學生：張立農

NUMERICAL STUDY ON ADJUSTING AND CONTROLLING EFFECT OF FOREST COVER ON PM10 AND O3

大綱

A new-coupled air quality numerical modeling system has been developed and applied to the study on the adjusting and controlling effect of forest cover on air quality. The modeling system is composed of Plant Canopy Layer Model (PCLM), Urban Scale Meteorological Model (USMM), and High-Resolution Chemical Model (HRCM).

Introduction

The system was applied to the study on the ecological adjusting and controlling effects on PM10 and O3 in Jinan City, China. The results show that the forest cover can adjust and control PM10 and O3 significantly by reducing the concentrations of PM10 while increasing the concentrations of O3 with the increase of forest cover.

Introduction

• 2.1. PCLM• PCLM provides the turbulent flux in forest canopy layer and lower

boundary conditions, and biological effect parameters for USMM, HRCM, and biogenic emission model. The PCLM differs considerably in various aspects from earlier canopy layer models.

• The momentum exchange in the canopy layer is based on a more realistic first-order closure momentum equation with an additional larger-scale diffusive term. The closure parameterizations are introduced not just for the sake of simplifying the computation of models using higher-order closure conditions, but also for enhancing the physical understanding of turbulent exchange processes in the canopy layer. There are pressure, viscosity, dispersion, and turbulence flux terms in the equation. The terms are correlated with sheltering factor, drag coefficient, forest foliage density and momentum gradient.

Coupled model system

• Integrating mass conservation equation with absorption factor, the model can simulate variations of uptake processes with height within the canopy layer. The vertical variations on the uptake factor are usually neglected in earlier models.

• The absorption factor is determined with vertical structure of forest foliage density and bulk resistance in the canopy layer. All the resistance parameters for different species, biological underlying surface patterns, seasons and atmospheric conditions have been given by field data

• The model simulates the combined effects of turbulent exchanges above and in the canopy layer, the canopy layer absorption, surface absorption and reflection, atmospheric stability, and diurnal variation of atmospheric elements on dry deposition processes. The concept of local deposition velocity and a new expression of mass sink height are adopted. A more in-depth description can be found in Lei .

Coupled model system

• 2.2. USMM• USMM simulates urban-scale synoptic processes and local circulations and

provides hourly 3-D atmospheric parameters (wind and temperature fields, and rainfall amount) for HRCM. The USMM is based on Meterorology Model version 5 (MM5) but differs considerably in various aspects from MM5 with several characteristics.

• First, with high spatial resolution, there are nine vertical grid levels in the Planetary Boundary Layer (PBL) and the lowest grid level is less than 10 m; nested grid and four-dimensional data assimilation are adopted with a nudging factor of 10−3–5×10−3(s−1). The horizontal grid distance is 1 km. Second, layering turbulent eddy diffusivity patterns are adopted; the eddy diffusivity given by parameterization relationship of turbulent statistics in the PBL , the MM5 relationship of vertical wind shear is used in troposphere above the PBL. Third, the ground temperature distribution patterns obtained by a large amount of measured data replace those derived from the energy budget equation. Fourth, owing to inputting data of high-resolution biological underlying surface patterns and terrain (Δx=Δy 1 km), the ⩽local circulations (mountain–valley breeze, sea–land breeze and urban heat–island circulation) and spatial evolution rules of ecological effects can be well reflected.

Coupled model system

• 2.3. HRCM

• HRCM (Lei and Lei, 2001) provides hourly 3-D concentrations as well as dry and wet depositions of chemical species. The HRCM is evolved from Regional Acid Deposition Model version 2 (RADM2) (NAPAP, 1990). The model solves a set of species conservation equations including comprehensive physical, chemical, and biological processes.

• The turbulent vertical structure of PBL and the effect of the canopy vertical structures on dry depositions are well reflected with highly resolved vertical layers in this model. The numerical diffusion is decreased by an order of magnitude by replacing the iterated finite difference scheme in the RADM2 with an advective scheme on conservation of second-order moment (Prather, 1986; Ge and Lei, 1997). The layered horizontal and vertical diffusion patterns are adopted. The expression of turbulent statistics (Lei, 1988) is used in PBL. The expression of inhomogeneous flow field (Kao, 1988) is used in the troposphere above PBL. A new dry deposition velocity pattern is used (Lei, 1996) which is a function of atmospheric stability, species characteristics, and characteristic parameters of the forest canopy layer (Wesely, 1988). Comprehensive gas-phase chemical processes with 40 chemical species (including acidic pollutants, primary and secondary pollutants, tropospheric oxidants, greenhouse gas, hydrocarbon, and PM10) and 79 chemical reactions are considered. Correlations on rate coefficients of the photolysis reactions (Gery et al., 1989) are made with experimental data of O3 concentration, ground temperature, rainfall, and radiation for recent years. The rate coefficients are more conformable to practical conditions than those used in RADM2.

Coupled model system

Coupled model system

Simulation scenarios

Simulation scenarios

Summary and conclusion

The Canopy Layer Model (PCLM), Urban Scale Meteorological Model (USMM), and High-Resolution Chemical Model (HRCM) system has been applied to the study of forecast cover effect on air quality in Jinan, China. The robustness of the modeling system was verified by both the routine air quality monitoring data and comprehensive measurement campaign data for PM10 and O3 across the city. Based on the Current Biological Underlying Surfaces (CBUS) in Jinan City, two afforestation scenarios—Afforestation of Southern Mountain Area (ASMA) and Protection Forest Circle (PFC) at the City surroundings—are studied. Both forest cover scenarios have a significant adjusting and controlling effect on PM10 and O3 in this city.

Both forest cover scenarios increase O3 concentrations and its polluting area while decreasing PM10 concentrations and its polluting area. However, the PFC scenario seems more beneficial to the air quality in this city, by which the increase of O3 concentrations is less and the decrease of PM10 concentrations is more than the ASMA scenario. The increase of O3 concentrations is attributed to the increase of biogenic VOC emissions (especially isoprene) by the forest.

This adverse effect of forecast cover increase can be compensated by reducing NOx emissions, which is another key precursor species of O3 production. The current NOx levels in the city are high due to fast increase of both population and industry. It is expected that the situation is improving due to strict emissions control measures and more efficient automobile combustion technologies.

The influence of increasing forest cover decreases with the increase of downwind distance and height, which can be neglected beyond 30 km from the city and 200 m above the ground.

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