Marwa M. H. El-Sayed

Participant: PROMISE AGEP Research Symposium


Marwa M. H. El-Sayed
: Chemical, Biochemical and Environmental Engineering
Institution: University of Maryland Baltimore County (UMBC)



An online method to characterize the reversibility of secondary organic aerosol formed in aerosol liquid water

A new method is presented for the characterization of the reversible/irreversible nature of secondary organic aerosol formed in aerosol water (aqSOA). The relative contribution of reversible and irreversible uptake processes is a major unknown in our understanding of atmospheric aqSOA formation. The method utilizes simultaneous measurements of water soluble organic carbon in the particle (WSOCp) and gaseous (WSOCg) phases as surrogates for SOA and for secondary oxygenated organic gases, respectively. The central feature of this measurement approach is the behavior of WSOCp under conditions of drying, the WSOCp sample is alternated between an unperturbed ambient channel (WSOCp) and a ‘dried’ channel in which the air sample passes through a silica gel diffusion dryer (WSOCp,dry). The enhancement in SOA formation due to aqSOA is inferred based on the enhancement in the fraction of the total WSOC in the particle phase, Fp, as a function of RH. A decrease in the WSOCp concentration through the dried channel indicates the evaporation of SOA due to water evaporation – and hence, reversible aqSOA. On the other hand, irreversible aqSOA is inferred if no statistically significant difference is observed in the WSOCp concentrations through the two channels. The completely automated system is able to run for weeks with minimal intervention. A single WSOCp-WSOCp,dry– WSOCg measurement cycle is completed in 14 min, allowing for the characterization of dynamic changes in the factors influencing reversible/irreversible uptake processes. Measures were undertaken to validate the method, minimize particle losses within the system and ensure reliable measurements across diverse ambient conditions.



Drying-induced evaporation of summertime organic aerosols: Measurement and regulatory implications
(Marwa M. H. El-Sayed, Dziedzorm Amenumey, and Christopher J. Hennigan,
Department of Chemical, Biochemical and Environmental Engineering, University of Maryland, Baltimore County, Baltimore, MD, USA.)

This study characterizes the effect of drying on the concentrations of water-soluble organic matter (WSOM) in PM2.5 during the summertime in the eastern U.S.  WSOM measurements were conducted alternately through an unperturbed ambient channel (WSOM) and through a ‘dried’ channel maintained at ~35% relative humidity (RH) (WSOMdry) to quantify changes in the organic aerosol due to drying.  We show, for the first time, that ambient WSOM evaporates as a result of drying.  Across July and August, the average evaporated WSOM concentration was 0.6 ug m-3; however, the maximum evaporated WSOM concentration exceeded 5 ug m-3, demonstrating the importance of this phenomenon on shorter (hourly to daily) time scales.  Evaporated WSOM concentrations increased with increasing RH.  The evaporation of WSOM was higher during the night due to this RH effect, when water soluble organic gases were taken up into aerosol liquid water forming WSOM. This observation implies that the secondary WSOM formed from the uptake of soluble organic gases into aerosol water occurred through a reversible process.  These results have important implications for measurements of PM2.5 using the EPA’s Federal Reference and Equivalent Methods (FRM and FEM), since these methods employ sample drying to 35% RH.  The average bias in hourly PM2.5 concentrations measured in Baltimore with FEM method 127 due to this effect was 5%.  The maximum bias in hourly PM2.5 concentrations was 21%, again demonstrating the importance of this phenomenon.  These results also have major implications for other widely used PM measurements that involve sample drying.


Direct atmospheric evidence for the irreversible formation of aqueous secondary organic aerosol (aqSOA)
(Marwa M. H. El-Sayed1, Yingqing Wang1, and Christopher J. Hennigan1)

This ambient study focuses on reversible partitioning of secondary organic aerosols (SOA) formed through the aqueous phase. In Baltimore during September, water soluble organic carbon (WSOC) in both the gas- (WSOCg) and particle (WSOCp) phases increased exponentially during the day with the increase in temperature, highlighting the effect of photochemistry on SOA production. The fraction of WSOC in the particle phase, Fp, increased with increasing relative humidity (RH) during the nighttime. These observations suggest that SOA formation occurred during the night through the absorption of semi-volatile organic compounds (VOCs) in liquid water (aqueous SOA – aqSOA). An enhancement of 71% in Fp values (0.161 to 0.275) was observed with the increase in RH from 55% to 85%, emphasizing the influence of particle water on the formation of SOA. To characterize the reversibility of aqSOA partitioning, the WSOCp measurement was alternated through an unperturbed (ambient) channel and through a channel where the sample was dried to ~40% RH. The WSOCp, dry to ambient ratio was unity under all ambient conditions: it did not exhibit any statistical difference with increasing RH in either the daytime or nighttime. This indicates that SOA formed through the uptake of water-soluble organic gases into aerosol liquid water remains in the particle phase upon the evaporation of aerosol water. Consequently, this suggests that the observed aqueous SOA undergoes further heterogeneous reactions in the aerosol phase inhibiting the reversibility of these particles and their partitioning back into the gaseous phase.

