Air Quality Guidelines - World Health Organization Regional Office ...

effects of ozone on vegetation
vegetation, or a 6-month growing season for trees. The appropriate months
to define the growing season will depend on the vegetation and climate in a
specific region or at a specific site. Since uptake of ozone by vegetation
occurs primarily during daylight hours when stomata are open, the calculation
of the AOT40 considers only those hours when radiation is higher
than 50 W/m 2 .
To define critical levels, the AOT40 is related to specific effects (2). A
reduction in economic yield (such as grain yield in wheat) is considered the
most relevant long-term effect of ozone on crop species, and a reduction in
biomass is chosen for tree species. For semi-natural vegetation, the effect of
ozone is expressed as the change in the species composition. The most
important short-term effect of ozone is the appearance of visible leaf injury.
The most sensitive species for each vegetation type for which adequate data
are available was selected to derive the critical level.
For crops, data on grain yield of spring wheat exposed in open-top field
chambers to different ozone concentrations over the growing season were
used to set the critical level, since the database is the largest (10 experiments
in 6 countries using 10 different cultivars) and most consistent, and wheat is
known to be a sensitive species. Statistical analysis of this pooled dataset
showed that the least significant deviation in yield that can be estimated
with 99% confidence is 4–5%. The critical level determined using this
criterion (Table 32) is 3 ppm·h (5, 7).
The critical level for short-term effects of ozone on crops (visible injury) is
derived from an extensive database of coordinated European field observations,
involving eight countries over two growing seasons using two clover
species (8). Using artificial neural network analysis, combinations of ozone
exposure and climatic conditions in the five days preceding the onset of
visible injury were identified and used to set critical levels (Table 32) of
0.2 ppm·h for humid air conditions (mean vapour pressure deficit below
1.5 kPa) and 0.5 ppm·h for dry air conditions (mean vapour pressure
deficit above 1.5 kPa).
For forests, the database available is small. Data sets from three different
European studies using open-top field chambers of the effects of ozone on
annual biomass increment in beech saplings have been used (9). Statistical
analysis of these data showed that the least significant deviation in biomass
increment that could be estimated with 95% confidence was about 10%,
and this criterion was used to determine a critical level of 10 ppm·h
(Table 32).
235