Evolution of Secondary Aerosol in Polluted Air: Case Studies for Tokyo and Beijing Nobuyuki Takegawa Takuma Miyakawa and Yutaka Kondo RCAST, University of Tokyo October 25, 2009

Acknowledgments M. Koike, H. Matsui, et al. (University of Tokyo) D. van Pinxteren, H. Herrmann (ifT) Y. Miyazaki (Hokkaido University) Y. Kanaya (JAMSTEC) A. Hofzumahaus, A. Wahner (FZJ) D. R. Blake (UC-Irvine) A. Takami, N. Sugimoto (NIES) T. Zhu, Y. H. Zhang (Peking University) … and many other collaborators

Megacities, Pollution, and Climate Intensive campaigns in Tokyo (IMPACT) and Beijing (CAREBeijing) - the two largest megacities in Asia. Similarity and difference in the evolution of secondary aerosol between Tokyo and Beijing Richter et al., Nature, 2005 SCIAMACHY NO 2 (Dec03-Nov04) Beijing and many other cities Tokyo PRD Seoul Shanghai High concentrations of pollutants over Asia → Significant influence on regional air quality, radiation, and cloud formation. Megacities are “hot spots” of pollutants. Need to understand the physical/ chemical processes in megacities Formation of secondary aerosol is a key issue.

Aerodyne Aerosol Mass Spectrometer Real-time measurements of PM 1 (< 1 µm) non-refractory aerosol (Jayne et al., 2000) Sizing by particle time-of-flight (calibrated by PSL) Chemical composition by EI-QMS (calibrated by NH 4 NO 3 ) Performance of the AMS (Takegawa et al., AST, 2005, 2009) Particle beam (aerodynamic lens) Sizing (Particle- TOF) Composition (QMS)

IMPACT Campaign (July-Aug 2004) Intensive measurements at an urban site and a rural site (50 km to the north). The airflow pattern was dominated by southerly (sea breeze) → the rural site was mostly downwind of the urban site. The Aerodyne AMS and many other instruments (SO 2, NO x, NO y, CO, VOCs, OH/HO 2, etc.) were deployed at both sites. Intensive measurements at an urban site and a rural site (50 km to the north). The airflow pattern was dominated by southerly (sea breeze) → the rural site was mostly downwind of the urban site. The Aerodyne AMS and many other instruments (SO 2, NO x, NO y, CO, VOCs, OH/HO 2, etc.) were deployed at both sites. Sea breeze Urban site Export of polluted air (secondary aerosol formation) Rural site (50 km north) 30 km

Increase in Aerosol at the Downwind Site Urban Downwind Sulfate and organics were the major fractions of PM 1. Large increase (and delay of peak appearance) in sulfate and organics was found at the downwind site compared to the urban site. Clearly illustrating photochemical production of secondary aerosol during transport. SO 2 + Oxidants (gas/aqueous) → Sulfate aerosol VOCs + Oxidants (gas/aqueous) → OVOCs → Organic aerosol Sulfate and organics were the major fractions of PM 1. Large increase (and delay of peak appearance) in sulfate and organics was found at the downwind site compared to the urban site. Clearly illustrating photochemical production of secondary aerosol during transport. SO 2 + Oxidants (gas/aqueous) → Sulfate aerosol VOCs + Oxidants (gas/aqueous) → OVOCs → Organic aerosol Takegawa et al., GRL, 2006 Miyakawa et al., JGR, 2008 Organics Sulfate

Evolution of Aerosol Mass and Size Data were systematically classified by air mass age (estimated from NO x /NO y ratio). Large increase in the mass of sulfate and SOA in aged air (factors of ~2 and ~3) No significant changes in peak diameters (slight change in the width) with air mass aging –Substantial amount of accumulation mode particles existed at age ~ 0 (← regional background) Data were systematically classified by air mass age (estimated from NO x /NO y ratio). Large increase in the mass of sulfate and SOA in aged air (factors of ~2 and ~3) No significant changes in peak diameters (slight change in the width) with air mass aging –Substantial amount of accumulation mode particles existed at age ~ 0 (← regional background) Miyakawa et al., JGR, 2008 SulfateSOA marker (m/z 44) ~12-h aged Fresh

Gas versus Aqueous Phase Processes The observed increase in sulfate: ~6 µg m -3 per 12 h –Gas-phase oxidation of SO 2 (~2 µg m -3 per 12 h) was substantial, but not sufficient –Aqueous phase reaction in cloud droplets was also important Possible explanations for the small change in the mass peak diameters –Sulfate or SOA formed in the gas phase were mostly condensed on the preexisting particles. –Sulfate or SOA were produced in cloud droplets (the preexisting particles may act as CCN) (A simple calculation suggests that only a small change in the peak diameter is expected in this case) The observed increase in sulfate: ~6 µg m -3 per 12 h –Gas-phase oxidation of SO 2 (~2 µg m -3 per 12 h) was substantial, but not sufficient –Aqueous phase reaction in cloud droplets was also important Possible explanations for the small change in the mass peak diameters –Sulfate or SOA formed in the gas phase were mostly condensed on the preexisting particles. –Sulfate or SOA were produced in cloud droplets (the preexisting particles may act as CCN) (A simple calculation suggests that only a small change in the peak diameter is expected in this case) The preexisting accumulation mode particles (especially condensation submode) may play an important role. Condensation submode Droplet submode

