A real-time secondary organic aerosol formation tool: development, characterization, and application in field studies

 

Authors

Jose-Luis Jimenez — University of Colorado
Amber V. Ortega — University of Colorado, Boulder
Laxmi Narasimha Reddy Yatavelli — Desert Research Institute
Douglas A. Day — University of Colorado, Boulder
Brett Palm — University of Colorado

Category

Aerosol Properties

Description

Recent field studies reveal large formation of secondary organic aerosol (SOA) under urban polluted ambient conditions, and there are indications of strong synergy between anthropogenic pollution and biogenic VOCs in increasing SOA formation. SOA formation in biomass burning smoke appears to be variable but sometimes substantial. Previous field studies depend on assumptions about mixing and air flow in order to relate the evolution of those SOA and its precursors, which limits the scientific insights under many conditions. To directly study SOA formation in ambient air in real-time, our group has deployed a Potential Aerosol Mass (PAM) photooxidation flow reactor in conjunction with an Aerodyne High Resolution Time-of-Flight Aerosol Mass Spectrometer, a Scanning Mobility Particle Sizer (SMPS) and a Proton Transfer Reaction-Mass Spectrometer (PTRMS). We have used this system to characterize SOA formation in (a) urban air during CalNex-LA-2010 in the Los Angeles area of California, (b) forest air at the USFS Manitou Forest in Colorado during BEACHON-RoMBAS-2011, and (c) biomass smoke in at the USFS Fire Science Lab in Missoula, Montana, during FLAME-3. The PAM reactor uses mercury lamps to create OH concentrations up to x10000 ambient levels. High oxidant concentrations accelerate the processing of volatile organic compounds and inorganic gases and their growth into the aerosol phase. PAM photochemical processing can represent up to approximately 20 days of equivalent atmospheric aging in the span of 4 minutes of residence time in the reactor, and PAM-processed aerosols have shown similar aging signatures and sulfate and SOA yields when compared to aging ambient aerosols. In some campaigns we used a gas-phase denuder to study heterogeneous OH processing of the pre-existing aerosol or injected O3 or N2O5 in PAM without lights to investigate SOA formation from O3 or NO3 oxidation. In all cases PAM OH photoxidation enhances SOA at intermediate exposure but results in net loss of OA at very long exposures. SOA formation greatly exceeds that calculated from the measured precursors in urban air, but differences are smaller in forest air. PAM oxidation also results in a similar slope in the Van Krevelen diagram than ambient oxidation. Lab experiments are ongoing to obtain SOA yields for the key precursors of the above campaigns under the same PAM conditions used in the field, and also to study SOA formation under conditions simulating the 2010 Gulf of Mexico oil spill.