Showing papers by "Prasad S. Kasibhatla published in 1997"

Evaluation and intercomparison of global atmospheric transport models using 222Rn and other short-lived tracers

Abstract: Simulations of 222Rn and other short-lived tracers are used to evaluate and intercompare the representations of convective and synoptic processes in 20 global atmospheric transport models. Results show that most established three-dimensional models simulate vertical mixing in the troposphere to within the constraints offered by the observed mean 222Rn concentrations and that subgrid parameterization of convection is essential for this purpose. However, none of the models captures the observed variability of 222Rn concentrations in the upper troposphere, and none reproduces the high 222Rn concentrations measured at 200 hPa over Hawaii. The established three-dimensional models reproduce the frequency and magnitude of high-222Rn episodes observed at Crozet Island in the Indian Ocean, demonstrating that they can resolve the synoptic-scale transport of continental plumes with no significant numerical diffusion. Large differences between models are found in the rates of meridional transport in the upper troposphere (interhemispheric exchange, exchange between tropics and high latitudes). The four two-dimensional models which participated in the intercomparison tend to underestimate the rate of vertical transport from the lower to the upper troposphere but show concentrations of 222Rn in the lower troposphere that are comparable to the zonal mean values in the three-dimensional models.

A three‐dimensional global model investigation of seasonal variations in the atmospheric burden of anthropogenic sulfate aerosols

Abstract: A global three-dimensional chemical transport model is used to investigate seasonal variations of anthropogenic sulfur in the troposphere. Particular emphasis is placed on detailed comparisons of the modeled surface sulfur dioxide (SO2) and sulfate (SO4) concentrations and sulfate wet deposition fluxes with measurements from the Eulerian Model Evaluation Field Study (EMEFS) and Cooperative Program for Monitoring and Evaluation of the Long Range Transmission of Air Pollutants in Europe (EMEP) field programs in North America and Europe, respectively. Initial comparisons of model results with measurements reveal a systematic tendency of the model to overestimate SO2 concentrations and underestimate SO4 concentrations while producing a reasonable fit to measured wet deposition fluxes. Through a series of sensitivity tests we find that the addition of a nonphotochemical pathway for converting SO2 to SO4 in the boundary layer with a pseudo first-order rate of constant of 1–2×10−6 s−1 provides the most reasonable method of bringing the model results into better agreement with the EMEFS and EMEP data sets. We propose that this additional pathway may be related to heterogeneous reactions between SO2 and atmospheric aerosols that typically are not included in models of the atmospheric sulfur cycle. Despite the vastly improved simulation of surface SO2 and SO4 when this hypothetical heterogeneous oxidation pathway is included, the model is unable to simultaneously simulate the large seasonal cycle in surface SO4 observed over eastern North America and the almost total absence of a seasonal cycle in surface SO4 over Europe. The seasonal cycles in model-predicted column SO4 burdens are similar, but not identical, to those for surface SO4 because of regional differences in transport, free tropospheric oxidation, and in-cloud removal. We find that the summer-to-winter ratio in column SO4 is larger over eastern North America than it is over Europe; however, both are larger than that for eastern Asia, where wintertime column SO4 is predicted to exceed summertime column SO4.

The global impact of human activity on tropospheric ozone

Abstract: Within a conceptual framework of stratospheric injection, CO-CH4 background tropospheric chemistry, parameterized pollution production in the continental boundary layer and surface deposition, we use an 11 level GCTM to simulate global distributions of present and pre-industrial tropospheric O3. The chemistry is driven by previously simulated present and pre-industrial NOx fields, while prescribed fields of CO, CH4 and H2O are held constant. An evaluation with measurements from 12 surface sites, 21 ozonesonde sites and 1 aircraft campaign finds agreement within ±25% for 73% of the observations while identifying systematic errors in the wintertime high-latitude Northern Hemisphere (NH), the Southern Hemisphere (SH) tropics during biomass burning, and the remote SH. We predict that human activity has increased the annual integral of tropospheric ozone by 39% with 3/4's of that increase in the free troposphere, though the boundary layer [BL] annual integral has increased by 66%. The 2 largest components of the global O3 budget are stratospheric injection at 696 TgO3/yr, and loss through dry deposition, which increases from 459 TgO3/yr to a present level of 825 TgO3/yr. While tropospheric chemistry's net contribution is relatively small, changing from a pre-industrial destruction of −236 TgO3/yr to a present production of +128 TgO3/yr, it is a balance between two much larger terms, −558 TgO3/yr of destruction in the background troposphere and +686 TgO3/yr of production in the polluted boundary layer. Human impact on O3 predominates in the summertime extratropical NH and in the tropics during their biomass burning seasons [increases of 50%–100% or more]. Conversely, there has been little increase in most of the upper troposphere [<20%], where ozone's influence on tropospheric climate is strongest.

