Grantee Research Project Results
Final Report: Aerosol Optical Properties and Biogenic SOA: Effect on Hygroscopic Properties and Light Absorption
EPA Grant Number: R835411Title: Aerosol Optical Properties and Biogenic SOA: Effect on Hygroscopic Properties and Light Absorption
Investigators: Khlystov, Andrey , Ramachandran, Subramanian
Institution: Desert Research Institute , Carnegie Mellon University
EPA Project Officer: Chung, Serena
Project Period: April 1, 2013 through March 31, 2016 (Extended to March 31, 2018)
Project Amount: $398,318
RFA: Anthropogenic Influences on Organic Aerosol Formation and Regional Climate Implications (2012) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Climate Change , Air
Objective:
Secondary organic aerosol (SOA) from biogenic sources is a major contributor to the global aerosol burden. It is estimated to have a profound effect on regional and global climate. There is strong evidence that biogenic SOA can influence optical properties of ambient aerosol by altering its hygroscopicity and contributing to light absorption directly via formation of brown carbon (BrC) and indirectly by enhancing light absorption by black carbon (“lensing effect”). The magnitude of these effects remains highly uncertain. It was suggested that organo-nitrogen (ON) compounds are the substances responsible for formation of brown carbon in biogenic SOA. No evidence exists yet for such a link in ambient aerosol. The goal of this project is to provide comprehensive characterization of optical properties of anthropogenically-influenced biogenic SOA, its contribution to aerosol hygroscopicity and light absorption via formation of brown carbon and “lensing” effect, and investigate the link between ON and brown carbon.
Summary/Accomplishments (Outputs/Outcomes):
A set of state-of-the-art instruments for measurements of optical properties of ambient aerosols was deployed during two field campaigns – one at the Centerville, AL, SEARCH site conducted during June-July 2013, another conducted during June 2015 in Duke Forest, near Chapel Hill, NC. The two sites are strongly influenced by biogenic sources, with the Duke Forest site being significantly more influenced by anthropogenic emissions than the Centerville site.
It was found that ambient relative humidity (RH) had a strong influence on aerosol light scattering at both sites. The aerosol was found to always contain water, even at RH down to 40%. Thus, if aerosol is assumed to be dry below deliquescence points of major inorganic compounds, aerosol light scattering and climate forcing can be significantly (~30% at RH = 60%) underestimated. No appreciable effect of SOA on the hygroscopic light scattering enhancement was detected, however.
Despite very low aerosol light absorption at the Centerville site, a large number of observations allowed detection of a statistically significant increase in light absorption of about 0.3 Mm-1 (~23%) due to the SOA coating of black carbon (BC) particles (the so-called “lensing effect”). Except for one brief biomass burning event, no BrC was detected at the Centerville site. Measurements at the Duke Forest site also showed an enhancement factor due to the lensing effect. The enhancement factor exhibited a clear diurnal pattern, reaching up to 30% during night hours. In the afternoon, the enhancement was lower, but the mass absorption coefficient (MAC) of the denuded aerosol exhibited a maximum, which indicated that the aerosol was enriched with low volatility light absorbing substances. Unlike the first campaign, several events were encountered at Duke Forest that suggested the presence of non-BC light absorbing material. While it was not possible to fully exclude presence of light absorbing dust, it is likely that these events were due to the presence of appreciable amounts of BrC. These events occurred in the afternoon, suggesting they were photochemically driven.
Aerosol volatility was qualitatively similar at both sites. An effective enthalpy of evaporation of bulk OA was found to be in the range 80-100 kJ mol-1, which is substantially higher than that typically assumed for simulating OA in atmospheric models (30-40 kJ mol-1). An evaporation coefficient was found to be approximately 0.5, which is lower than generally assumed value of one. Such a value of evaporation coefficient suggests that equilibration time scales under common atmospheric conditions are on the order of several minutes to an hour under atmospheric conditions.
Aerosol volatility measurements also provided an insight into the mixing state of BC particles. Both single particle soot photometer (SP2) and the tandem-differential mobility analyzer (TDMA) measurements indicated that BC particles at Duke Forest site were coated with a material that has volatility similar to that of non-BC particles. TDMA measurements indicate that there was, on average, a 40 nm coating on 60 nm diameter BC particles and these particles comprised approximately 25% of 100 nm particles.
Due to a poor performance of the total organic carbon / total nitrogen analyzer, it was not possible to adequately investigate the contribution of organo-nitrogen compounds to BrC. Based on the optical measurements, there was no appreciable amount of BrC at the Centerville site. There were, however, events detected at the Duke Forest that suggested presence of BrC. Measurements of organo-nitrogen would have been beneficial to identifying the source and nature of BrC. Measurements of inorganic aerosol constituents were, however, available and were useful in checking for the presence of crustal material in aerosols at the site.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 12 publications | 1 publications in selected types | All 1 journal articles |
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Type | Citation | ||
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Saha PK, Khlystov A, Yahya K, Zhang Y, Xu L, Ng NL, Grieshop AP. Quantifying the volatility of organic aerosol in the southeastern US. Atmospheric Chemistry and Physics 2017;17(1):501-520. |
R835411 (2014) R835411 (2015) R835411 (2016) R835411 (Final) R835403 (2015) R835403 (Final) |
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Supplemental Keywords:
ambient air, global climate, tropospheric, particulates, organics, environmental chemistry, physics, analytical, measurement methods, southeastProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.
Project Research Results
- 2016 Progress Report
- 2015 Progress Report
- 2014 Progress Report
- 2013 Progress Report
- Original Abstract
1 journal articles for this project