1. The semi-volatile nature of secondary organic aerosol (SOA) in the Mexico City Metropolitan Area November 2, 2007 EAS Graduate Student Symposium Christopher J. Hennigan

2. SOA background Carbonaceous particulate matter Elemental carbon (EC, Black carbon, soot) Organic carbon (OC) Primary organic aerosol (POA) Secondary organic aerosol (SOA)

3. SOA importance Can be a major fraction of PM 2.5 –10-80% of OC Still poorly understood –Sources –Formation mechanisms –Chemical composition –Impact on climate change (e.g., role as CCN)

4. MIRAGE field study MIRAGE – part of multi- agency field study in 2006 T1 ground site: located at Universidad Tecnológica de Tecámac (Tecamac University), approximately 30 km northeast and downwind of Mexico City center PM 2.5 online measurements Inorganic ions: Na +, NH 4 +, Ca 2+, Mg 2+, Cl -, NO 3 -, SO 4 2-, (PILS-IC) Water Soluble Organic Carbon (PILS-WSOC) Gas-phase measurements included CO, NOx, HNO 3, NH 3, OH, VOCs Meteorology parameters

5. PILS-WSOC measurement Measures water soluble fraction of OC aerosol on-line PILS-WSOC vs. filter-extracted WSOC compare favorably (slopes = 0.88 – 1.35) WSOC and SOA by EC tracer method highly correlated (R 2 = 0.70 - 0.79) observed WSOC/SOA 0.67 – 0.75 Excellent agreement (R 2 = 0.86 - 0.93) between oxygenated organic carbon (OOC) aerosol via AMS and WSOC; 88% of OOC water soluble Less than 10% of primary OC is water soluble (summer, fall, and winter in Tokyo) WSOC is an approximate measure of SOA [Sullivan et al., 2004; Zhang et al., 2005; Miyazaki et al., 2006; Kondo et al., 2007]

6. March 27-29 chemical composition Typical diurnal patterns Correlation suggests similar sources and atmospheric processing Use nitrate behavior to investigate SOA?

7. Nitrate production Average NO 3 - concentration increase (7:00am – 11:00am) = 13.8 μg m -3 ∫ 11am 7am k * [OH] * [NO 2 ] * dt = 15 μg m -3 Average morning HNO 3 (g) production ISORROPIA thermodynamic equilibrium model prediction: NO 3 - (NO 3 - + HNO 3 ) > 0.90 NO 3 - from secondary photochemical production; WSOC from SOA formation Avg. increase (7:00am – 11:00am): NO 3 - = 13.8 μg m -3 (300%); WSOC = 1.6 μg C m -3 (50%)

8. Nitrate, WSOC concentration decrease Possibly due to: –Boundary layer dilution –Advection –Atmospheric processing (thermodynamics) –Combination of the above Avg. decrease (11:00am – 12:45pm): NO 3 - = 14.9 μg m -3 (82%); WSOC = 2.68 μg C m -3 (56%)

9. Modeling nitrate thermodynamics Semi-volatile NH 4 NO 3 well understood –Exists in gas (HNO 3, NH 3 ) or particle (NH 4 NO 3 ) phase at ambient conditions –NH 4 NO 3 : T, RH, pH, aerosol composition (SO 4 2- ) dependent ISORROPIA-II thermodynamic equilibrium model –Predicts aerosol H 2 O content –Predicts equilibrium gas-particle partitioning of inorganic species (including NH 3 (g)/NH 4 +, HNO 3 (g)/NO 3 - ) [Nenes et al., 1998; Fountoukis et al., 2007]

10. ISORROPIA model output [Fountoukis et al., 2007] ISORROPIA predicts observed drop in NO 3 - just before noon

11. Correlation to SOA? Dramatic decrease in semi-volatile NO 3 - concentration was due at least in part to thermodynamics (NO 3 - evaporation) Was observed WSOC concentration decrease also due to the semi-volatile nature of SOA? Assess the possible impacts of BL dilution and advection by using a conservative tracer (CO in this case)

12. WSOC:CO and NO 3 - :CO ratios WSOC:CO and NO 3 - :CO R 2 = 0.46 Appears as if meteorology is driving the afternoon WSOC concentration, not thermodynamics (observed on all three days)

13. Summary Nitrate and WSOC (SOA) highly correlated (R 2 = 0.80) in Mexico City Metropolitan Area (MCMA), indicating similar sources and atmospheric processing Nitrate experienced a rapid phase shift from the particle to the gas phase around noon, with observations and model results agreeing well; not so for SOA Fresh SOA with strong anthropogenic influence in the MCMA was less volatile than NH 4 NO 3 –Semi-volatile intermediates more stable products –Oligomer formation? –Thermodynamic threshold not reached? Fast (2 - 3hrs)

14. Acknowledgements NSF grant: ATM 0513035 Amy P. Sullivan, Arsineh Hecobian, Rodney J. Weber, Athanasios Nenes, Christos I. Fountoukis, Oscar Vargas, Anne T. Case Hanks, L. Gregory Huey Georgia Institute of Technology Barry L. Lefer University of Houston