Current NERC-funded Research Projects :
New Project starting Jan 2016: Air Pollution in Beijing - Sources and Processes
Part of the NERC Air Pollution and Human Health programme, see here for further details.
New Project 2015: Sources of Nitrous Acid in the Atmospheric Boundary Layer
Led by Birmingham, joint with Eiko Nemitz, Ute Skiba (CEH) and Steve Ball (Leicester)
React-SCI: Reactions of Stabilised Criegee Intermediates - Implications for Atmospheric Chemistry and Climate
Joint with Andrew Rickard, Mat Evans (York), in collaboration with Luc Vereecken (MPI Mainz) and EUPHORE Labs
Chemical reactions govern the rate of removal of many primary species emitted into the atmosphere, and control the production of secondary species. The dominant atmospheric oxidant is the OH radical; reaction with OH initiates the removal of many organic compounds, nitrogen oxides and other species such as sulphur dioxide (SO2). In the case of SO2, gas-phase oxidation by OH produces sulphuric acid, which increases aerosol mass, and may also act as a nucleating agent, forming new particles in the atmosphere – affecting climate by directly scattering solar radiation, and indirectly by affecting could droplet formation, making very substantial cooling contributions. Understanding oxidation rates is critical to accurate prediction of the impacts of these factors upon atmospheric composition and climate.
This project will determine the importance of an additional potential atmospheric oxidant: reactions with stabilised criegee intermediates (SCIs), formed from the ozonolysis of alkenes.
Ozone can act as a direct oxidising agent, reacting with alkenes (species with one or more double bonds). This class of compounds includes most biogenic reactive carbon emissions, which dominate the organic compounds released to the atmosphere. Gas-phase ozone-alkene reactions produce reactive intermediates, SCIs, which have lifetimes of a few seconds (or less – this is a critical uncertainty) in the atmosphere. It has been known for some time that SCIs can react with other species, notably including SO2; however the current generally accepted wisdom is that reaction with water vapour, or decomposition, dominates the removal of SCIs in the troposphere, and so they are not considered to be important oxidants.
A number of recent pieces of evidence are changing this picture – model studies pointing to missing SO2 oxidation mechanisms; field and chamber studies pointing to enhanced SO2 oxidation in the presence of elevated levels of alkenes, and recent lab. studies which found that reactions of at least one SCI species with SO2 and NO2 are very fast, and with H2O very slow (at least under the specific experimental conditions considered). If this conclusion is generalised, simple calculations indicate that SCI reactions would be comparable to those of OH for the gas-phase oxidation of SO2 in the boundary layer. The associated sulphate aerosol increase would imply a significant change to radiative forcing calculations. Similarly, enhanced oxidation of NO2 would lead to increased nitrate production. Critically however, the recent results are not consistent with previous laboratory studies of the SCI reaction system, potentially as a consequence of differences in approach and conditions (reagent abundance, pressure, timescales etc.) which diverge substantially from those of relevance to the atmosphere.
In this project, we will apply a new approach to this critical and timely issue: application of an atmospheric simulation chamber to directly assess the importance of SCIs as oxidants. We will use the EUPHORE (European Photoreactor) chamber, which will allow us to replicate ambient conditions (using both artificial and real air samples), produce SCIs in a manner identical to their formation in the atmosphere (i.e. through alkene ozonolysis) and directly monitor their impacts upon SO2 and NO2. This approach will avoid the uncertainties of (large) extrapolation which affect interpretation of previous studies.
Our experiments will confirm (or otherwise) the importance of SCI reactions through experiments which replicate the real atmosphere and may be analysed by direct inspection; in addition we will determine kinetic parameters for the reactions of a range of SCI species, which will be used to revise the mechanism for SCI formation in atmospheric chemical models. We will then apply to such models (the MCM and GEOS-Chem) to quantify the contribution of SCI reactions to atmospheric oxidation on both local and global scales.
ICOZA: Integrated Chemistry of Ozone in the Atmosphere
Joint with Claire Reeves (UEA), Dwayne Heard (York).
Tropospheric ozone is an important air pollutant, harmful to human health, agricultural crops and vegetation. It is the main precursor to the atmospheric oxidants which initiate the degradation of most reactive gases emitted to the atmosphere, and is an important greenhouse gas in its own right. As a consequence of this central role in atmospheric chemistry and air pollution, the capacity to understand, predict and manage tropospheric ozone levels is a key goal for atmospheric science research. This goal is hard to achieve, as ozone is a secondary pollutant, formed in the atmosphere from the complex oxidation of VOCs in the presence of NOx and sunlight, and the timescale of ozone production is such that a combination of in situ chemical processes, deposition and transport govern ozone levels. Uncertainties in all of these factors affect the accuracy of numerical models used to predict current and future ozone levels, and so hinder development of optimal air quality policies to mitigate ozone exposure. Here, we will address this problem by measuring the local chemical ozone production rate, and directly determining the ozone production regime.
We will achieve this aim by building upon an existing instrument for the measurement of atmospheric ozone production rates (funded through a NERC Technology Proof-of-Concept grant, and deployed in the recent ClearfLo “Clean Air for London” NERC Urban Atmospheric Science programme). Within this project, we will develop our existing ozone production instrument to include this capability, and validate the measurements, through comparison with ozone production from VOC oxidation in a large simulation chamber, and by measurement of the key oxidant OH radicals, and their precursors, within the system.
We will then apply the instrument to compare the measured ozone production rates with those calculated using other observational and model approaches, and to characterise the ozone control regime, in two contrasting environments: In the outflow of a European megacity (at Weybourne Atmospheric Observatory, WAO, in the UK), and in a rural continental location (at Hohenpeissenberg, HPB, in southern Germany). At WAO, we will compare the measured ozone production rate with that calculated through co-located measurements of HO2 and RO2 radicals (using a newly developed approach to distinguish between these closely related species), and with that simulated using a constrained photochemical box model.
The project will develop and demonstrate a new measurement approach, and apply this to improve our understanding of a fundamental aspect of atmospheric chemical processing. Future applications have considerable potential both to support atmospheric science research, but also as an important air quality tool, alongside existing measurement and modelling approaches, to inform the most effective emission controls to reduce ozone production in a given location. In the context of global crop yield reductions arising from ozone exposure of 7 - 12 % (wheat), 6 - 16 % (soybean) and 3 - 4 % (rice), this is an important societal as well as scientific goal.