4.5.1.4 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 4.2.3.3 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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