Effects of Sectional PM Distribution on PM Modeling in the Western US

1
Effects of Sectional PM Distribution on PM
Modeling in the Western US

• Ralph Morris and Bonyoung Koo
• ENVIRON International Corporation
• Novato, CA
• (rmorris_at_environcorp.com)
• National RPO Modeling Meeting
• Denver, Colorado
• May 24-25, 2004

2
Outline

• Introduction
• Previous Results in SoCal
• WRAP 1996 Western US Modeling
• Conclusions

3
CAMx4 PM Treatment

• PM Size Distribution
• Mechanism 4 (M4) 2-section (fine/coarse)
• Sectional (N-Section, N10, 4,)
• Allows side-by-side comparisons of Sectional
  versus fine/coarse PM size distribution treatment
  in same platform
• Aqueous-Phase Chemistry
• RADM Bulk Module
• Variable Size Resolution Module (VSRM)
• Allows analysis of the effects of treating PM
  size distribution in aqueous-phase chemistry
  (buffering issues)
• Aerosol Thermodynamics
• ISORROPIA
• PM Size Distribution
• Equilibrium, Dynamic, Hybrid
• Secondary Organic Aerosol
• SOAP

4
Sectional vs. Fine/Coarse Comparisons in Southern
California (presented previously)

• South Coast Air Basin (SoCAB) of Southern
  California
• October 17-19, 1995 PTEP episode
• High NO3 Episode
• Analyze Effect of PM Size Resolution in VSRM
  Aqueous-phase Chemistry
• CMU 1-section Bulk Module
• CMU Variable Size Resolution Model (VSRM)
• Analyze Effects of 10-Section Versus 2-Section
  (Fine/Coarse) Representation on PM Size
  Resolution
• M4 Mechanism 4 Fine/Coarse
• EQUI 10-Section
• PM Representation

5
VSRM (Multi-Section) vs. Bulk Aqueous
ChemistryPercent Increase in Sulfate ()
By second day, VRSM estimates 15-30 more
sulfate across the SoCAB with gt 50 increase
offshore and around Long Beach
6
VSRM (Multi-Section) vs. Bulk Aqueous Chemistry
VRSM can form significantly more sulfate than the
bulk 1-section
aqueous-phase chemistry module
7

• 24-Hour Nitrate (?g/m3)
• October 18, 1995
• M4 peak NO3 83 ?g/m3
• EQUI peak NO3 54 ?g/m3
• Observed NO3 peak at Riverside 40 ?g/m3
• Differences due to assuming all nitrate is fine
  vs. PM nitrate represented by 10 size sections
  (EQUI)

M4
EQUI
8

• Differences in 24-Hour Nitrate (?g/m3)
• October 18, 1995
• M4 peak NO3 83 ?g/m3
• EQUI peak NO3 54 ?g/m3
• EQUI 10-Section grows PM NO3 into coarser
  sections where it dry deposits faster than M4 NO3
  that is assumed to be fine
• Result is less NO3 in downwind Riverside area
  that agrees better with observations

M4
9
WRAP/CRC Modeling Analysis

• 1996 36 km Western US
• (Section 309 Database)
• Evaluate Four Models
• CMAQ V4.3
• REMSAD V7
• CAMx_M4 Mechanism 4 Fine/Coarse
• CAMx_4sec 4-Sections, Section 4 is Coarse
• Use same emissions, IC/BC, vertical layers, etc.
• No Sea Salt or Explicit Calcium Emissions in WRAP
  1996 Database
• How much SO4 and NO3 in Coarse Model?

10
Example SO4 Model Performance
Fractional Gross Error
Fractional Bias
Winter overestimation compensates for summer
underestimation results in annual bias lt 10
Yellow 1996 Annual Blue January 1996 Red
July 1996
11
Average 1996 PM Concentrations Across IMPROVE
Monitors (Section 4 Coarse)
12
Conclusions Western US Sectional vs. Fine Coarse
PM Distribution Representation

• Sectional 1996 Annual Modeling (4-Sections)
  Estimates that 10 of the Sulfate and 5 of the
  Nitrate is in the Coarse Mode (PM2.5-10)
• SoCal Analysis Suggests
• Sectional Approach Affects Aqueous-Phase
  Chemistry
• Biggest Effect is on Dry Deposition Rates
• Several Caveats Should be Mentioned
• Sea Salt was not included in the WRAP 1996
  database which is known to form coarse Sodium
  Nitrate
• Important at Coastal Sites and Desert Areas
• Other Soil buffering compounds (e.g., Calcium)
  were not explicitly treated