Showing papers by "Prasad S. Kasibhatla published in 1998"

DMS oxidation in the Antarctic marine boundary layer: Comparison of model simulations and held observations of DMS, DMSO, DMSO2, H2SO4(g), MSA(g), and MSA(p)

Abstract: A sulfur field study (SCATE) at Palmer Station Antarctica (January 18 to February 25) has revealed several major new findings concerning (dimethyl sulfide) DMS oxidation chemistry and the cycling of sulfur within the Antarctic environment. Significant evidence was found supporting the notion that the OH/DMS addition reaction is a major source of dimethyl sulfoxide (DMSO). Methane sulfonic acid (MSA(g)) levels were also found to be consistent with an OH/DMS addition mechanism involving the sequential oxidation of the products DMSO and methane sulfinic acid (MSIA). Evidence supporting the hypothesis that the OH/DMS addition reaction, as well as follow-on reactions involving OH/DMSO, are a major source of SO2 was significant, but not conclusive. No evidence could be found supporting the notion that reactive intermediates (i.e., SO3) other than SO2 were an important source of H2SO4. Quite clearly, one of the major findings of SCATE was the recognition that a large fraction of the Antarctic oxidative cycle for DMS (near Palmer Station) took place above the boundary layer (BL) in what we have labeled here as the atmospheric buffer layer (BuL). Although still speculative in places, the overall picture emerging from the SCATE field/modeling results is one involving major coupling between chemistry and dynamics in the Antarctic. At Palmer the evidence points to frequent episodes of rapid vertical transport from a very shallow marine BL into the overlying BuL. Due to the combination of a long photochemical lifetime for DMS and the frequency of shallow convective events, a large fraction of ocean released DMS is transported into the BuL while still in its unoxidized state. There, in the presence of elevated OH and low aerosol scavenging, high levels of oxidized sulfur accumulate. Parcels of this BuL air are then episodically entrained back into the BL, thereby providing a controlling influence on BL SO2, DMSO, and DMSO2. Additionally, because SO2 and DMSO are major precursors to H2SO4 and MSA, BuL chemistry, in conjunction with vertical transport, also act to control BL levels of the latter species. Although many uncertainties remain in our understanding of Antarctic DMS chemistry, the above picture already suggests that previous chemical interpretations of Antarctic field data may need to be altered.

Relationships between regional ozone pollution and emissions of nitrogen oxides in the eastern United States

Abstract: This study examines the relationships between regional ozone (O 3 ) pollution and emissions of nitrogen oxides (NO x ) in the eastern United States during summer. Using measurements from rural sites during the summer of 1995, three 4-day time periods are identified during which significant enhancements of surface O 3 occurred on spatial scales ranging from 0.5 to 1.8 million km 2 . Each of these episodes was characterized by relatively stagnant meteorological conditions conducive to the photochemical formation and accumulation of O 3 in the boundary layer. The surface ozone accumulation efficiency (SOAE), a parameter which relates the O 3 accumulation to the NO x emission density in a given region, is estimated to range from 1 to 2 ppbv O 3 kg -1 N km -2 in the eastern United States during summer. This result is discussed in the context of regional NO x -based O 3 control strategies. In addition, the net ozone production efficiency (OPE) is estimated to range from 2 to 3 ppbv O 3 ppbv -1 NO x in this region.

Development and implementation of a seasonal model for regional air quality

Abstract: As part of its five Cooperative Agreements with the US EPA, the Southern Oxidant Study (SOS) Science Team has promoted the development and application of a unique regional air quality modeling system: The Seasonal Model for Regional Air Quality (SMRAQ). This paper describes the SMRAQ development process, the resulting model formulation, and model performance during an initial thirty-day simulation. The SMRAQ modeling system was developed using the Environmental Decision Support System (EDSS). The meteorological driver for SMRAQ is a modified version of the PSU/NCAR Mesoscale Model, MM5. Emissions are processed using the Sparse Matrix Operator Kernel Emissions (SMOKE) modeling system. The SMRAQ air quality model was built using the Multiscale Air Quality SImulation Platform (MAQSIP). The unique configuration of the SMRAQ modeling system attempts to address processes important to the formation, transport, and removal of ozone on a seasonal and regional basis. An initial thirty-day SMRAQ simulation has been performed for July 7, 1995 to August 5, 1995 and the results analyzed. On average, this initial simulation shows an average error for eight-hour average ozone ranging from 20 to 25 percent. Average bias ranges from -15 to +15 percent. Both error and bias are worse than these averages in casesmore » where observed eight-hour average ozone concentrations exceed 100 ppb. Based on the analysis of the initial simulation, modifications are being made to the SMRAQ model and a full four-month simulation of the summer of 1995 will be carried out during the spring of 1998. The successful completion of this seasonal simulation will pave the way for future modeling studies that avoid the limitations of episodic modeling. These future studies will provide a better understanding of the ozone formation process and will yield more robust and effective control strategies for abatement of the ground-level ozone problem.« less