M. R. Alfarra

Evolution of Organic Aerosols in the Atmosphere

Framework for Change Organic aerosols make up 20 to 90% of the particulate mass of the troposphere and are important factors in both climate and human heath. However, their sources and removal pathways are very uncertain, and their atmospheric evolution is poorly characterized. Jimenez et al. (p. 1525; see the Perspective by Andreae) present an integrated framework of organic aerosol compositional evolution in the atmosphere, based on model results and field and laboratory data that simulate the dynamic aging behavior of organic aerosols. Particles become more oxidized, more hygroscopic, and less volatile with age, as they become oxygenated organic aerosols. These results should lead to better predictions of climate and air quality. Organic aerosols are not compositionally static, but they evolve dramatically within hours to days of their formation. Organic aerosol (OA) particles affect climate forcing and human health, but their sources and evolution remain poorly characterized. We present a unifying model framework describing the atmospheric evolution of OA that is constrained by high–time-resolution measurements of its composition, volatility, and oxidation state. OA and OA precursor gases evolve by becoming increasingly oxidized, less volatile, and more hygroscopic, leading to the formation of oxygenated organic aerosol (OOA), with concentrations comparable to those of sulfate aerosol throughout the Northern Hemisphere. Our model framework captures the dynamic aging behavior observed in both the atmosphere and laboratory: It can serve as a basis for improving parameterizations in regional and global models.

Ubiquity and dominance of oxygenated species in organic aerosols in anthropogenically-influenced Northern Hemisphere midlatitudes

[1] Organic aerosol (OA) data acquired by the Aerosol Mass Spectrometer (AMS) in 37 field campaigns were deconvolved into hydrocarbon-like OA (HOA) and several types of oxygenated OA (OOA) components. HOA has been linked to primary combustion emissions (mainly from fossil fuel) and other primary sources such as meat cooking. OOA is ubiquitous in various atmospheric environments, on average accounting for 64%, 83% and 95% of the total OA in urban, urban downwind, and rural/remote sites, respectively. A case study analysis of a rural site shows that the OOA concentration is much greater than the advected HOA, indicating that HOA oxidation is not an important source of OOA, and that OOA increases are mainly due to SOA. Most global models lack an explicit representation of SOA which may lead to significant biases in the magnitude, spatial and temporal distributions of OA, and in aerosol hygroscopic properties.

Cloud forming potential of secondary organic aerosol under near atmospheric conditions

An Aerodyne quadrupole aerosol mass spectrometer (QAMS) was used to provide on-line, quantitative measurements of the chemical composition and mass size distributions of the non-refractory fraction of the SOA particles at a temporal resolution of two minutes. In brief, the AMS utilizes an aerodynamic lens [Zhang et al., 2004, 2002] to produce a collimated particle beam that impacts on a porous tungsten surface heated typically to 600◦C under high vacuum (∼10−8 Torr), causing the non-refractory fraction of the particles to flash vaporize. The vapor plume is immediately ionized using a 70 eV electron impact (EI) ionization source, and a quadrupole mass spectrometer (QMA 410, Balzers, Liechtenstein) is used to analyze the resultant ions with unit mass-to-charge (m/z ) resolution. More detailed descriptions of the AMS measurement principles and various calibrations [Jayne et al., 2000], its modes of operation [Jimenez et al., 2003] and data processing and analysis [Allan et al., 2004, 2003] are available in other publications.

Technical note : Description and use of the new jump mass spectrum mode of operation for the aerodyne quadrupole aerosol mass spectrometers (Q-AMS)

A new mode of operation for the Aerodyne Quadrupole Aerosol Mass Spectrometer (Q-AMS) has been developed and used to improve the detection limits and time resolution of the instrument. The Jump Mass Spectrum (JMS) mode works by stepping through a small number of specific user defined positions within the mass spectrum, increasing the time spent scanning specific m/zs . The JMS mode is conceptually similar to the “Selected Ion Monitoring” mode of some commercial quadrupole-based instrumentation and can be used for direct quantification when the fragmentation pattern is known. The JMS mode can also be used to augment the standard Q-AMS operation in Mass Spectrum mode when the fragmentation pattern is not known, improving the effective signal-to-noise ratio (SNR) and in turn the detection limits and time resolution. A decrease in detection limits for the Q-AMS by factors of 4.6, 3.9, 1.3, and 3.5 for nitrate, sulphate, total organics, and m/z 43 mass loadings respectively was achieved for 1 minute sampling (20 s in each of the three Q-AMS modes, monitoring 10 m/z in JMS mode). Although the benefit to the SNR of the total organic mass concentration measured by the Q-AMS is smaller, sensitivity to organic fragments which can act as markers for various sources and processes (such as fresh primary anthropogenic emissions, aged secondary organics, and biomass burning aerosol), is greatly increased by the JMS mode. Example data from applications that benefit from this technique are presented, including an aircraft platform and in smog chamber experiments, alongside high time-resolution, ground-based data.

