Photochemistry of biogenic emissions over the Amazon Forest

The boundary layer chemistry over the Amazon forest during the dry season is simulated with a photochemical model. Results are in good agreement with measurements of isoprene, NO, ozone, and organic acids. Photochemical reactions of biogenic isoprene and NO_x can supply most of the ozone observed in the boundary layer. Production of ozone is very sensitive to the availability of NO_x, but is insensitive to the isoprene source strength. High concentrations of total odd nitrogen (NO_y,) are predicted for the planetary boundary layer, about 1 ppb in the mixed layer and 0.75 ppb in the convective cloud layer. Most of the odd nitrogen (≈70%) is present as PAN-type species, which are removed by dry deposition to the forest. The observed daytime variations of isoprene are explained by a strong dependence of the isoprene emission flux on sun angle. Nighttime losses of isoprene exceed rates of reaction with NO₃ and O₃ and appear to reflect dry-deposition processes. The 24-hour averaged isoprene emission flux is calculated to be 38 mg m^-2 d^-1. Photooxidation of isoprene could account for a large fraction of the CO enrichment observed in the boundary layer under unpolluted conditions and could constitute an important atmospheric source of formic acid, methacrylic acid, and pyruvic acid.