Evolution of trace gases and particles emitted by a chaparral fire in California

Biomass burning (BB) is a major global source of trace gases and particles. Accurately representing the pro- duction and evolution of these emissions is an important goal for atmospheric chemical transport models. We measured a suite of gases and aerosols emitted from an 81 hectare pre- scribed fire in chaparral fuels on the central coast of Cali- fornia, US on 17 November 2009. We also measured physi- cal and chemical changes that occurred in the isolated down- wind plume in the first 4 h after emission. The measure- ments were carried out onboard a Twin Otter aircraft outfit- ted with an airborne Fourier transform infrared spectrome- ter (AFTIR), aerosol mass spectrometer (AMS), single par- ticle soot photometer (SP2), nephelometer, LiCor CO2 an- alyzer, a chemiluminescence ozone instrument, and a wing- mounted meteorological probe. Our measurements included: CO2; CO; NOx; NH3; non-methane organic compounds; or- ganic aerosol (OA); inorganic aerosol (nitrate, ammonium, sulfate, and chloride); aerosol light scattering; refractory black carbon (rBC); and ambient temperature, relative hu- midity, barometric pressure, and three-dimensional wind ve- locity. The molar ratio of excess O3 to excess CO in the plume (1O3/1CO) increased from 5.13 (±1.13)◊ 10 3 to 10.2 (±2.16)◊ 10 2 in 4.5 h following smoke emis- sion. Excess acetic and formic acid (normalized to excess CO) increased by factors of 1.73± 0.43 and 7.34± 3.03 (re- spectively) over the same time since emission. Based on the rapid decay of C2H4 we infer an in-plume average OH concentration of 5.27 (±0.97)◊ 10 6 molec cm 3 , consistent with previous studies showing elevated OH concentrations in biomass burning plumes. Ammonium, nitrate, and sulfate all increased over the course of 4 h. The observed ammo- nium increase was a factor of 3.90± 2.93 in about 4 h, but accounted for just 36 % of the gaseous ammonia lost on a molar basis. Some of the gas phase NH3 loss may have been due to condensation on, or formation of, particles be- low the AMS detection range. NOx was converted to PAN and particle nitrate with PAN production being about two times greater than production of observable nitrate in the first 4 h following emission. The excess aerosol light scattering in the plume (normalized to excess CO2) increased by a fac- tor of 2.50± 0.74 over 4 h. The increase in light scattering was similar to that observed in an earlier study of a biomass burning plume in Mexico where significant secondary forma- tion of OA closely tracked the increase in scattering. In the California plume, however, 1OA/1CO2 decreased sharply for the first hour and then increased slowly with a net de- crease of 20 % over 4 h. The fraction of thickly coated rBC particles increased up to 85 % over the 4 h aging period. Decreasing OA accompanied by increased scattering/particle coating in initial aging may be due to a combination of par- ticle coagulation and evaporation processes. Recondensation of species initially evaporated from the particles may have contributed to the subsequent slow rise in OA. We compare our results to observations from other plume aging studies