Figure 2. The upper panel shows total NO_y concentrations for the period September 16-22 along with the contribution from NO, NO_x, and NO_x + PAN. The lower panel shows isoprene concentrations above the forest canopy at two heights. The divergence between these lines is proportional to the emission flux. Most of the NO_y is present as NO_x on days 259-261 when there is little isoprene present and the solar intensity (not shown) is low. On the sunny days with active isoprene emission the proportion of NO_y present as PAN increases as does the remainder due to HNO₃ possibly other organic nitrates, which were not measured at this time.