Sources of Nitrous Acid in the Atmospheric Boundary Layer

Atmospheric chemical processing drives the removal of emitted pollutants, and leads to the formation of ozone and secondary aerosol, which are harmful to human and environmental health, and contribute to climate forcing. Reaction with the OH radical is the primary driver of these oxidation processes; OH abundance must be quantitatively understood in order to accurately predict such effects. In the free troposphere, ozone photolysis is the principal net OH source (neglecting NO-driven HOx cycling); however in the boundary layer a large body of evidence shows that nitrous acid (HONO) is an important, and sometimes the dominant, net OH precursor.

Well-understood gas-phase HONO chemistry is not able to explain observed levels of HONO in the boundary layer: large additional sources, forming up to an order of magnitude more HONO, are required - however their identity remains elusive. Recent laboratory work (Su et al., Science 2011; Oswald et al., Science 2013) has identified soils as a globally significant source of HONO - driven, in part, by microbial action (analogous to the well known NO, N2O production), alongside surface NO2-to-HONO conversion mechanisms - but this microbial source has not been explored in the real environment. In urban areas, there is also increasing evidence, from field and chamber studies, that vehicles dominate HONO production - yet no data on HONO production from the UK vehicle fleet exist.

Past studies have attempted to constrain HONO production through steady-state approaches, applied to co-located point measurements of OH, NO and HONO. Such analyses are however potentially hampered by the very different atmospheric lifetimes of these species, which dictates that they may not be in equilibrium in complex (spatially heterogeneous) environments. There is an urgent need for robust quantification of HONO sources, in order to quantitatively predict boundary layer HONO and OH abundance, and atmospheric chemical processing affecting air quality.


Within SNAABL, we will directly measure HONO production from (1) natural ground surfaces (including soil production), and (2) road traffic emissions. Our approach will focus upon real-world environmental behaviour, and will avoid the uncertainties associated with analyses of ambient HONO concentrations.

(1) Natural Ground Surfaces. We will measure surface HONO fluxes from contrasting agricultural and unmanaged environments, and relate these to NOx and N2O fluxes and physical, chemical atmospheric and soil parameters. Fertiliser manipulation experiments will assess the impact of nutrient addition at a unique field location permitting simultaneous measurement of perturbed- and control systems. We will also perform laboratory studies of natural surface HONO production, using soil cores from our field sites and other UK locations. Through manipulation and selective sterilisation, we will isolate and characterise the potential abiotic and microbial HONO production mechanism(s), including surface processes.

(2) Traffic Emissions. We will directly determine HONO production from traffic, through measurement of HONO, NOx and CO2 in a road tunnel, an approach which provides a single, well characterised (video monitoring) source term, and removes the confounding factors of multiple sources, dispersion and photochemistry found in the ambient atmosphere. This approach will reflect the real-world fleet emissions, rather than potentially artificial results from dynamometer driving cycles.

We will use our data to parameterise the resulting HONO source terms, and assess their accuracy, and implications for boundary layer air quality, using photochemical box and regional chemistry-transport modelling. SNAABL will deliver quantitative understanding of HONO production from natural surfaces and vehicle traffic, and so substantially improve the accuracy of predictions of boundary layer atmospheric chemical processing.

The project will deliver new scientific insight into our current understanding of the sources and abundance of nitrous acid in the lower atmosphere, and hence of boundary layer oxidation chemistry and secondary pollutant formation. It will also constrain a potentially important anthropogenic air pollution source, vehicular emission, of relevance to urban air quality. These outcomes relate to the Benefiting from Natural Resources and Managing Environmental Change themes of the new NERC strategy, and will be of direct use to a wide range of external beneficiaries :

Principal Beneficiaries:
-Research scientists working across atmospheric chemistry, alongside researchers studying soil processes, reactive nitrogen cycling, and urban air pollution.
-The air quality community, and policy makers involved in the development and formulation of air pollution control measures.

Methods to Ensure Impact
To ensure these outcomes are achieved, we will communicate our results through traditional academic routes (conference presentations; journal publications), and additionally through:
1. Direct engagement with the atmospheric science research community
2. Production of a policy brief summarising the outcomes of the project
3. A focussed discussion workshop to disseminate the project science outcomes to relevant stakeholders

Wider Beneficiaries: General Public
The research proposed here will lead, both directly and through improved general scientific understanding, to improved air pollution control strategies / policies, and hence to improved protection of human and environmental health. The outcomes of this work will therefore address two of the key RCUK definitions of Impact, namely "increasing the effectiveness of public policy" and "enhancing quality of life and health".

Milestones and Measures of Success
-Publication of high profile papers and delivery of presentations to the scientific community
-Use of (reference to) the project results by the air quality policy / expert advice groups
-Position paper summarising the state of the science in this area, arising from the end-of-project workshop