Keywords: aqueous secondary organic aerosols (aqSOA), reversible partitioning, air quality, water soluble organic carbon (WSOC)



Marwa El-Sayed was born and grew up in Cairo, Egypt where she received her B.Sc. in Chemical Engineering with honors from Cairo University and came in second in class 2003. Immediately after graduation, she joined the National Research Center (NRC) in Cairo as a research assistant in the field of Cleaner Production. In 2006, she obtained her M.Sc. in chemical engineering at Cairo University with honors and came second in her class. Her thesis was entitled: Cleaner Production in Textile Wet Processing in Small and Medium-sized Enterprises. El-Sayed attended, organized several international symposia, workshops and conferences as a member of the organizational and editorial board of the conference unit at NRC. Believing that no progress for the environment will be achieved without understanding the economic, political as well as technological contexts, El-Sayed joined another Masters program at University of Cambridge, UK in Engineering for Sustainable Development where she received her MPhil in 2009. Having this passion for the environment, she shortly afterwards moved to the US in 2011 to pursue a doctorate degree in Chemical Engineering. El-Sayed is currently a PhD candidate at the Chemical Engineering Department University of Maryland, Baltimore County (UMBC) working on an air pollution project under the supervision of Dr. Chris Hennigan that focuses on characterizing and better understanding the formation of secondary organic aerosols present in the atmosphere especially in the Baltimore area.



Air pollution is a significant problem with several negative effects on society. Our current research focuses on one important class of pollutants known as aerosols and in particular the organic portion of aerosols.  Aerosols are extremely small solid or liquid particles suspended in the atmosphere. They have many detrimental consequences on human health, visibility reduction, climate change as well as effects on Earth’s climate as they scatter and absorb solar radiation and ultimately impact the radiative properties of clouds.  Of particular interest are secondary organic aerosols (SOA) which comprise approximately 70% of the annual global production of total organic matter. The present work focuses on the formation of secondary organic aerosols through aqueous processes (aqSOA) that has been identified as an important route in forming organic aerosols; however, many aspects of aqSOA formation are still uncertain. State-of-the-art literature shows clear discrepancies in explaining SOA formation, possibly due to major uncertainties in understanding the pathways, sources as well as the precursors involved in the formation process. Thus, identification of reactions forming these particles requires a better understanding of the pathways that govern their formation. Our research aims to better understand the sources and processes that affect atmospheric aerosol concentrations in order to formulate solutions that could help reducing the associated undesired impact of air pollution. In particular, the aim of this work is to characterize the reversible and irreversible formation of aqSOA to provide insight into the main factors which govern the uptake of organic gases into aerosol liquid water.



  1. El-Sayed M.M.H., Amenumey D., Hennigan C.J, Drying-induced evaporation of secondary organic aerosol during summer, Sci. Technol., 2016.
  2. El-Sayed M.M.H.; Wang, Y.; Hennigan, C. J. Direct atmospheric evidence for the irreversible formation of aqueous secondary organic aerosol. Res. Lett. 2015, 42, 5577–5586; DOI 10.1002/2015GL064556.
  3. El-Sayed M.M.H., Hennigan C.J., Seasonal Characterization of SOA formed through the uptake of water-soluble gases to aerosol liquid water (aqSOA), American association for Aerosol Research (AAAR) Portland, OR, USA October 2016.
  4. El-Sayed M.M.H., Amenumey D., Hennigan C.J., Drying-induced evaporation of summertime organic aerosols: measurement and regulatory implications, Air Quality Measurement Methods and Technology Conference (AWMA), Chapel Hill, NC, USA March 2016.
  5. El-Sayed M.M.H.; Wang, Y.; Hennigan, C. J., Direct atmospheric evidence for the irreversible formation of aqueous secondary organic aerosol (aqSOA), International Conference on Carbonaceous Particles in the Atmosphere, Lawrence Berkeley National Laboratory, Berkeley, CA, USA August 2015.


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