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Characterization of Organic Aerosol Formation and Processing in California

This study examines the formation and processing of organic aerosols (OA) in California using airborne measurements. The research focuses on flights in the Los Angeles area and compares OA production on weekdays and weekends. The results show that combustion sources are important precursors of OA, and photochemical processing contributes to OA production and increases in OA/CO ratio. The study also suggests that emissions from diesel vehicles may not be major contributors to SOA formation.

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Characterization of Organic Aerosol Formation and Processing in California

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  1. Characterization of Organic Aerosol Formation and Processing in California from Airborne Measurements R. Bahreini, A.M. Middlebrook, C. Warneke, J. de Gouw, J. Holloway, S. Brown, A. Neuman, J.M. Roberts, A. Perring, J. Schwarz, R. Spackman, C.A. Brock, M.T. Trainer, T.B. Ryerson, D.D. Parrish CalNex Data Workshop May 19, 2011 Picture by H. Stark

  2. Introduction • CalNex-2010: • April 30- June 22 • NOAA WP-3D, based in Ontario • Day/night, Weekend/weekday flights in LA • San Joaquin and Sacramento Valley flights • Mix of local sources • Outline • Emphasis on LA flights • 6 day flights; 4 night flights • OA formation and processing (in BL) • Weekend vs. weekday OA • Predictability of SOA using measured VOCs and known SOA yields

  3. AMS Quantification- LA flights • Sum of AMS and BC mass well correlated with mass estimated from size distributions

  4. OA vs. CO- LA flights, Altitude < 1500 m • Sampled air masses have OA/CO greater than previously estimated primary ratio (de Gouw et al.,JGR, 2008)

  5. OA Processing- Daytime, <1500 m de Gouw et al., 2008, Warneke et al., 2007 Toluene/Benzene=3.7 [OH]=2e6 molec./cm3 Downtown LA Redlands Long Beach Daytime, Fresh: NOx/(NOx+HNO3+PAN) > 0.8 Daytime, Aged : NOx/(NOx+HNO3+PAN) < 0.3 • SOA production during aging (2-3 days) increases OA/CO • by factor of ~2

  6. OA Processing- Nighttime, < 500 m Downtown LA Redlands Long Beach Nighttime, Fresh: NOx/(NOx+HNO3+PAN+2*N2O5+NO3) > 0.8 • OA/CO in nighttime-fresh BL comparable to daytime-fresh air

  7. OA Processing- Oxygenated OA Downtown LA Redlands Long Beach m44: fragment from highly oxygenated OA f44=m44/OA • Increase in OA/CO brings along increase in fraction of oxygenated species

  8. OA Processing Downtown LA Redlands LV-OOA SV-OOA Long Beach HOA f44: fraction from highly oxygenated OA f43: fraction from less oxygenated OA • OA data fall into OOA-regime • Aircraft never really sampled primary-dominated OA

  9. OA Processing- Daytime Aging • Continuous increase in OA oxidation with photochemical processing

  10. Weekday vs. Weekend- LA Basin • Less diesel traffic on weekends • Decreased emission of BC on weekends

  11. Weekday vs. Weekend- LA Basin Aging • Faster chemical processing on weekends

  12. Weekday vs. Weekend- LA Basin • Similar emissions of aromatics on weekends • Gasoline dominant source of aromatics

  13. Weekday vs. Weekend- LA Basin HCs from diesel vehicles not major contributors to SOA??! Daytime, Fresh: NOx/(NOx+HNO3+PAN) > 0.8 • Less diesel traffic on weekends • OA production not lower on weekends!

  14. SOA Production from VOCs • With aging: • OA/CO↑ • VOC/CO↓ OA (mg m3) D(OA/CO)obs. SOA Yield D(SOAi/CO)pred. 1) D(VOCi/CO)obs. CO (ppbv) 2) Total Predicted=SD(SOAi/CO)pred. D(OA/CO)obs. 1 ?= 3) SD(SOAi/CO)pred. D(VOC/CO)obs. VOC (pptv) CO (ppbv)

  15. VOC Processing- Daytime *Ng et al., ACP, 2007; Kroll et al., GRL 2005 • Toluene, C8- and C9- aromatics contributing most to SOA

  16. SOA Production from VOCs- Daytime D(OA/CO)obs. ?= 1 SD(SOAi/CO)pred. 1:1 line HCs from diesel vehicles not major contributors to SOA!! • Observed D(OA/CO) higher than predicted by factor of ~10 • No significant change in predictability of SOA with/without diesel emissions!

  17. Conclusions • High correlation of OA with CO shows combustion sources as precursors of OA • Photochemical processing (2-3 days) contributes to • SOA production and increase in OA/CO by a factor of 2 • Increase in degree of OA oxidation by 25% • Faster chemical processing over weekends leads to higher OA/CO • Observed SOA are a factor of 10 higher than predicted from measured VOCs • HCs from diesel emissions not so important for SOA formation?!

  18. SOA Yields

  19. the missing precursor?

  20. SOA Production- Daytime D(OA/CO)=0.02 mg m-3/ppb

  21. SOA Production- Weekday vs. Weekend D(OA/CO)=0.024 mg m-3/ppb D(OA/CO)=0.010 mg m-3/ppb • No significant change in predictability of SOA with/without diesel emissions!

  22. Weekday vs. Weekend • Lower diesel-vehicle traffic on weekend- observations: • Lower NOx but higher O3 • Slightly lower VOCs (Marr et al., EST, 2002; Murphy et al., ACP, 2008) • PM10 lower on weekend in Mexico City (Stephens et al, ACP, 2008) and LA (Qin et al., Atmos. Env., 2004) • Higher OC/EC (Harley et al., EST, 2005) • Driven by lower EC on weekend • Gasoline vs. diesel emission differences: • Ban-Weiss et al. (Atmos. Env., 2008) • PM2.5 emission factor higher for diesel vs. gasoline • BC/OM higher for diesel • PM2.5 dominated by BC for diesel • Jakober et al. (EST, 2008) • Less carbonyls in POC of diesel vs. gasoline exhaust • Miguel et al. (EST, 1998) and Geller et al. (Atmos. Env., 2006) • Lighter PAHs from diesel, ultrafine and accumulation mode • Heavier PAHs from gasoline, ultrafine mode

  23. Weekday vs. Weekend- LA Basin HCs from diesel vehicles not major contributors to SOA??! Daytime, Fresh: NOx/(NOx+HNO3+PAN) > 0.8 • Less diesel traffic on weekends • OA production not lower on weekends!

  24. OA Processing- Daytime, <1500 m

  25. Weekday vs. Weekend- LA Basin • Faster chemical processing during weekend

  26. OA Processing- Daytime

  27. Weekday vs. weekend

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