Sources of Nitrous Acid in the Atmospheric Boundary Layer

Lead Research Organisation: University of Leicester
Department Name: Chemistry


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.

Planned Impact

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
Description Our measurements indicate that nitrous acid (HONO) is emitted directly from road vehicles, along with NO2 and aerosol particles. The amount of HONO emitted as a percentage of NOx emitted is 0.85% (0.72 to -1.01% at 95% confidence interval). Additionally, higher HONO percentages were observed when traffic flow using the tunnel contained a high fraction of diesel-fuelled vehicles. This work shows that HONO is directly emitted by road vehicles - in areas with high traffic density, vehicle exhaust emissions are likely to be the dominant HONO source into the boundary layer. Photolysis of HONO by sunlight makes OH radicals which are the main oxidant removing numerous emitted gases from the atmosphere, and thus vehicular HONO is an important source of OH which should be included in atmospheric models.
Exploitation Route Impact on policy. Traffic management. Modelling of atmospheric oxidation chemistry and trace gas lifetimes.
Sectors Environment,Transport

Description CENTA Doctoral Training Partnership (Leicester University)
Amount £80,000 (GBP)
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 09/2015 
End 03/2019
Description Proof-of-concept funding, University of Leicester
Amount £25,000 (GBP)
Organisation University of Leicester 
Sector Academic/University
Country United Kingdom
Start 08/2017 
End 02/2018
Title HONO and NO2 measurements inside Queensway tunnel, Birmingham 
Description Measurements of nitrous acid (HONO), nitrogen dioxide (NO2) and aerosol optical extinction in air the south-bound bore of the A38 Queensway tunnel, Birmingham. Instruments were located in a maintenance layby approx. 435 metres into the tunnel from the tunnel's entrance. Measurements cover the period 02:30 29/07/2016 to 23:45 08/08/2016. Measurements made by broadband cavity enhanced absorptions spectroscopy (BBCEAS at wavelengths between 370-395 nm). Data at 20 second time resolution and averaged to 15 minutes. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
Impact Publication based on this dataset: Nitrous acid (HONO) emissions under real-world driving conditions from vehicles in a UK road tunnel, L. J. Kramer et al., Atmos. Chem. Phys., 20, 5231-5248, 2020, 
Description Birmingham University 
Organisation University of Birmingham
Country United Kingdom 
Sector Academic/University 
PI Contribution Measurements of nitrogen dioxide (NO2), nitrous acid (HONO) and aerosol optical extinction inside the A38 Queensway tunnel. Measurements made by broadband cavity enhanced absorption spectroscopy instrument built & developed at Leicester University. Discussion & synthesis of results.
Collaborator Contribution Co-deployment of chemiluminescence instrument (NO and NO2), ozone instrument and aerosol samplers. Discussion & synthesis of results.
Impact Time series of NO2, HONO and aerosol optical extinctions measured inside A38 Queensway tunnel. BBCEAS dataset runs 29 July to 08 Aug 2016. A separate dataset of NO2 and HONO measurements versus distance through the tunnel were obtained by driving a different BBCEAS instrument in a van through the tunnel, 12 to 14 Feb 2017.
Start Year 2015
Description Atmospheric Chemistry and Air Pollution "Masterclass" 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Postgraduate students
Results and Impact Theme science "master class" for second year PGR students funded by CENTA doctoral training program. Event gave basis training in atmospheric chemistry to PGR students from across the NERC remit.
Year(s) Of Engagement Activity 2016,2017