The formation and characterisation of secondary organic aerosol and its contribution to PM10 concentrations in the UK atmosphere

Project Details

Description

OBJECTIVES

To promote a better understanding of the formation, composition and concentrations of secondary organic aerosol (SOA) through the following:

(1) To sample airborne particulate matter and to use the measured ratio of organic to elemental carbon to estimate the airborne concentrations of SOA.

(2) To use carbon-14 as a tracer of biogenic SOA in the collected aerosol samples.

(3) To determine for a series of anthropogenic and biogenic organic species the key low molecular weight oxidation products of VOC which can act as monomer building blocks using comprehensive gas chromatography with mass spectrometric detection.

(4) To resolve the molecular structures of oligomeric compounds formed via heterogeneous reactions of monomers using high performance liquid chromatography coupled to tandem mass spectrometry, and apply this to samples collected from both controlled chamber experiments and the atmosphere.

(5) To propose chemical pathways and parameterisations for the formation of macromolecules based on observed reagents and products.

(6) To conduct modelling studies using the UK photochemical trajectory model and the Master Chemical Mechanism to predict concentrations of both specific oxidation products of biogenic and anthropogenic VOC for different airmass trajectories and to provide model data for comparison with the atmospheric measurements.

Layman's description

Air pollution has important adverse effects on the health of the public. These include premature mortality, additional hospital admissions and worsening symptoms for people with respiratory diseases such as asthma. A number of pollutants are responsible for these effects but the one with the biggest impact is known as particulate matter. This term describes tiny particles invisible to the naked eye floating in the air, which can be breathed into the lungs. These particles arise from a range of sources, the best known being road traffic. However, emission sources are not the only source of particles and a contribution of growing importance is from particles which form in the atmosphere from chemical reactions of gases. These include chemical substances known as sulphates and nitrates, which form from emissions of sulphur dioxide and oxides of nitrogen by pathways that are reasonably well understood. In addition, there is a class of chemical substances within airborne particles known as secondary organic compounds. These are formed through chemical processing in the atmosphere of organic vapours arising from both manmade sources (e.g. petrol vapour) and natural sources, especially from compounds released by trees. These secondary organic compounds are very diverse in their chemical composition and their contribution to the mass of particles in the air is not well understood.



This project is concerned with measuring secondary organic compounds in the atmosphere of the UK, so as to estimate their contribution to the total amount of airborne particles. In addition, the project will measure the chemical composition of such particles, and use this to understand which organic vapours they are formed from. Through such studies it will be possible to infer which parts of the secondary organic compound mass can be controlled through abatement of anthropogenic sources, and which part arises from natural compounds released from vegetation, which is not readily amenable to control.

Key findings

High volume samples of PM2.5 (58) were collected from a site in Birmingham in summer 2007 and winter/spring 2008 along with other gaseous and particulate pollutant metrics. Organic and elemental carbon were analysed by a Sunset Laboratories analyser and the EC-tracer method was applied to estimate the primary/secondary organic carbon (OC) split in each sample.



Selected filter samples collected as part of the sampling programme were subjected to detailed chemical analysis using the off-line methods of GCxGC-MS and LC-MS. This combination of techniques allowed for the volatile organic and less volatile water soluble organic fraction of aerosol to be examined in a speciated manner. GCxGC-MS analysis showed key signatures of different organic sources including direct petrochemical emissions and secondary oxygenated species, notably from aromatic oxidation. A comprehensive inventory of nitrated organics found in urban aerosol was also made. Good agreement was found between the absolute amount of organic carbon observed by the Sunset analyser techniques at U. Birmingham and the GCxGC-MS methods at U. York LC-MS analyses highlighted that irrespective of absolute concentration of organic components in the urban aerosol ( or other properties such as back trajectory origin) , the mass range of species encountered was highly stable. This was observed to peak at around 260 amu, with a FWHM in the peak maximum distribution of around 25 amu. Such behaviour suggests that organic aerosol is self maintaining in molecular weight, limited at lower masses by the minimum vapour pressure required to drive gas to aerosol partitioning and at higher molecular weights by cleavage of longer chain organics into shorter chain more oxygenated species. Limited high resolution (FT-ICR-MS) analysis of the urban samples confirmed the complete absence of oligomeric molecules. This analysis indicated the presence of species with a range of O:C ratios between 0.2 to 0.7 broadly in line with bulk estimates. Molecular analyses indicated a peak in this ratio around 0.4, with some indication that this may also be a self maintaining property which is independent of absolute aerosol concentration.



The 14C content in the bulk total carbon (TC), and in the organic carbon (OC) and elemental carbon (EC) fractions separately, was determined in a subset of samples by combustion to CO2, graphitisation, and quantification by accelerator mass spectrometry. The split between OC and EC for the 14C analyses was assigned methodologically by combustion at 340 C for 20 min, and at 850 C for 4 h, respectively. The mean fraction present as contemporary TC was 0.51 (range 0.28-0.67, n = 26). There was no seasonality to the data. However, there was a positive trend between fraction contemporary carbon and magnitude of secondary organic carbon to total carbon ( SOC/TC) ratio, and for the high values of these two parameters to be associated with air-mass back trajectories arriving from over the European continent. Using a five-compartment mass balance model on fraction contemporary carbon in OC and EC, the following average source apportionment percentages were derived for these PM2.5 samples: 26% fossil EC; 19% fossil OC; 2% biomass EC; 10% biomass OC; and 43% biogenic OC. The findings from this work are consistent with those from 14C analyses of PM10 and PM2.5 samples elsewhere in Europe and support the conclusion of a significant and ubiquitous contribution from non-fossil sources to the carbon in terrestrial aerosol.



The modelling work used MCM v3.1 in a Photochemical Trajectory Model (PTM) to examine the chemical evolution of air masses arriving at the EROS site (Birmingham), with prior optimisation using TORCH-2003 data (Utembe et al., 2009). SOA formation was represented by absorptive partitioning of less-volatile species between gas and aerosol phases (the “Pankow model”). The simulated trends in total concentrations of both OA and SOA were in qualitative agreement with those observed, but with consistent underestimation owing in part to comparison of simulated boundary layer averages with ground level measurements. The highest simulated OA concentrations typically showed elevated levels of SOA, with important contributions from both anthropogenic and biogenic sources, whereas the lowest simulated OA concentrations were dominated by the prescribed background imported into the model domain (which is probably aged OA of both primary and secondary origin). Characterisation of this background was identified as an important goal for future work.


StatusFinished
Effective start/end date20/08/0719/08/09

Funding

  • NERC: £118,580.00