When Esperanza first came to work at the park, in 1982, the air pollution was worse than it is today and the smoggy days more numerous. The San Joaquin Valley Air Control District, the responsible agency, has been under unrelenting pressure to improve things. It has pushed diesel-engine retrofits and electric-powered lawn mowers; it has promoted carpooling and educated drivers about unnecessary idling. Last year the air in the valley was the cleanest on record for ozone and fine particulate matter.