Cloud condensation nucleation activities of calcium carbonate and its atmospheric ageing products

Aerosol particles can serve as cloud condensation nuclei (CCN) to form cloud droplets, and its composition is a main factor governing whether an aerosol particle is an effective CCN. Pure mineral dust particles are poor CCN; however, changes in chemical composition of mineral dust aerosol particles, due to heterogeneous reactions with reactive trace gases in the troposphere, can modify their CCN properties. In this study we investigated the CCN activities of CaCO3 (as a surrogate for mineral dust) and its six atmospheric ageing products: Ca(NO3)2, CaCl2, CaSO4, Ca(CH3SO3)2, Ca(HCOO)2, and Ca(CH3COO)2. CaCO3 has a very low CCN activity with a hygroscopicity parameter (κ) of 0.001-0.003. The CCN activities of its potential atmospheric ageing products are significantly higher. For example, we determined that Ca(NO3)2, CaCl2 and Ca(HCOO)2 have κ values of ∼0.50, similar to that of (NH4)2SO4. Ca(CH3COO)2 has slightly lower CCN activity with a κ value of ∼0.40, and the κ value of CaSO4 is around 0.02. We further show that exposure of CaCO3 particles to N2O5 at 0% relative humidity (RH) significantly enhances their CCN activity, with κ values increasing to around 0.02-0.04. Within the experimental uncertainties, it appears that the variation in exposure to N2O5 from ∼550 to 15,000 ppbv s does not change the CCN activities of aged CaCO3 particles. This observation indicates that the CaCO3 surface may be already saturated at the shortest exposure. We also discussed the atmospheric implications of our study, and suggested that the rate of change in CCN activities of mineral dust particles in the troposphere is important to determine their roles in cloud formation.

EXPOSURE TO ULTRAFINE PARTICLES FROM TRAFFIC IN CITY STREETS AND THE URBAN ATMOSPHERE

Mass-based emission controls are successfully reducing pollutant levels of fine particles from road vehicles, but may have actually increased the emission of ultrafine particles and their persistence in the atmosphere just as growing evidence indicates that these ultrafine particles present the greatest threat to health upon inhalation. These particles have so little mass that they barely register in measurements made by current urban air quality monitoring networks, which measure mass of particulate matter (PM-sub-10). Current daily/hourly monitoring of PM-sub-10 for the purposes of air quality management fails to represent the wide variation and episodicity in exposure of an urban population to the threat from traffic-sourced ultrafine particles. In order to quantify and interpret this variability, data from experiments employing sophisticated, high-resolution instrumentation in U.K. cities is offered. This data illustrates the variability on time scales from minutes to hours of ultrafine particle exposure on busy streets, and shows how meteorological factors and urban topography determine the exposure to traffic particle emissions in the surrounding urban environment. Such information has key consequences for the assessment of future emission reduction and air quality improvement strategies, especially localized ones.

Contribution of biogenic emissions to carbonaceous aerosols in summer and winter in Switzerland: A modelling study

The MM5/CAMx model system was applied to the complex terrain of Switzerland for winter and summer periods. The focus in this paper is on the formation and transport of particulate matter (PM) and the contribution of biogenic sources to the aerosol formation. Both model results and measurements indicate that particulate nitrate and organic aerosols are the major components of the aerosol composition in winter in northern Switzerland. Organic aerosols dominate the aerosol composition in summer and they are mostly secondary. Measurements show that biogenic emissions in Zurich contribute about 60% and 27% to organic carbon (OC) in summer and winter, respectively. The model predictions of the biogenic contribution are very close to the measurements in summer. The biogenic precursors of secondary organic aerosols are mainly monoterpenes emitted from Norway Spruce forests in northern Switzerland. The model predictions suggest that biogenic emissions contribute predominantly to the secondary organic aerosols (SOA) in winter as well, although concentrations are lower than in summer. The fraction of biogenic SOA is much lower in the south, around the polluted region of Milan.