Measurements of the dry deposition velocity of PAN above grass

The modified Bowen ratio method was used to measure the dry deposition velocity of peroxyacetyl nitrate (PAN) during July and October above grass. Ambient air samples were collected in bags at heights of 0.92 and 3.0 m for periods of 30 min and analyzed within 30 min of collection using high-resolution gas chromatography. By comparison to water vapor gradients and fluxes measured concurrently with an energy balance Bowen ratio system, the gradients in PAN concentrations were evaluated to yield the vertical flux densities and deposition velocities. On average, the deposition velocity was 0.13 cm/s during daytime conditions. A theoretical analysis suggested that most of the uptake was through the plant leaf stomata at a rate that is commensurate with the physicochemical properties of PAN. These results indicate that the deposition velocity for PAN above vegetation appears to be a factor of 3-5 smaller than that for ozone and that the removal of PAN by dry deposition is considerably smaller than the loss by thermal decomposition in the planetary boundary layer during warm conditions.