RESEARCH COMMUNICATIONS CURRENT SCIENCE, VOL. 77, NO. 11, 10 DECEMBER 1999 1537 An Eulerian photochemical model for tropospheric ozone over the tropics S. B. Debaje* and D. B. Jadhav Indian Institute of Tropical Meteorology, Pune 411 008, India A time-dependent Eulerian photochemical model for the highly reactive tropospheric trace species is formulated to gain insight into the observed trace species (ozone, NOx, PAN, HOx, etc.) over the tropics. In the present study, the model is designed to simulate a dirunal variation of surface ozone and vertical profile of the tropospheric ozone up to 15 km by considering the chemical and physical processes. The basis for this model is the mass balance of the concerned species (for example, ozone in this study) and it is solved by using Euler’s numerical techniques assuming quasi steady state approximation (QSSA) as suggested by Hov 1 . The various terms like advection, turbulent diffusion, chemical transformations, emission and removal in mass balance equation can be solved independently. Therefore, in this study, emphasis is on the chemistry of the mass balance equation of tropospheric ozone. The model results are compared with ozone measurements made at Pune. The simulated ozone concentrations for clear sky agree within less than 20% differences except for monsoon season (cloudy days). The high tropospheric ozone observed usually in March (summer season) is shifted to monsoon season in model results. This shift in ozone is due to the neglect of the impact of cloud and aqueous phase processes on tropospheric ozone production in the model. MATHEMATICAL models are needed to study the highly reactive tropospheric trace species as their data are sparse and difficult to measure. These increasing highly reactive trace species (for example, tropospheric ozone (O 3 ), nitrogen dioxide (NO 2 ), nitric oxide (NO), peroxyacetyl nitrate (PAN) and reactive hydrocarbons) are mainly responsible for adverse impact on our biosphere. Predictions from mathematical models provide the means to assess the response of these highly reactive tropospheric trace species on future changes in the environment. Models can also be used to study the effect of an increase or decrease in the emissions of the highly reactive species on photochemical processes. The Eulerian photochemical model is designed to simulate the concentrations of chemically highly reactive tropospheric trace species by simulating the physical and chemical processes in the troposphere. The chemistry and transport parts of the mass balance equation can be solved independently 1 . Therefore, the main emphasis is on an accurate description of the chemistry of the mass balance equation in this study. The development and validation of the model are presented in three parts: (i) Formulation of the model; (ii) Inventory of trace species emissions; and (iii) Evaluation of the model. The formulation of a model for predicting the dynamic behaviour of a highly chemically reactive tropospheric trace species in an urban atmosphere is studied. The basis for the model is the mass balance equation of the concerned highly reactive tropospheric trace species. This equation represents a mass balance in which all of the relevant emissions, transport, diffusion, chemical transformations and removal processes are expressed in mathematical terms as follows: δ c i / δ t = – δ (uc i )/δ x – δ (v c i )/δ y – δ (wc i )/δ z + δ K H / δ x δ c i / δ x + δ K H / δ y δ c i / δ y + δ K V / δ z δ c i / δ z + R i + S i + L i , (1) where c i represents the concentration of species i and is a function of space (x, y, z) and time (t ). Subscript i denotes the number of species (i = 1, 2, . . ., n; where n is the number of species to be studied) simulated in the model. For simplicity in the chemical scheme the single species (ozone) is studied by putting n = 1 in the model eq. (1). The other terms in eq. (1) are: u, v , w are meridional and zonal wind speed components; K H , K V are horizontal and vertical turbulent diffusion coefficients; R i is net rate of production of species i by chemical transformations; S i is emission rate of species i , and L i is net rate of removal of species i by surface uptake processes (dry and wet deposition). Equation (1) is integrated forward in time using an Euler numerical technique 1 . Numerical solution of eq. (1) requires specification of initial conditions, together with time- and space-resolved descriptions of meteorology and emissions at the grid point. Equation (1) can be simplified, for the study of ozone initially by putting n = 1 for a single cell and applying the following assumptions: (i) advection term is neglected; (ii) concentration of the species is uniform in the grid, i.e. turbulent diffusion is neglected; and (iii) emission and deposition terms are neglected. Then, eq. (1) reduces to δ c i / δ t = R i ({P K }t , T ) (2) where R i is the net generation term of the ozone (balance of chemical production and destruction) and is a function of space and time. In eq. (2), {P K } stands for a detailed photochemical mechanism, including all the relevant species participating in the O 3 generation and destruction, and is a function of space, time, temperature (T ) and solar zenith angle (z). *For correspondence. (e-mail: debaje@tropnet.ernet.in) RESEARCH COMMUNICATIONS brought to you by CORE View metadata, citation and similar papers at core.ac.uk provided by Ministry of Earth Sciences, Government of India