Composition and Morphology of Individual Combustion, Biomass Burning, and Secondary Organic Particle Types Obtained Using Urban and Coastal ATOFMS and STXM-NEXAFS Measurements

Abstract

Complementary single particle measurements of organic aerosols using aerosol time-of-flight mass spectrometry (ATOFMS) and Scanning Transmission X-ray Microscopy—Near Edge X-ray Absorption Fine Structure (STXM-NEXAFS) are compared to examine the relationships between particle morphologies and chemical composition of particles having similar sources. ATOFMS measurements provide size-resolved chemical composition information for single particles. Measurements from field campaigns in polluted or urban (Riverside/SOAR 2005; Mexico City/MILAGRO 2006; Port of Long Beach 2007) and clean or marine (Arabian Sea/INDOEX 1999; Sea of Japan/ACE-Asia 2001; Trinidad Head/CIFEX 2004) locations illustrate regional differences. The majority (≥ 85 %) of the number of submicron particles are carbonaceous (including elemental and organic carbon), but represent less than 10% of the number of supermicron particles. Organic carbon (OC) particles are classified into three meta-classes corresponding to (1) combustion generated OC/EC internal mixtures, (2) biomass burning generated K/OC mixtures, and (3) OC/High Mass OC (HMOC) mixtures containing secondary markers of atmospheric processing. Normalized dot products are used to quantify similarity among fragment spectra and indicate that OC particle types are consistent across (and within) platforms. Single particle carbon STXM-NEXAFS measurements during ACE-Asia 2001 and MILAGRO 2006 yield similar source categories based on relative abundances of aromatic, alkane, and carboxylic acid functional groups. All three organic particle types correspond to a variety of very heterogeneous particle morphologies, although the highly oxygenated OC particles with likely secondary organic contributions frequently are nearly spherical, liquid-like particles. Size-resolved number fractions of the major ATOFMS OC particle types show qualitative agreement with OC particle types from STXM-NEXAFS analysis, indicating a correspondence of the OC/EC type with the presence of strong aromatic groups, of the OC/HMOC type with high carboxylic acid groups, and of the biomass burning OC type with aromatic and carbonyl groups. ATOFMS measurements can be used to establish robust statistics for offline single particle techniques, providing the atmospheric context for the functional group and morphological information obtained from STXM-NEXAFS for an improved understanding of the climate impact of organic aerosols.

Acknowledgments

This comparative study was funded by a grant from BP. Additional support from the National Science Foundation, the James S. McDonnell Foundation, and the California Air Resource Board funded our participation in the field studies used in this work. The authors would like to acknowledge all the members of the Russell and Prather research groups at UCSD responsible for collecting the measurements summarized here, including Satoshi Takahama, Ryan Moffett, Sergio Guazzotti, Sharon Quin, Keith Coffee, John Holecek, Matt Spencer, Andy Ault, Cassandra Gaston, David Sodeman, and Monica Rivera, as well as collaborators Patricia Quinn and Tim Bates of NOAA PMEL.