Problems to be solved:
It is presently accepted that nitrous acid (HONO) plays an important role for the oxidation capacity of the atmosphere. In addition, HONO is an important indoor pollutant, which can react with amines leading to carcinogenic nitrosamines. However, many questions concerning the formation and degradation of this trace gas in the atmosphere are still poorly understood. Problems to be solved by the consortium address the following questions:
1. What are the weights of the various HONO formation pathways in urban, rural and polar regions of the troposphere;
2. In particular, is the aerosol surface (soot, secondary organic, aqueous aerosol, cloud droplets) an important HONO source or is HONO formed only on the ground;
3. What is the quantitative relevance of HONO photolysis to the OH budget and consequently to the oxidation capacity of the atmosphere?
Scientific objectives and approach:
Significant progress towards answering these questions can only result from an integrated research project which combines field, laboratory and modelling studies. The field studies primarily focus on the formation of HONO in urban areas and take into account transport phenomena. HONO daytime formation rates are determined to clarify the importance of HONO photolysis to the oxidation capacity of the atmosphere not only at sunrise but also at noon. In addition, the vertical gradient of HONO is measured to differentiate between HONO formation on aerosols and on the ground. Finally, HONO is also measured in polar regions to provide a database, which can be used to validate the assumption that the oxidation capacity in the polar region is controlled by HONO photolysis. In the laboratory studies kinetic and mechanistic investigations of the relevant heterogeneous reactions leading to conversion of nitrogen compounds, such as NOx, into HONO are performed. The study focus on HONO formation and loss processes on soot, secondary organic aerosol and aqueous surfaces. It is assumed that oxidisable surface groups can lead to rapid HONO formation. Organic aerosol particles, which constitute a major fraction of the atmospheric aerosol are believed to contain such oxidisable surface groups and hence may be a significant source of HONO in the atmosphere. Consequently, a key objective is to provide laboratory data needed to evaluate the significance of the organic aerosol as a source of atmospheric HONO. In the model studies tropospheric HONO formation is simulated by box and 3D calculations and compared to the field measurements. The model includes the present state of knowledge of HONO formation on different surfaces and is closely linked to the laboratory studies. A sensitivity analysis is performed to quantify the effect of uncertainties in the rates of the various HONO formation processes upon the concentrations of HONO, ozone and other important pollutants.
As a result of this project it is expected to be in a significantly better position to describe the formation pathways of HONO and its influence on the oxidation capacity of the atmosphere. In addition, the project contributes to the database necessary for estimating the toxicological risks and the radical source strength of the polluted and polar atmosphere due to the occurrence of nitrous acid.
Funding SchemeCSC - Cost-sharing contracts
B15 2TT Birmingham