Investigating the Impacts of Aqueous-Phase Processing on Organic Aerosol Chemical Climatology Using ARM and ASR Observations

Abstract

Secondary organic aerosol (SOA) is an important but poorly characterized component of the Earth’s climate system. Large uncertainties exist in current model predictions of SOA concentrations and properties due to poor understanding and thus inadequate model representation of SOA’s sources and formation mechanisms. SOA can be formed via both gas-phase reactions (gas SOA) and aqueous-phase reactions within hydrated aerosols and cloud/fog droplets (aqueous SOA). There has been mounting evidence suggesting that aqueous SOA is abundant in the atmosphere and that aqueous SOA likely consists of molecules with distributions of molecular weight, oxygen to carbon ratio, and physical properties (light absorption, water-uptake, and volatility) that are significantly different from those of gas SOA. Cloud/fog processing is also responsible for forming aerosols in the droplet accumulation-mode diameter range, which is generally unattainable through gas-phase reactions and condensation during the typical ambient aerosol lifetime. Since aerosols’ optical and cloud nucleating properties are controlled by their composition and size distribution, a thorough understanding of the aqueous-phase chemistry of SOA is necessary for elucidating aerosol-cloud interactions and for improving accuracy in predicting aerosols’ effects on the global radiative energy budget. However, so far, the impacts of aqueous-phase chemistry on ambient SOA remain to be characterized and the treatments of aqueous SOA in regional and global models are poorly constrained.

The main goal of this research is to unravel the influences that aqueous-phase chemistry has on the evolution of SOA concentration and modification of aerosol properties. We will achieve this goal by performing advanced and integrated analyses of Aerosol Mass Spectrometer (AMS) and Aerosol Chemical Speciation Monitor (ACSM) data acquired from ASR- and ARM-supported field campaigns and long-term measurement studies. The specific objectives of this research include:

1. Investigate the role of aqueous-phase chemistry in ambient SOA formation and evolution through analyses of aerosol mass spectrometer measurements and associated observations from DOE campaigns.
2. Identify and evaluate aerosol mass spectral signatures for aqueous SOA through analyses of organic matter in cloud/fog waters and ambient aerosols subjected to significant aqueous-phase processing.
3. Develop a new Multilinear Engine–based organic aerosol factor analysis value added product (ME-OAComp) to improve understanding of organic aerosol chemical climatology and aqueous SOA processes at ARM’s Southern Great Plains (SGP) site.
4. Perform integrated analysis of worldwide aerosol mass spectrometry datasets to gain a broader understanding of aqueous SOA from biomass burning emissions.
5. Synthesize the results generated from Objectives (1) – (4) into phenomenological descriptions and data products against which modeling results can be evaluated. 

The proposed research is expected to lead to an improved and more quantitative understanding of aqueous SOA in the atmosphere and provide data products that may be useful for developing parameters and evaluating regional and global models with improved accuracy for simulating atmospheric organic aerosol concentrations and properties.