A coupled diffusion-chemistry model was developed for the turbulent transport of reactive trace gases in and above a forest canopy. The one-dimensional model was used to study daytime vertical profiles of gaseous concentrations and fluxes and the influences of vertical distributions of solar irradiation and uptake and emission at leaves and the forest floor. The upper boundary of the model was extended to the top of the atmospheric boundary layer to allow adequate coupling at the atmosphere-foi'est interface. To study the effects of biogenic nonmethane hydrocarbons, chemical reactions for isoprene and its atmospheric oxidation products were linked with reactions for inorganic species and the oxidation of CO and CH4. Isoprene emission rates at various heights in the canopy were calculated as a function of local radiation, temperature, and leaf density distribution. Photolysis rates for photochemical reactions were allowed to vary with height according to the change in solar irradiation in the canopy. Vertical profiles of 03, NO, NO 2, NOx, NOy, OH, HNO 3, H20 2, and isoprene concentrations and fluxes simulated for a specified deciduous forest were examined with a single set of measured and computed daytime micrometeorological conditions. Results show that for strongly depositing gases like O3, HNO3, and H20 2, deposition velocities appear to be insensitive to chemistry and to have a profile similar to those predicted for a nonreactive case (simulation without chemistry), although the fluxes are influenced by concentration changes caused bY chemistry. Simulated profiles of is0prene concentration and flux agree closely with results for the nonreactive case, largely because of the dominant effects of emission and turbulent mixing. Chemical reactions have the most important influence on profiles of NO, NO2, and NOx concentrations and fluxes. With a small and representative NO emission forced at the forest floor, NO Concentration decreases quickly with height near the ground and falls to a minimum value near the middle of the canopy because the chemical transformation of NO is fast while photodecomposition of NO 2 is weak inside the canopy.As a result, the NO2 concentration becomes higher inside the canopy than above, and an upward NO2 flux occurs near the canopy top despite NO2 deposition in the canopy. The total flux of NOx near the canopy top appears to be downward because of strong downward NO flux. The flux of NOy above the canopy is almost invariant with height as chemical interchanges create no net effect on the total nitrogen flux, although a large flux divergence caused by dry deposition occurs inside the canopy. 1985; Chameides et al., 1988]. For nonreactive or conserved species the atmosphereforest exchange is modulated primarily by •3hysical and biological processes. These processes involve transport in the atmospheric boundary layer (ABL), turbulent diffusion within the canopy, and molecular diffusion through leaf stomata, whose openings are adjusted by radiation, leaf temperature, and water stress. For ch...
PM, PM, precursor gas, and upper-air meteorological measurements were taken in Mexico City, Mexico, from February 23 to March 22, 1997, to understand concentrations and chemical compositions of the city's particulate matter (PM). Average 24-hr PM concentrations over the period of study at the core sites in the city were 75 H g/m. The 24-hr standard of 150 μ g/m was exceeded for seven samples taken during the study period; the maximum 24-hr concentration measured was 542 μ g/m. Nearly half of the PM was composed of fugitive dust from roadways, construction, and bare land. About 50% of the PM consisted of PM, with higher percentages during the morning hours. Organic and black carbon constituted up to half of the PM. PM concentrations were highest during the early morning and after sunset, when the mixed layers were shallow. Meteorological measurements taken during the field campaign show that on most days air was transported out of the Mexico City basin during the afternoon with little day-to-day carryover.