Results from the Intergovernmental Panel on Climatic Change Photochemical Model Intercomparison (PhotoComp)

Abstract: Results from the Intergovernmental Panel on Climatic Change (IPCC) tropospheric photochemical model intercomparison (PhotoComp) are presented with a brief discussion of the factors that may contribute to differences in the modeled behaviors of HOx cycling and the accompanying O-3 tendencies. PhotoComp was a tightly controlled model experiment in which the IPCC 1994 assessment sought to determine the consistency among models that are used to predict changes in tropospheric ozone, an important greenhouse gas, Calculated tropospheric photodissociation rates displayed significant differences, with a root-mean-square (rms) error of the reported model results ranging from about +/-6-9% of the mean (for O-3 and NO2) to up to +/-15% (H2O2 and CH2O). Models using multistream methods in radiative transfer calculations showed distinctly higher rates for photodissociation of NO2 and CH2O compared to models using two-stream methods, and this difference accounted for up to one third of the rms error for these two rates, In general, some small but systematic differences between models were noted for the predicted chemical tendencies in cases that did not include reactions of nonmethane hydrocarbons (NMHC). These differences in modeled O-3 tendencies in some cases could be identified, for example, as being due to differences in photodissociation rates, but in others they could not and must be ascribed to unidentified errors. O-3 tendencies showed rms errors of about +/-10% in the moist, surface level cases with NOx concentrations equal to a few tens of parts per trillion by volume. Most of these model to model differences can be traced to differences in the destruction of O-3 due to reaction with HO2. Differences in HO2, in turn, are likely due to (1) inconsistent reaction rates used by the models for the conversion of HO2 to H2O2 and (2) differences in the model-calculated photolysis of H2O2 and CH2O. In the middle tropospheric ''polluted'' scenario with NOx concentrations larger than a few parts per billion by volume, O-3 tendencies showed rms errors of +/-10-30%. These model to model differences most likely stem from differences in the calculated rates of O-3 photolysis to O(D-1), which provides about 80% of the HOx source under these conditions. The introduction of hydrocarbons dramatically increased both the rate of NOx loss and its model to model differences, which, in turn, are reflected in an increased spread of predicted O-3. Including NMHC in the simulation approximately doubled the rms error for O-3 concentration.

Gas‐to‐particle conversion of tropospheric sulfur as estimated from observations in the western North Pacific during PEM‐West B

Abstract: Aircraft observations during the Pacific Exploratory Mission in the western Pacific Ocean, phase B (PEM-West B), taken in February–March 1994, have been used to constrain a numerical model that calculates local concentrations of gaseous H2SO4 rates of homogeneous nucleation, and concentrations of newly formed, nanometer-sized particles The data was selected from 13 flights over the western Pacific Ocean that covered an altitude range from the boundary layer (BL) to the upper troposphere (UT) and latitudes from 10°S to 60°N The largest nucleation rates were calculated for the data from the flights over the temperate latitudes (λ>30°N) Within these latitudes, homogeneous nucleation rates averaged about 1–100 particles cm−3 s−1 Significantly smaller nucleation rates were calculated for the tropical (λ<20°N) and subtropical (20°N<λ<30°N) regions In the tropics, average nucleation rates in excess of 10 particles cm−3 s−1 were limited to the UT In the subtropics, large average nucleation rates in excess of 1 particle cm−3 s−1 were obtained in the BL and in the UT, and average rates of about 10−1 particles cm−3 s−1 were obtained for the rest of the troposphere The relatively large nucleation rates calculated for the temperate latitudes could be largely attributed to the cold temperatures encountered in this region during the PEM-West B flights For the data from the tropical and subtropical flights, little or no homogeneous nucleation was calculated for the average conditions encountered in the BL and midtroposphere (MT) Instead, significant nucleation was limited either to the UT or to several small-scale events These enhanced nucleation events were generally characterized by spikes in relative humidity and low aerosol surface density However, the strongest nucleation events, with homogeneous nucleation rates of about 10 particles cm−3 s−1, were associated with high concentrations of SO2, most likely as a result of pollution from the Asian continent Our results imply that in regions in which homogeneous nucleation is dominated by small-scale fluctuations, approaches that attempt to infer nucleation rates using average or typical conditions will grossly underestimate the actual average rate of nucleation

Impact of inert organic nitrate formation on ground-level ozone in a regional air quality model using the carbon bond mechanism 4

Abstract: A regional air quality model is used to assess the impact of inert organic nitrate formation on ground-level ozone in the eastern United States during summer. The chemical mechanism used is the Carbon Bond Mechanism 4 (CBM4), which is widely used by regulatory agencies in the United States in air quality modeling applications. Recently, modifications were made to the reaction mechanism involving the organic peroxy radicals which form inert organic nitrates without a critical scientific review of the effects of these changes. In this study, we demonstrate for the first time that the simulated large-scale distribution of ground-level ozone is extremely sensitive to these mechanism changes. Inclusion of radical-radical reactions involving the organic peroxy radicals suppresses inert organic nitrate formation, and leads to significant increases in nitrogen oxide levels over large parts of the model domain. As a consequence of increased rates of ozone photochemical production, ozone mixing ratios are enhanced by as much 10–25 ppbv when these additional radical termination pathways are considered in the model.