Working Group I: The Scientific Basis

Other reports in this collection Uncertainties in the tropospheric O3 budget

An updated survey of global tropospheric CTM studies since the SAR focuses on the tropospheric O3 budget and is reported in Table 4.12. In this case authors were asked for diagnostics that did not always appear in publication. The modelled tropospheric O3 abundances generally agree with observations; in most cases the net budgets are in balance; and yet the individual components vary greatly. For example, the stratospheric source ranges from 400 to 1,400 Tg/yr, while the surface sink is only slightly more constrained, 500 to 1,200 Tg/yr. If absolute production is diagnosed as the reactions of HO2 and other peroxy radicals with NO, then the globally integrated production is calculated to be very large, 2,300 to 4,300 Tg/yr and is matched by an equally large sink (see Sections and 4.2.6). The differences between the flux from the stratosphere and the destruction at the surface is balanced by the net in situ photochemical production. In this survey, the net production varies widely, from -800 to +500 Tg/yr, indicating that in some CTMs the troposphere is a large chemical source and in others a large sink. Nevertheless, the large differences in the stratospheric source are apparently the driving force behind whether a model calculates a chemical source or sink of tropospheric O3. Individual CTM studies of the relative roles of stratospheric influx versus tropospheric chemistry in determining the tropospheric O3 abundance (e.g., Roelofs and Lelieveld, 1997; Wang et al., 1998a; Yienger et al., 1999) will not represent a consensus until all CTMs develop a more accurate representation of the stratospheric source consistent with observations (Murphy and Fahey, 1994).

Table 4.12: Tropospheric ozone budgets for circa 1990 conditions from a sample of global 3-D CTMs since the SAR.
CTM STE Prod Loss P-L SURF Burden Reference
  (Tg/yr) (Tg)  
MATCH 1440 2490 3300 -810 620   Crutzen et al. (1999)
MATCH-MPIC 1103 2334 2812 -478 621   Lawrence et al. (1999)
ECHAM/TM3 768 3979 4065 -86 681 311 Houweling et al. (1998)
ECHAM/TM3a 740 2894 3149 -255 533 266 Houweling et al. (1998)
HARVARD 400 4100 3680 +420 820 310 Wang et al. (1998a)
GCTM 696     +128 825 298 Levy et al. (1997)
UIO 846     +295 1178 370 Berntsen et al. (1996)
ECHAM4 459 3425 3350 +75 534 271 Roelofs and Lelieveld (1997)
MOZARTb 391 3018 2511 +507 898 193 Hauglustaine et al. (1998)
STOCHEM 432 4320 3890 +430 862 316 Stevenson et al. (2000)
KNMI 1429 2864 3719 -855 574   Wauben et al. (1998)
UCI 473 4229 3884 +345 812 288 Wild and Prather (2000)
STE = stratosphere-troposphere exchange (net flux from stratosphere) (Tg/yr).
Prod & Loss = in situ tropospheric chemical terms, P-L = net. (Tg/yr).
SURF = surface deposition (Tg/yr). Burden = total content (Tg, 34DU = 372Tg).
Budgets should balance exactly (STE+P-L=SURF), but may not due to roundoff.
a Results using CH4-only chemistry without NMHC.
b Budget/burden calculated from surface to 250 hPa (missing part of upper troposphere).

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