[1] On the basis of observations from the 1997-2002 Photochemical Ozone Budget of the Northeast Pacific (PHOBEA) experiments, we have identified 11 transpacific long-range transport (LRT) episodes, which contain significantly elevated levels of CO, O 3 , and aerosol scattering. The LRT episodes were determined from aircraft and ground-based observations of CO, O 3 , aerosol scattering coefficient, and 281 whole air samples analyzed for nonmethane hydrocarbons (NMHC). The ratio of excess O 3 to excess CO (DO 3 /DCO) for the 11 LRT episodes ranged from À0.06 to 1.52. Lower DO 3 /DCO ratios (<0.10) are characteristic of LRT episodes transported in the boundary layer or in the presence of substantial mineral dust. These events lack O 3 enhancements, even though O 3 precursors (CO, NMHCs) are elevated. Ratios of DO 3 /DCO of 0.2-0.5 are characteristic of LRT episodes of industrial and/or biomass burning where excess CO is coincident with expected excesses in O 3 . High DO 3 /DCO ratios (>0.50) are found in LRT episodes transported higher in the free troposphere and are probably due to a mixing of LRT pollution plumes with ozone-rich upper tropospheric air. Using PHOBEA observations, backward trajectories, and data from other experiments in the North Pacific (TRACE-P, ACE-Asia, PEM-West B) we calculate OH concentrations using two different methods. For the LRT episodes we obtain mean OH concentrations between 1.9 Â 10 5 and 1.3 Â 10 6 molecules cm À3 . We also present a method using dispersion models and observations to calculate the rate of dilution, k dil , with background air during LRT. A low k dil indicates less mixing with background air during transport, while a high value represents more entrainment with background air. For the April 2001 LRT episode we calculate a mean k dil of 0.010 ± 0.004 hr À1 and an OH radical concentration of 2 Â 10 5 molecules cm À3 . On the basis of these calculations we find that the large mineral dust transport episode, which took place in April 2001, was associated with the lowest OH concentration of the 11 episodes considered, implicating a strong role for heterogeneous chemistry during LRT.
Airborne observations of NMHCs, O3, CO, and aerosol scatter were made near the coast of Washington State from 29 March to 6 May 2001 as part of the Photochemical Ozone Budget of the Eastern North Pacific‐II (PHOBEA‐II) experiment. These observations overlapped the time period of the TRACE‐P (24 February to 10 April 2001) and ACE‐ASIA (27 March to 30 April 2001) experiments operating in the Western Pacific. Measurements were made during 12 flights at 48.31 ± 0.03°N latitude, 124.63 ± 0.08°W longitude at altitudes from 0 to 6 km. On several flights, significant enhancements in all species were observed and are attributed to transport from the Eurasian continent, including a long‐range transport event observed on 14 April 2001. This event contained substantial CO, NMHC, and aerosol loadings and was identified by the Total Ozone Mapping Spectrometer (TOMS) aboard the Earth Probe Satellite and airborne and surface measurements throughout North America. This airmass was unique in that it contained the highest levels of aerosol scatter, CO, and various NMHCs observed in 2001, was the only flight with a low Ångstrom coefficient (0.7) indicating dominance of super micron aerosols, and had a negative relationship between ozone and aerosol scatter (r = −0.30). Within this mineral dust and pollution layer, aerosol scatter, propane, and CO were enhanced by 1054%, 85%, and 36%, respectively, over the observed spring 2001 median values between 3.5 and 6 km. A comparison of our previous aircraft campaign in 1999 with 2001 observations shows that ozone, aerosol scatter, and most NMHCs were significantly lower in the spring of 2001. The exact cause is still under investigation, but the combination of elevated ozone, aerosol scatter, and NMHCs suggests a combustion source that was enhanced and/or transported more efficiently during the spring of 1999.
[1] Ozone production efficiency (OPE) can be defined as the number of ozone (O 3 ) molecules photochemically produced by a molecule of NO x (NO + NO 2 ) before it is lost from the NO x À O 3 cycle. Here we consider observational and modeling techniques to evaluate various operational definitions of OPEs using aircraft and surface measurements taken as part of the 1999 Southern Oxidant Study field campaign in Nashville, Tennessee. A key tool in our analysis is a Lagrangian box model, which is used to quantitatively describe the effects of emissions, dilution, dry deposition, and photochemistry in an urban air parcel as it was advected downwind. After evaluating the model using the observed downwind concentrations of several key species, we show that the modeled NO x oxidation and O 3 production rates as well as the associated instantaneous and cumulative OPEs depend on the time of day and the photochemical age of the air parcel. The observationbased OPEs are found to be consistent with the modeled values with the expected biases. A model sensitivity study suggests that downwind O 3 concentrations in the Nashville plume are more sensitive to NO x emissions than anthropogenic VOC emissions. Because the OPE exhibits a nonlinear dependence on emissions and meteorological effects, it would be difficult to rely only on observations to map out the nonlinear response of O 3 to a wide span of NO x and VOC emission changes. Properly constrained and well-evaluated models using a variety of observations are therefore necessary to reliably predict O 3 -NO x -VOC sensitivity for designing effective O 3 control strategies.
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