La Niña doesn't cool the earth but preferentially stores heat in the ocean. The east Pacific cold tongue gets colder and collects sensible heat from solar insolation, but because it is cold does not have much evaporation (latent heat loss from the ocean). In the west Pacific evaporation from 30 degree water causes significant heat loss, and rainfall, and hurricanes. during el Niño, 30 degree water sloshes back to the east, producing a much larger area where evaporation occurs and movement of heat to the atmosphere.
The aerosols over China may affect the El Niño cycle and thus how heat moves around the surface ocean, but only indirectly. However, they also block incoming solar radiation and help to hide some of the greenhouse gas effects.