P1254325939Nauej

1
Primary and secondary organic aerosols over the
United States estimates on the basis of
observations and modeled primary OC/EC ratios

•  1.   INTRODUCTION    
• Organic carbon (OC) is a complex mixture of
  hundreds of different compounds. Particulate OC
  exists in two forms primary OC (OCpri), which is
  directly emitted, and secondary OC (OCsec), which
  is formed through atmospheric oxidation of
  reactive organic gases and subsequent
  gas-to-particle conversion processes. It is very
  difficult to quantify the relative contributions
  of OCpri and OCsec in the atmosphere due to the
  lack of a direct chemical analysis technique.
  Four different indirect methods have been used to
  predict OCpri and OCsec, i.e., use elemental
  carbon (EC) as a tracer of OCpri (Empirical
  primary OC/EC ratio approach, Turpin and
  Huntzicker, 1995 Strader et al., 1999), use
  chemical transport models to describe the
  formation of OCsec (Chemical modeling of OCsec,
  Pandis et al., 1992), use non-reactive transport
  models to estimate OCpri (Transport/emission of
  OCpri method, Hildemann et al., 1993), and use
  source-oriented and receptor-oriented methods for
  OCpri (Source-apportionment of OCpri approach,
  Schauer et al., 1996).
• In this paper a hybrid approach is developed that
  combines the empirical primary OC/EC ratio method
  with a transport/emission model of OCpri and EC,
  to estimate the relative contributions of OCsec
  and OCpri. Our approach is termed the
  emission/transport of primary OC/EC ratio method.
  The approach is applied to the observations over
  the US  from 15 June to August 31, 1999.
• 2. Description of the Observational Databases and
  Model
• 2.1. Observational Databases
• Both IMPROVE and SEARCH used thermal optical
  reflectance (TOR).
• IMPROVE two 24-hour PM2.5 OC and EC measured at
  63 sites over the US on quartz filters each week,
  on Wednesday and Saturday, beginning midnight
  local time (Sisler and Malm, 2000).
• SEARCH daily PM2.5 OC and EC were measured at 8
  sites,Figure 1.
• Southern Oxidants Study (SOS)/Nashville 99
  Experiment (6/15-7/15, 1999) the hourly PM2.5
  EC and OC measured by the Magee scientific
  Aethalemeter, and flash vaporization carbon
  analyzer of ADI (Aerosol Dynamics, Inc.),
  respectively.
• SOS/Atlanta 99 Supersite Experiment (August
  3-September 1, 1999) The hourly PM2.5 OC
  concentrations determined by Rutgers in situ
  thermal-optical carbon analyzer were used and
  PM2.5 EC concentrations determined by the
  Aethalemeter were used (Lim et al., 2003).
• 2.2.Models-3/CMAQ
• EPA Models-3/CMAQ (2003 release) The EPA
  Models-3/CMAQ (Byun and Ching, 1999) the 32-km
  model domain covers the whole United States
  (178x124 32-km grid cells) 22 layers.
• The model was driven by meteorological fields
  from the MM5.
• Emissions of gas-phase SO2, CO, NO, NO2, NH3,
  and VOC were based on the 1999 EPA NEI99. The
  primary anthropogenic PM2.5 emissions were
  separated into different speciations including
  particle SO42-, NO3-, OC, EC. Table 1 lists the
  top 10 speciation profiles for PM2.5 OC and EC
  emissions in the NEI99.

• Figures 8 and 9, and Table 5 some diurnal
  variations of each OC component. Secondary OC
  make largest contributions to OC during sunny
  period (1300 to 1800 LST) at both sites (54.9
  for Nashville and 82.4 for Atlanta) because
  secondary OC formation is highly dependent on
  temperature and relative humidity (Seinfeld et
  al., 2001). On average, secondary OC made more
  contribution to OC at Atlanta (79.1) than that
  (44.7) at Nashville. The mean primary (OC/EC)
  ratios at Nashville site were higher than 1.0
  whereas their mean values at Atlanta site were
  lower than 1.0.
•  
• 4.      Conclusions
• To the authors knowledge, this is first attempt
  to estimate the spatial distributions of OCpri
  and OCsec over the continental US. Since the
  emission and transport assisted approach used in
  this study can provide primary OC/EC ratios at
  any time and any place, OCpri and OCsec
  concentrations can now be determined
  quantitatively at any time and location where EC
  and OC measurements are available. Our results
  reveal significant temporal and geographic
  variability in the relative contribution of OCpri
  and OCsec.

