing funds (forget if it's yet a requirement) to replace school buses with ZEVs in the most polluted districts, and eventually that will apply across the state, as will be the case with buses generally. What CARB's been doing seems to be working pretty well, although much more remains to be done:
https://www.sfchronicle.com/business/ar ... 066821.php
California slashes emissions, hits major greenhouse gas goal years early