What odor reminds Palmer of Pigeon Day?

The smell of smoke reminds Palmer of Pigeon Day long after it is over.

"... he began smelling the gray and sour odor even when his father wasn't there, even when Pigeon Day was over. It might happen in the morning as he sat in school or at night as he lay in bed. It could even happen in his father's lap in the middle of winter, when the shotgun had been locked away for months."