6.2 Attribution of ozone changes to chemistry and transport
6.2.3 Attribution of long-term changes in ozone
96
6.2. ATTRIBUTION OF OZONE CHANGES TO CHEMISTRY AND TRANSPORT this explains the enhanced ozone fluxes despite invariable mass fluxes.
A reduction in the ozone mass flux from middle to high latitudes of about 20%
occurs in the southern hemisphere in the stratosphere. This inhibited ozone mass flux does not coincide with an inhibited annual mean decrease in mass flux. As ozone concentrations in mid-latitudes increase, this discrepancy can in this case not easily be explained by changes in ozone concentrations. However, note that the fluxes are the integrated flux over a large range in altitude. As the distribution of mass and of ozone mass differs (more mass is situated at lower levels, while ozone mass maximises at the model top), changes in mass fluxes of different signs at different levels might result in a net decrease in ozone mass flux but not in air mass flux. Also, only the annual mean is shown, and as ozone concentrations do have a pronounced annual cycle this might also obscure the results. The changes in (ozone) mass fluxes from middle to high southern latitudes will be discussed in more detail below.
6.2.3 Attribution of long-term changes in ozone
−80 −60 −40 −20 0 20 40 60 80 101
102
103
65±9
29±7
28±6 17±5
1±0
85±7 18±5
15±5
35±9 35±9
1±0 2±1
40±31
2±1
−22±11
latitudes [°]
pressure [hPa]
O3 mass fluxes Diff [1Tg/year]
−80 −60 −40 −20 0 20 40 60 80
101
102
103
17±7
−48±45
−47±42
4±3 34±12
22±9 13±11
11±8
14±12
latitude [°]
pressure [hPa]
Mass fluxes Diff [1015kg/year]
Figure 6.13: Difference in annual mean ozone mass fluxes [109kg/year] (top) and air mass fluxes [1015kg/year] (bottom) between decades 2040-49 and 2000-09. Shown are changes only if the difference is larger than one joint standard deviation (which is given together with the flux changes).
98
6.2. ATTRIBUTION OF OZONE CHANGES TO CHEMISTRY AND TRANSPORT
It can be seen from this equation that if there were only changes in the destruction rate, the resulting changes in ozone are RDO3 = Dp1/Dp2 −1 = (Dp1 −Dp2)/Dp2. Accordingly, changes in production cause changes in ozone ofRPO3 = (Pp2−Pp1)/(Pp1+ Tp1) and changes in transport cause ozone changes of ROT3 = (Tp2 −Tp1)/(Pp1 +Tp1).
Changes in the destruction rate are directly transferable to changes in ozone (i.e. a 10% reduction in the local destruction rate translates to 10% more ozone). Changes in production or transport, on the other hand, have to be seen relative to the total amount of a potential ozone ’source’, i.e. the sum of production and transport.
The relative change in ozone can then be written as:
RO3 =(RDO3+ 1)(RPO3+RTO3+ 1)−1
=RDO3 +RPO3 +RTO3 +RDO3(RPO3+RTO3) (6.6)
When relative changes are small (≈≤0.1) the last term is by one magnitude smaller than the first terms and can be neglected, so that the total relative change in ozone is approximately equal to the sum of the relative changes due to destruction, production and transport changes:
RO3 ≈RDO3 +RPO3 +RTO3 (6.7) The change in ozone due to transport can further be split up into transport of ozone from different regions by using the ozone origin diagnostic. AsT =P9
i=1Ti, the change in ozone due to transport equals RTO3 = P9
i=1(Tpi2 −Tpi1)/(Pp1 +Tp1). This allows to specify whether changes in ozone are due to changes in export or due to changes in import, and for the latter the region ozone is imported from.
Results
The relative difference of the mean ozone mixing ratios in each region was calculated between the decades 2040-49 and 2000-09 from the SCN-B2d simulation. The relative changes in ozone due to changes in production, destruction and transport were calcu-lated using Eq. 6.5. Fig. 6.14 shows the results for each region with the transport term split up into changes due to import and export of ozone. The first two bars in each plot show the relative difference in ozone calculated from the right and left hand side of Eq. 6.5. If the assumption in Eq. 6.4 of ozone being in balance was correct, the two bars should be equal. It can be seen that within the uncertainty bounds of the directly calculated difference the two bars can not be distinguished in the stratosphere.
