Observational constraints on the global organic aerosol budget

Understanding climate change is one of the most important challenges facing science today. Atmospheric particles (aerosol) impact the Earth's climate through absorbing and scattering sunlight and through changing the properties of clouds. Aerosols from human activity (e.g., from car exhausts and industrial activity) are the dominant cooling forcing of the Earth's climate and have partly counteracted greenhouse gas warming over the industrial period. Quantifying this aerosol cooling is a critical step to making more accurate predictions of climate change.
Over the past few decades substantial effort has been spent trying to better understand how aerosols impact climate. Much of this early effort focussed on sulfate which was known to be an important anthropogenic aerosol. However, in the past few years new instruments have allowed a detailed evaluation of the chemical composition of aerosol. A surprising result from these studies was the importance of organic carbon aerosol. In fact, in many places in the atmosphere it was found that organic carbon aerosol actually dominated the mass loading of fine (submicron) particles.

Climate and atmospheric aerosol models have been unable to reproduce the amount of organic aerosol observed in the atmosphere with the model underprediction being greatest in polluted regions. Evidence points to the major problem with the models being in the treatment of secondary organic aerosol (SOA): that is organic aerosol that is formed in the atmosphere from gas-phase precursors. Improving our understanding of SOA and organic aerosol will be challenging because many 1000s of organic components and 10 000s reactions are involved in their formation. However, it is an essential step in improving both air quality and climate predictions. A further complication comes from radio-carbon observations that suggest that the majority of the organic carbon aerosol in the atmosphere is modern (non-fossil). The typical interpretation of this observation is that the dominant source of organic carbon aerosol is from biogenic (vegetation) sources. If this were the case it would be expected that the highest concentrations of organic carbon aerosol would be found over forests. However, observations show the opposite, with high concentrations over polluted regions and much lower concentrations over forests. It has been suggested that this apparent contradiction could be due to organic vapours from biogenic sources more efficiently converting to organic carbon aerosol in the presence of anthropogenic pollution. In a recent study we used a model and organic carbon observations to suggest that as much as 60% of global organic carbon aerosol could be formed through this enhancement mechanism. If this is true it has substantial implications for our understanding of aerosol climate forcing.

In this proposal we will synthesise observations of organic carbon aerosol from a wide range of different instruments. The different instruments complement each other in the information they provide and together provide a rigorous and demanding test for the model. We will use this database to evaluate a global aerosol model and to improve the treatment of the sources of organic aerosol in the model. It will be the most comprehensive test of organic carbon aerosol in a global model to date. We will test whether there is strong evidence for a substantial enhancement of secondary organic aerosol by anthropogenic pollution. This work will greatly improve our understanding of the sources and processes controlling organic carbon aerosol and will help guide future field and laboratory measurements.

The proposal is well placed to have a direct impact on national and international climate prediction centres, climate policy and the Intergovernmental Policy on Climate Change (IPCC). The proposal will develop and improve a global aerosol model (GLOMAP) that is embedded in the UK Chemistry and Aerosol (UKCA) Model and as part of the Met Office Unified Model and the European Centre for Medium-Range Forecasting (ECMWF) Integrated Forecasting System. Developments to GLOMAP feed directly into these models resulting in improved representation of aerosol processes and climate impacts. The Institute for Climate and Atmospheric Science (ICAS) has established collaborations with the Met Office. ICAS has been working with the Met Office for several years on a new chemistry-aerosol-climate model which will be used for climate prediction including future IPCC assessments. The new Met Office academic partnership is a cluster of research excellence that includes the University of Leeds. Improving our understanding of aerosol processes is one of 3 key areas identified for the Leeds - Met Office collaboration. This proposal will contribute directly to this partnership. ICAS also collaborates with the ECMWF through the EU Global Monitoring for Environment and Security MACC project.

Specifically in this project, our user engagement will include:

1. Met Office: Engagement with the Met Office will continue through ongoing collaborations including the Met Office - Leeds academic partnership and the NCAS-funded collaboration (UKCA). We propose a comprehensive evaluation of organic aerosol in the GLOMAP model. Through this process we will improve treatment of organic aerosol resulting in improvement to the UK's climate modelling capability.

2. ECMWF: The project will improve the understanding and model treatment of secondary organic aerosol (SOA) which contributes to air quality issues in many regions of the world. The extensive dataset of organic aerosol observations that we will synthesise in this project will be useful for assessment of organic aerosol and SOA in the ECMWF IFS model. Through the MACC project improved knowledge from this project will feed into the ECMWF model. Therefore our project will ultimately help ECMWF provide publically available air quality forecasts.

3. SOA research community: The modelling results from this project will be directly useful to the wider SOA research community. Results from our project will be presented at national and international meetings and published in leading scientific journals. We will make model datasets available through the BADC. Through interactions with the wider SOA community our project will help guide future field and laboratory observations.

4. Public outreach: We will make the results of our research applicable and accessible to the general public. We will publish information on our work on the University department homepage and through a dedicated website for GLOMAP model research (http://researchpages.net/glomap/). In collaboration with the University and the NERC press office we will submit press releases relating to important results from our project. The PI has a strong track record of engagement with the public through local, national and international media (including the BBC, Guardian, Times and Los Angeles Times: see http://homepages.see.leeds.ac.uk/~eardvs/).