Nitrogen Oxides :: Freshwaters


[See also: Nitrogen deposition :: Rivers and Streams | Nitrogen Deposition :: Standing Open Water and Canals | Acid deposition :: Rivers and Streams | Acid deposition :: Standing Open Water and Canals]

Effects and Implications

There are no studies that have looked at the effects of gaseous nitrogen oxides on freshwaters. Most studies have looked at effects from eutrophication and acidification effects of nitrogen deposition. Links to these Records are shown at the top of the page.

Overview: evidence, process and main impacts

Direct effects of NOx (the sum of nitric oxide NO and nitrogen dioxide NO2) have been recognised in the setting of a critical level concentration (Ashmore and Wilson 1994, Sanders et al. 1995). The gases are considered together partly because their concentrations in air are inextricably linked through their atmospheric chemistry, and partly because little is known of the direct effects of NO alone. However, rates of NOx deposition are rather slow, so that local impacts are likely to be less than for other pollutant gases such as ozone or ammonia.

The major role of NOx is as a transboundary pollutant, and its conversion in the atmosphere to nitric acid (HNO3) vapour and nitrate particles which are deposited directly or in precipitation many hundreds of km from sources. NOx is also a key precursor for ozone production in the atmosphere. Emission controls (in addition to those set out to protect human health) are driven by its role as an ozone precursor rather than because of its direct effects. However, direct effects may occur in the immediate vicinity of major roads and in urban areas, caused by high NOx emissions from vehicles. NOx may also lead to ground flora changes related to eutrophication.

However, no studies have looked at freshwater habitats.

Pollutant type and risk

Type of N deposition

Form of N

Risk areas

Dry deposition


NOx as NO2 or NO

  • Habitats adjacent to motorways and large highways (within 1km of the road)
  • Habitats in and around urban centres

The measured NO2 concentrations across the UK highlight the predominance of traffic and urban sources, with the largest concentrations in the large conurbations and adjacent to the motorway network, with annual mean concentrations in excess of 20 µg m-3 in these areas. Urban centres are hot spots due to the high density of traffic emissions and the smaller contribution from the residential and commercial sectors, as well as other area sources such as mobile machinery (e.g. excavators, bulldozers, and front and back loaders) (RoTAP, 2012). However, in rural areas, away from industry and roads NOx concentrations are usually low and not a concern to ecosystems.

Indicators of NOx impacts

Biochemical changes have only been used as additional indicators of potentially relevant ecological responses. In an ecological context, growth stimulation and reduction are both potentially negative responses. For instance, NOx (and NH3 and NH4 +) generally cause an increase in the shoot:root ratio, which may or may not be beneficial (WHO, 2000). However, no studies have looked at freshwater habitats.

Examples of species specific responses

None available.

What factors modify NOx impacts?

Responses to nitrogenous pollutants can be further modified and exacerbated by interactions with other environmental factors, including frost, drought and pest organisms. These interactions generally include increased susceptibility to these factors, which may in turn lead to major ecological changes.

Nitrogen oxides are known to have greater adverse effects in the presence of SO2 or O3, and hence the critical level should apply where these pollutants are also close to their critical level.

Environmental limit: 
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). The level for NOX should only be applied where levels of SO2 and O3 are close to their critical levels.


This page was accessed on Wednesday, January 16, 2019 12:59