Showing papers by "Prasad S. Kasibhatla published in 1999"

Effects of ship emissions on sulphur cycling and radiative climate forcing over the ocean

Abstract: The atmosphere overlying the ocean is very sensitive—physically, chemically and climatically—to air pollution Given that clouds over the ocean are of great climatic significance, and that sulphate aerosols seem to be an important control on marine cloud formation1, anthropogenic inputs of sulphate to the marine atmosphere could exert an important influence on climate Recently, sulphur emissions from fossil fuel burning by international shipping have been geographically characterized2, indicating that ship sulphur emissions nearly equal the natural sulphur flux from ocean to atmosphere in many areas3 Here we use a global chemical transport model to show that these ship emissions can be a dominant contributor to atmospheric sulphur dioxide concentrations over much of the world's oceans and in several coastal regions The ship emissions also contribute significantly to atmospheric non-seasalt sulphate concentrations over Northern Hemisphere ocean regions and parts of the Southern Pacific Ocean, and indirect radiative forcing due to ship-emitted particulate matter (sulphate plus organic material) is estimated to contribute a substantial fraction to the anthropogenic perturbation of the Earth's radiation budget The quantification of emissions from international shipping forces a re-evaluation of our present understanding of sulphur cycling and radiative forcing over the ocean

Simulated tropospheric NOx : Its evaluation, global distribution and individual source contributions

Abstract: Using the 11-level Geophysical Fluid Dynamics Laboratory global chemical transport model, we simulate global tropospheric fields of NO x , peroxyacetyl nitrate (PAN), HNO 3 , and NO y , as well as the deposition of nitrate, extensively evaluate them against available observations from surface stations and aircraft missions, and quantify the contributions of individual natural and anthropogenic sources The patterns and magnitudes of simulated and observed HNO 3 wet deposition are generally in good agreement around the globe Scatterplots of model simulations versus aircraft observations for NO x and NO y find ∼50% of the points within ±25%, find -75% within ±50%, and show no systematic global biases Both simulated and observed vertical profiles have similar shapes with high levels (∼1 ppbv or greater) in the polluted boundary layer (BL), very low values in the remote BL, and values increasing from the middle to the upper troposphere Simulated NO y HNO 3 , and NO x + PAN are also in good agreement with extensive lower free tropospheric ( ) observations made at Mauna Loa Observatory In general, the level of agreement between simulation and observation is as good as the agreement between separate, but simultaneous, observations of NO, NO x or NO y As previous studies have shown, fossil fuel combustion and biomass burning control NO x levels in most of the lower half of the troposphere with a significant contribution from biogenic emissions The exceptions are the remote low-NO x regions where BL and FT sources make comparable contributions Unlike most previous studies, we find that the much smaller in situ FT sources generally dominate in the upper half of the troposphere Lightning dominates in the tropics and summertime midlatitudes, and stratospheric injection is the major source in the summer high latitudes The exception is transported emissions from fossil fuel combustion, which dominate in winter high latitudes Though seldom dominant, aircraft emissions do have a significant impact on the upper troposphere and lower stratosphere of the northern hemisphere extratropics

Is aerosol production within the remote marine boundary layer sufficient to maintain observed concentrations

Abstract: To evaluate the impact of aerosols on climate we must consider the aerosol dynamics of the remote marine atmosphere. Marine aerosols are subject to losses due to precipitation, dry deposition, and coagulation; yet, observed remote marine aerosol concentrations and size distributions are relatively constant. This maintenance of the aerosol distribution requires a particle source. This work focuses on the potential of H2SO4 nucleation within the marine boundary layer (MBL) to supply these particles. Spatial and temporal variability in meteorology and species concentrations are considered in a mathematical model to evaluate the effect of natural deviations from average MBL conditions on the highly nonlinear aerosol system. A dynamic, vertically dimensioned, size-resolved aerosol model is used with parameterized heterogeneous chemical processes. The results suggest that MBL nucleation may be an important source of new particle number in the remote MBL. However, though our model shows that typical remote MBL aerosol distributions can on average be maintained by MBL nucleation and sea-salt emissions, large oscillations in particle number concentration occur. Because such oscillations are only occasionally reported in measurements, MBL nucleation may not be the dominant source of new particles in the remote MBL. The nucleation events, which cause these oscillations, are predicted to occur at the top of the MBL after rain and/or entrainment of clean free tropospheric air. Predictions are particularly sensitive to the H2SO4 accommodation coefficient, nucleation tuner, and washout efficiency. Reduction of the accommodation coefficient is shown to increase the predicted accumulation mode concentration because the nucleation rate is enhanced. Entrainment of clean free tropospheric air is shown to increase the frequency of nucleation events within the MBL and may help to explain the observed correlation between subsidence and MBL small particles. A small constant addition of particles from the free troposphere, ocean, etc. suppresses H2SO4 nucleation and can lead to a reduction in total predicted aerosol number. This is because nucleation events require low total aerosol surface area and the constant addition of particles reduces the severity of aerosol surface area minimums. Larger external aerosol sources can maintain the observed remote MBL aerosol distribution. However, the temporal and spatial variability of such sources could have a large impact on aerosol concentrations and requires further investigation.

A mass-balance/photochemical assessment of DMS sea-to-air flux as inferred from NASA GTE PEM-West A and B observations

Abstract: This study reports dimethyl sulfide (DMS) sea-to-air fluxes derived from a mass-balance/photochemical-modeling approach. The region investigated was the western North Pacific covering the latitude range of 0°–30°N. Two NASA airborne databases were used in this study: PEM-West A in September-October 1991 and PEM-West B in February-March 1994. A total of 35 boundary layer (BL) sampling runs were recorded between the two programs. However, after filtering these data for pollution impacts and DMS lifetime considerations, this total was reduced to 13. Input for each analysis consisted of atmospheric DMS measurements, the equivalent mixing depth (EMD) for DMS, and model estimated values for OH and NO3. The evaluation of the EMD took into account both DMS within the BL as well as that transported into the overlying atmospheric buffer layer (BuL). DMS fluxes ranged from 0.6 to 3.0 μmol m−2 d−1 for PEM-West A (10 sample runs) and 1.4 to 1.9 μmol m−2 d−1 for PEM-West B (3 sample runs). Sensitivity analyses showed that the photochemically evaluated DMS flux was most influenced by the DMS vertical profile and the diel profile for OH. A propagation of error analysis revealed that the uncertainty associated with individual flux determinations ranged from a factor of 1.3 to 1.5. Also assessed were potential systematic errors. The first of these relates to our noninclusion of large-scale mean vertical motion as it might appear in the form of atmospheric subsidence or as a convergence. Our estimates here would place this error in the range of 0 to 30%. By far the largest systematic error is that associated with stochastic events (e.g., those involving major changes in cloud coverage). In the latter case, sensitivity tests suggested that the error could be as high as a factor of 2. With improvements in such areas as BL sampling time, direct observations of OH, improved DMS vertical profiling, direct assessment of vertical velocity in the field, and preflight (24 hours) detailed meteorological data, it appears that the uncertainty in this approach could be reduced to ±25%.