Wildfires are a large source of gases and particles to the atmosphere, both of which impact human health and climate. The amount and composition of particles from wildfires can change with time in the atmosphere; however, the impact of aging is not well understood. In a series of controlled laboratory experiments, we show that the particles are oxidized and a significant fraction of the gas-phase carbon (24 %–56 %) is converted to particle mass over the course of several days in the atmosphere.