AN ANALYSIS OF MEASUREMENTS AND MODELING OF AIR-SURFACE EXCHANGE OF NO-NO2-O-3 OVER GRASS

This study lends support to the view that a dry deposition model for N O, NO2 and O-3 should include the appropriate chemical reactions in th e surface layer concentration equation. Here, we attempt to include th is mechanism in a model that is based on a K-theory (a rough approxima tion that has been criticized in the literature), structured to be use d within regional air quality models. The model accepts input of routi ne meteorological measurements and is evaluated using numerical sensit ivity tests and flux measurements that were collected over grass in No rthern Colorado and are available for about 20 summer days. A comparis on of this model's estimates of fluxes with observations shows that th e impact on the NO and NO2 fluxes is strong when both the chemical rea ctions and the NO emissions are included at some low height in the sur face layer and this impact increases with height. For O-3 fluxes at th e low height of 6 m this impact is not as large, primarily due to the existence of large O-3 fluxes compared to NO + NO2. Numerical sensitiv ity tests show that it is necessary to include chemical reactions as w ell as NO emission for modelling the NO, NO2 and O-3 dry deposition fl uxes, particularly for NO and NO2. Under some circumstances when the O -3 concentration is much larger than that of NO + NO2 and at a low ref erence height, the Big-Leaf model based on the constant flux assumptio n outperforms the K-theory model for O-3 dry deposition but this is no t the case for NO and NO2 where reasonably accurate NO emissions need to be made available. This conclusion is based on the limited observat ions of the present study and may not apply to chemically reaction mod els using higher-order closures. (C) 1998 Elsevier Science Ltd. All ri ghts reserved.