Characteristics of the Tokyo Outflow Study period: July-August 2004 Temporal scale: < 1 day (diurnal variation dominated) Spatial scale: ~100 km Sulfate and SOA were the major fractions of PM 1 Large increases in the mass of sulfate and SOA with aging –Factors of ~2 and ~3, respectively, after ~12 h No significant changes in the mass peak diameters with aging –Possible influence from preexisting particles in regional background air even in Tokyo –Origin of regional background air could be very much complicated (mixtures of various sources and processes in East Asia)

CAREBeijing Campaign (Aug-Sep 2006) Yufa site Intensive measurements were made at an urban site and a rural site (~50 km south of the urban site) The AMS and many other advanced instruments were deployed at the rural site. Google map AMS at the Yufa site Matsui et al., JGR, 2009

2-Day Backward Trajectory NOAA HYSPLIT Starting at 20:00LT, 500 m Generally weak southerly, occasionally high winds from north (mostly due to the passage of a cyclone) Characterized as a cycle of low wind speed periods over the course of a few days (stagnant periods) followed by rapid advection of clean air Generally weak southerly, occasionally high winds from north (mostly due to the passage of a cyclone) Characterized as a cycle of low wind speed periods over the course of a few days (stagnant periods) followed by rapid advection of clean air Confined within ~500 km during the preceding 2 days Streets et al., 2003 BC emission

Variability of Submicron Aerosol Mass Sulfate and organics were the major fractions of PM 1 (similar to Tokyo and other cities). Variability of aerosol mass was observed to be associated with the cycle of stagnant period and clean air episode. Dominance of day-to-day variations over diurnal variations (regional scale). Sulfate and organics were the major fractions of PM 1 (similar to Tokyo and other cities). Variability of aerosol mass was observed to be associated with the cycle of stagnant period and clean air episode. Dominance of day-to-day variations over diurnal variations (regional scale). Clean air (cyclone) Stagnant 1Stagnant 2Stagnant 3Stagnant 4 Clean airClean air (cyclone) 1 week Takegawa et al., JGR, 2009

Evolution of Aerosol Mass and Size (Period 3) Large, gradual increases in the mass and size of sulfate and SOA (m/z 44) during the stagnant period 3 (by an order of magnitude after 3 days). Sulfate and SOA showed similar size distributions throughout the aging process. Influence of preexisting large particles in background air was minor. Large, gradual increases in the mass and size of sulfate and SOA (m/z 44) during the stagnant period 3 (by an order of magnitude after 3 days). Sulfate and SOA showed similar size distributions throughout the aging process. Influence of preexisting large particles in background air was minor. SulfateSOA marker (m/z 44) 3-day aged After clean air episode

Evolution of Aerosol Mass and Size (Period 4) SulfateSOA marker (m/z 44) Similar features were found for the other periods 3-day aged

Gas versus Aqueous Phase Processes The observed increase in sulfate (diurnal average): ~7 µg m -3 d -1 –Gas-phase oxidation of SO 2 (diurnal average ~8 µg m -3 d -1 ) was sufficiently fast, but condensation alone cannot explain the observed feature. Gas and aqueous phase processes took place simultaneously and interactively –New particle formation and fast growth by condensation supplied a large number of CCN in this region (Wiedensohler et al., JGR, 2009) → Enhance the cloud formation → … –We may not be able to separate the importance of condensation and cloud process. The observed increase in sulfate (diurnal average): ~7 µg m -3 d -1 –Gas-phase oxidation of SO 2 (diurnal average ~8 µg m -3 d -1 ) was sufficiently fast, but condensation alone cannot explain the observed feature. Gas and aqueous phase processes took place simultaneously and interactively –New particle formation and fast growth by condensation supplied a large number of CCN in this region (Wiedensohler et al., JGR, 2009) → Enhance the cloud formation → … –We may not be able to separate the importance of condensation and cloud process. The rapid evolution of large (droplet mode) particles An evidence for aqueous- phase reaction. Growth of newly formed particles

Characteristics of the Beijing Regional Pollution Study period: August-September 2006 Temporal scale: 3-4 days; Spatial scale: ~500 km –Much larger than the Tokyo case Sulfate and SOA showed similar size distributions throughout the aging process –Also found in Tokyo and other cities. Maybe a common feature in regions of anthropogenic influence Large, gradual increases in the mass and size of sulfate and SOA during air mass stagnation (by an order of magnitude after 3-4 d) High concentrations of precursors, few preexisting particles, and stagnation of air masses may have resulted in complicated interaction of new particle formation, condensation, and cloud process –Significantly different from Tokyo. This is the most drastic evolution of aerosol among those previously reported.

Future Directions More quantitative analysis of aerosol formation processes in/around megacities using a state-of-the-art 3D model –Current models reproduce the mass of sulfate relatively well (a factor of ~2) but largely underestimate organics (a factor of ~5) (Matsui et al., JGR, 2009a, 2009b). –Detailed size-resolved model should be incorporated because evolution of aerosol size is likely a key issue. –Identification of important SOA precursors and their reaction pathways