Impact Type: Direct exposure to pollutant
Key Concerns:
Work by Caporn et al. (2000) has shown that Calluna exposed to NO2 and SO2 throughout the growing season caused an increase in both root and shoot growth. However this was offset by a increased risk of damage by frost and an imbalance in root/shoot growth make the plants more vulnerable to water stress.
Additional Comments:
No work has been published as to the separate effects of these two pollutant gases (NO2 and SO2) on Calluna. It is possible that locally elevated NOX from roads could impact Calluna on roadside verges. In a study of the A31 dual carriageway in the New Forest, Hampshire, Angold (1997) showed that there was an effect on vegetation (including Calluna) up to 200m from the dual carriageway. While they attributed most of the effect to NOX, the effect of salt and heavy metals was acknowledged. With smaller roads the distance of impact will be proportionately less.
| Habitat/ Ecosystem Type | Critical Load/ Level | Status | Reliability | Indication of exceedance | Reference |
|---|---|---|---|---|---|
| all vegetation categories |
30 µg NOX (as NO2) m-3 annual mean; 75 µg NOX (as NO2 ) m-3 24-hour mean |
UNECE 2004 | Uncertainty: quite reliable i.e. the results of some studies are comparable |
The concentration units are referenced as if all the NOX were in the form of NO2. (see Unit Conversion). Nitrogen oxides are known to have greater adverse effects in the presence of SO2 and O3, and hence the critical level should apply where these pollutants are close to their critical level too. |
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