Shaocai Yu, Robin Dennis, Prakash Bhave, and
Brian K. Eder National Exposure Research
Laboratory, U.S. Environmental Protection Agency,
RTP, NC 27711 Air Resources Laboratory,
National Oceanic and Atmospheric Administration,
RTP, NC 27711

• 2.3. Methodology
• Primary (OC pri) and secondary OC (OC sec) can be
  estimated (Turpin and Huntzicker, 1995 Strader
  et al., 1999)
• (OC pri) (OC Bpri)
  (OC/EC)pri(EC)
• (OC sec) (OC tot) -
  (OC pri)
• (OC/EC)pri ratio of primary OC/EC (OC
  Bpri) primary biogenic OC, (OC tot) total
  OC. (OC/EC)pri values are from model.
• 3. Comparison of the modeled primary OC and EC
  with observations over the US
• Figure 3 model captures most of the
  observations within a factor of 2. Domain mean
  of modeled EC is 0.60?0.64 mg m-3, close to
  observation.
• Figures 4 and 5 model captures daily variations
  of EC very well at all SEARCH sites with
  exception at the BMH site. The model also
  captures the average daytime variations of EC but
  overpredicts early evening EC.
• (3) Table 2 General agreement between the model
  and observation-based results for both primary OC
  and primary OC/EC ratios. Mean modeled primary
  OC and primary OC/EC ratio at the OAK site are
  2.20 mg C m-3 and 4.31, respectively, very close
  to the observations.
• 4. Applications of the modeled primary OC/EC
  ratio approach
• 4.1. Regional analysis of modeled primary OC/EC
  ratios over the US
• (1) Figure 2 Six regions (NE, SE, Midwest,
  Central, West and West Pacific).
• (2) Figures 1b and 2a, and Table 3 primary
  OC/EC ratios are the highest over the West and
  West Pacific (3.49) regions with the lowest value
  over the Northeast (1.16). The mean primary
  OC/EC ratios can vary substantially from 0.78 at
  Washington D.C. (WASH) in Southeast, to 5.63 at
  Redwood National Park (REDW) in CA.

• (3) Default natural EC and OC (EPA, 1999) for the
  Regional Haze Rule 0.02 mg C m-3 for EC, 0.35
  mg C m-3 (West) and 1.0 mg C m-3 (East) for OC.
  Our estimated primary OC are significantly higher
  than these values (EPA, 1999), except the
  Northeast, whose primary OC (0.39 mg C m-3) is
  60 lower than 1.0 mg C m-3 (East), and
  Southeast, whose primary OC (1.02mg C m-3) is
  close to default value .
• 4.2. Time-series of primary and secondary OC at 8
  SEARCH sites, and Nashville and Atlanta sites
• Fig. 7 and Table 4
• Daily means of primary OC/EC ratios can vary more
  significantly at the rural sites (CTR, OAK, and
  YRK) than those at 4 urban sites. For example,
  the primary OC/EC ratios varied from 1.59 to 5.11
  (mean 3.66?1.01) at CTR whereas their values
  changed from 0.73 to 1.27 (mean 0.83?0.11) at
  JFT.
• The mean daily concentrations of secondary OC
  varied from 0.26 mg C m-3 at GFT to 1.75 mg C m-3
  at JFT while the primary OC mean concentrations
  changed from 0.78 mg C m-3 at OLF to 2.68 mg C
  m-3 at BHM. The secondary OC can make a
  significant contribution to OC as large as
  65.8?11.1 at JST. Secondary OC made more
  contributions to OC on weekends (46.1) than that
  on weekdays (42.3).

 
Figure 8 (Nashville)
Time (EST)
Figure 9 (Atlanta)
Acknowledgements The authors wish to thank other
members at ASMD of EPA for their contributions to
the 2002 release version of EPA Models-3/CMAQ
during the development and evaluation. This work
has been subjected to US Environmental Protection
Agency peer review and approved for publication.
Mention of trade names or commercial products
does not constitute endorsement or recommendation
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