SNAABL (Sources of Nitrous Acid in the Atmospheric Boundary Layer) directly measured HONO production from (1) natural ground surfaces (including soil production), and (2) road traffic emissions. The approach focused upon real-world environmental behaviour, and avoided the uncertainties associated with analyses of ambient HONO concentrations.

(1) Natural Ground Surfaces. The project measured 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. The project also performed 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. The project directly determined 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 reflected the real-world fleet emissions, rather than potentially artificial results from dynamometer driving cycles.

The project used 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 delivered quantitative understanding of HONO production from natural surfaces and vehicle traffic, and so substantially improve the accuracy of predictions of boundary layer atmospheric chemical process

Objectives:
The overall aim of this project was to quantify natural ground surface and road traffic exhaust emissions of nitrous acid (HONO), in order to improve our understanding of boundary layer chemical processing, air pollution and reactive nitrogen cycling.

The specific objectives were:

1. To measure natural ground surface fluxes of HONO and related species (NO, N2O) in situ at well-characterised field sites with contrasting soil type, land use, fertiliser regimes and climates

2. To combine the ambient flux data with laboratory measurements under controlled conditions to identify the mechanism(s) and factors controlling HONO production from soils, and to extrapolate the field flux results to a larger range of environmental conditions.

3. To directly determine HONO emissions from road traffic, through field measurements within a controlled environment, and link these to well-understood emission indices (NOx, CO2).

4. To develop parameterisations for natural surface and traffic HONO sources, and assess their impact upon atmospheric composition through complementary modelling approaches.