Dry deposition of pan to grassland vegetation

Peroxyacetyl nitrate or PAN (CH{sub 3}C(O)OONO{sub 2}) is formed in the lower troposphere via photochemical reactions involving nitrogen oxides (NO{sub x}) and non-methane hydrocarbons (NMHCs). PAN has a lifetime in the free troposphere of about three months and is removed by photolysis or reaction with OH. Dry deposition will decrease its lifetime, although the few measurements that have been made indicate that this process is slow. Measurements of the uptake of PAN by alfalfa in growth chambers indicated that the dry deposition velocity (downward flux divided by concentration at a specified height) was 0.75 cm s{sup {minus}1}. Garland and Penkett measured a dry deposition velocity of 0.25 cm s{sup {minus}1} for PAN to grass and soil in a return-flow wind tunnel. Shepson et al. (1992) analyzed trends of PAN and O{sub 3} concentrations in the stable nocturnal boundary layer over mixed deciduous/coniferous forests at night, when leaf stomata were closed, and concluded that the deposition velocity for PAN was at least 0.5 cm s{sup {minus}1}. We measured the dry deposition velocity of PAN to a grassland site in the midwestern United States with a modified Bowen ratio technique. Experiments were conducted on selected days during September, October, and November of 1990. An energy balance Bowen ratio station was used to observe the differences in air temperature and water vapor content between heights of 3.0 and 0.92 m and to evaluate the surface energy balance. Air samples collected at the same two heights in Teflon {reg_sign} bags were analyzed for PAN by a gas chromatographic technique. We present an example of the variations of PAN concentrations and gradients observed during the day and compare measurements of the dry deposition velocity to expectations based on the physicochemical properties of PAN

Dynamics of ozone and nitrogen oxides at Summit, Greenland. II. Simulating snowpack chemistry during a spring high ozone event with a 1-D process-scale model

Observed depth profiles of nitric oxide (NO), nitrogen dioxide (NO2), and ozone (O3) in snowpack interstitial air at Summit, Greenland were best replicated by a 1-D process-scale model, which included (1) geometrical representation of snow grains as spheres, (2) aqueous-phase chemistry confined to a quasi-liquid layer (QLL) on the surface of snow grains, and (3) initialization of the species concentrations in the QLL through equilibrium partitioning with mixing ratios in snowpack interstitial air. A comprehensive suite of measurements in and above snowpack during a high O3 event facilitated analysis of the relationship between the chemistry of snowpack and the overlying atmosphere. The model successfully reproduced 2 maxima (i.e., a peak near the surface of the snowpack at solar noon and a larger peak occurring in the evening that extended down from 0.5 to 2 m) in the diurnal profile of NO2 within snowpack interstitial air. The maximum production rate of NO2 by photolysis of nitrate (NO3-) was approximately 108 molec cm-3 s-1, which explained daily observations of maxima in NO2 mixing ratios near solar noon. Mixing ratios of NO2 in snowpack interstitial air were greatest in the deepest layers of the snowpack at night and were attributed to thermal decomposition of peroxynitric acid, which produced up to 106 molec NO2 cm-3 s-1. Highest levels of NO in snowpack interstitial air were confined to upper layers of the snowpack and observed profiles were consistent with photolysis of NO2. Production of nitrogen oxides (NOx) from NO3- photolysis was estimated to be two orders of magnitude larger than NO production and supports the hypothesis that NO3- photolysis is the primary source of NOx within sunlit snowpack in the Arctic. Aqueous-phase oxidation of formic acid by O3 resulted in a maximum consumption rate of ~106–107 molec cm-3 s-1 and was the primary removal mechanism for O3