Showing papers by "Prasad S. Kasibhatla published in 1996"

Simulated global tropospheric PAN: Its transport and impact on NO x

Abstract: Using the 11-level Geophysical Fluid Dynamics Laboratory (GFDL) global chemical transport model (GCTM) with all known sources of tropospheric NOx, we simulate the global tropospheric distribution of peroxyacetyl nitrate (PAN) and quantify its impact on tropospheric NOx. The model's global distribution of PAN is in reasonable agreement with most available observations. In the atmospheric boundary layer, PAN is concentrated over the continental sites of NOx emissions, primarily the midlatitudes in the northern hemisphere and the subtropics in the southern hemisphere. PAN is distributed relatively zonally throughout the free troposphere of the northern hemisphere, with the maximum levels found in the coldest regions, while in the southern hemisphere the maximum PAN levels are found in an equator to 30°S belt stretching from South America to Australia. Overall, the simulated three-dimensional fields of seasonal PAN are a result of the interaction of the type of transport meteorology (convective or synoptic scale storms) occurring in the PAN formation regions and PAN's temperature-dependent lifetime. We find the impact of PAN chemistry on NOx to be rather subtle. The magnitude and the seasonal cycle of the global tropospheric integral of NOx, which has its maximum in January and the formation of HNO3 as its dominant loss path, are barely affected by the inclusion of PAN chemistry, however PAN, as a result of its temperature sensitivity and transport, regionally provides an efficient mechanism for redistributing NOx far from its source areas. With the inclusion of PAN chemistry, monthly mean NOx concentrations increase by up to a factor of 5 in the remote lower troposphere and show a spring maximum over areas of the North Atlantic and North Pacific Oceans. In contrast, PAN has only a minor impact in the upper half of the troposphere (±10%). Examining local time series of NOx and PAN, the monthly mean mixing ratios in remote regions are shown to be composed of numerous short-term (1–2 days) large magnitude events. These episodes are large enough to potentially result in ozone production even when the monthly mean NOx values are in the ozone destruction range. While both the direct transport of NOx and its indirect transport as PAN contribute to the elevated NOx episodes over the remote extratropical oceans, events over the remote subtropical oceans are dominated by midtropospheric PAN that sinks anticyclonically equatorward and decomposes to NOx in the warmer air.

A global three‐dimensional time‐dependent lightning source of tropospheric NOx

Abstract: The spatial and temporal distribution for a global three-dimensional, time-dependent lightning source of NO x is constructed from a general circulation model's (GCM) deep moist convection statistics [Manabe et al., 1974 ; Manabe and Holloway, 1975], observations of cloud-to-cloud and intracloud lightning fractions and the vertical distribution of lightning discharge [Proctor, 1991], and empirical/theoretical estimates of relative lightning frequency resulting from deep moist convection over ocean and over land [Price and Rind, 1992]. We then bracket the annual global emission of NO x from lightning between 2 and 6 Tg N/yr, with a most probable range of 3 to 5 Tg N/yr, by comparing tropospheric NO x simulations from the Geophysical Fluid Dynamics Laboratory Global Chemical Transport Model with measurements of NO x and/or NO y in the mid and upper troposphere where lightning is a major, if not the dominant, source. With this approach, the global magnitude of the lightning source is constrained by observed levels of NO x , while the temporal and spatial distributions of the source are under the control of the parent GCM. Although our lightning source is smaller than many previous estimates, it is still the major source of NO x and NO y in the mid and upper troposphere for a latitude belt running from 30°N to 30°S, an important contributor to summertime free tropospheric levels over the midlatitudes, and a major contributor, even in the lower troposphere, to the low NO x and NO y levels over the remote oceans.

Transport-induced interannual variability of carbon monoxide determined using a chemistry and transport model

Abstract: Transport-induced interannual variability of carbon monoxide (CO) is studied during 1989–1993 using the Goddard chemistry and transport model (GCTM) driven by assimilated data. Seasonal changes in the latitudinal distribution of CO near the surface and at 500 hPa are captured by the model. The annual cycle of CO is reasonably well simulated at sites of widely varying character. Day to day fluctuations in CO due to synoptic waves are reproduced accurately at remote North Atlantic locations. By fixing the location and magnitude of chemical sources and sinks, the importance of transport-induced variability is investigated at CO-monitoring sites. Transport-induced variability can explain 1991–1993 decreases in CO at Mace Head, Ireland, and St. David's Head, Bermuda, as well as 1991–1993 increases in CO at Key Biscayne, Florida. Transport-induced variability does not explain decreases in CO at southern hemisphere locations. The model calculation explains 80–90% of interannual variability in seasonal CO residuals at Mace Head, St. David's Head, and Key Biscayne and at least 50% of variability in detrended seasonal residuals at Ascension Island and Guam. Upper tropospheric interannual variability during October is less than 8% in the GCTM. Exceptions occur off the western coast of South America, where mixing ratios are sensitive to the strength of an upper tropospheric high, and just north of Madagascar, where concentrations are influenced by the strength of offshore flow from Africa.

Three‐dimensional view of the large‐scale tropospheric ozone distribution over the North Atlantic Ocean during summer

Abstract: A global chemical transport model is used to study the three-dimensional structure of the tropospheric ozone (O3) distribution over the North Atlantic Ocean during summer. A simplified representation of summertime O3 photochemistry appropriate for northern hemisphere midlatitudes is included in the model. The model is evaluated by comparing simulated O3 mixing ratios to summertime O3 measurements taken in and near the North Atlantic Ocean basin. The model successfully reproduces (1) the means and standard deviations of ozonesonde measurements over North America at 500 mbar; (2) the statistical characteristics of surface O3 data at Sable Island off the coast of North America and at Bermuda in the western North Atlantic; and (3) the mean midtropospheric O3 measured at Bermuda and also at the Azores in the eastern North Atlantic. The model underestimates surface O3 in the eastern North Atlantic, overestimates O3 in the lower free troposphere over the western North Atlantic, and also has difficulty simulating the upper tropospheric ozonesonde measurements over North America. An examination of the mean summertime O3 distribution simulated by the model shows a significant continental influence on boundary layer and free-tropospheric O3 over the western North Atlantic. The model has also been exercised using a preindustrial NOx emission scenario. By comparing the present-day and preindustrial simulations, we conclude that anthropogenic NOx emissions have significantly perturbed tropospheric O3 levels over most of the North Atlantic. We estimate that present-day O3 levels in the lower troposphere over the North Atlantic are at least twice as high as corresponding preindustrial O3 levels. We find that the anthropogenic impact is substantial even in the midtroposphere, where modeled present-day O3 mixing ratios are at least 1.5 times higher than preindustrial O3 levels.