In the tropospheric regions, especially in the northern extratropics, the relative change in ozone calculated from the right hand side is slightly lower than the directly calcu-lated difference (from the left hand side). However, overall it can be concluded that the assumption of ozone being in equilibrium is fulfilled good enough as for the following results to be valid.
The ozone mixing ratios increase from 2000 to the mid 21st century in most parts of the atmosphere, only in the tropical lower stratosphere the difference is close to zero.
The attribution method allows to contribute the changes in ozone to chemical and dynamical processes. In the stratospheric regions the changes in ozone are generally largely driven by changes in chemistry. A decrease in the destruction rates leads to higher ozone mixing ratios in the mid-latitudes, the tropical middle stratosphere and the polar regions. In the southern polar region, the largest relative difference in ozone in the stratosphere occurs (about 10%) and it can be seen that reductions in destruction rates would lead to an increase in ozone mixing ratios of approximately 20%. The increase is, however, counteracted by a negative effect on ozone due to less import of ozone into the southern polar stratosphere. The changes in the southern polar stratosphere are examined in more detail below. Another region in the stratosphere in which changes in transport play an important role is the tropical lower stratosphere. Even though the overall changes in ozone are close to zero here, it turns out that this is due to the cancellation of changes in transport and chemistry. While production is increasing, the overall change due to transport results in a reduction of ozone due to enhanced export of ozone. The finding that transport has a larger impact in the lower stratosphere compared to regions that include the middle stratosphere can be expected, as short life times of ozone in the middle stratosphere compared to timescales of transport cause ozone concentrations to be chemically controlled there. In the lower stratosphere, where ozone life times are longer, ozone concentrations are controlled by dynamics to a larger degree.
In the troposphere, ozone increases by about 10 to 20%. This is largely due to an increase in production, counteracted by more export. The increase in export can simply be understood as increased ozone mixing ratios will lead to an increase in export even if the air mass flux remains the same (see Sec. 6.2.2). In the southern hemisphere, enhanced import of ozone is the largest contributor to the positive trend.
Employing the ozone origin diagnostic allows to separate not only import and ex-port, but also the region ozone is imported from. In Fig. 6.15 the changes in ozone mixing ratios in the southern polar stratosphere are shown with the changes due to transport partitioned into the 9 ozone origin regions. As noted above, the strong in-crease in ozone due to dein-creased destruction rates is partly counteracted by dein-creased import of ozone. The partitioning reveals that the decrease in import is a decrease of ozone imported from the southern mid-latitudes (denoted as region 7 in Fig. 6.15), while import of ozone from all other regions does not cause changes in ozone in the southern polar region.
100
6.2. ATTRIBUTION OF OZONE CHANGES TO CHEMISTRY AND TRANSPORT
dO3l dO3r P D Im Ex
−0.2
−0.1 0 0.1 0.2 0.3
NH Troposphere
dO3l dO3r P D Im Ex
−0.2
−0.1 0 0.1 0.2 0.3
SH Troposphere
dO3l dO3r P D Im Ex
−0.2
−0.1 0 0.1 0.2 0.3
tropical Troposphere dO3l dO3r P D Im Ex
−0.15
−0.1
−0.05 0 0.05 0.1 0.15
tropical lower Stratosphere dO3l dO3r P D Im Ex
−0.15
−0.1
−0.05 0 0.05 0.1 0.15
tropical Stratosphere
dO3l dO3r P D Im Ex
−0.15
−0.1
−0.05 0 0.05 0.1 0.15
Northern mid−lat Stratosphere
dO3l dO3r P D Im Ex
−0.15
−0.1
−0.05 0 0.05 0.1 0.15
Southern mid−lat Stratosphere
dO3l dO3r P D Im Ex
−0.15
−0.1
−0.05 0 0.05 0.1 0.15
Northern polar Stratosphere
dO3l dO3r P D Im Ex
−0.2
−0.1 0 0.1 0.2 0.3
Southern polar Stratosphere
Figure 6.14: Relative differences 2040s - 2000s in mean ozone mixing ratios of each region and changes in ozone due to chemistry (production and destruction) and dynamics (import and export). The errorbars denote the 1 sigma uncertainty in the differences. For more details see text.
dO3l P D 1 2 3 4 5 6 7 8 Ex
−0.2
−0.1 0 0.1 0.2 0.3
Southern polar Stratosphere
Rel. difference in O3
Figure 6.15: As in Fig. 6.14 but for the southern polar stratosphere only and with the changes due to import split up into import from each region.