Photochemical Model Performance for PM2.5 Sulfate, Nitrate, Ammonium, and pre-cursor species SO2, HNO3, and NH3 at Background Monitor Locations in the Central and Eastern United States Kirk Baker October 2006 Lake Michigan Air Directors Consortium University of Illinois - Chicago

Ammonia Monitor Network 9 rural and 1 urban (MI) Measure SO4, SO2, NH4, NH3, NO3, HNO3 24-hr average sample every 6 days Samples at most sites use URG manual Data collected and validated for entire year of 2004

Horizontal Grid Domains Meteorological Domain 36 km Photochemical Domain 36 km Vertical atmosphere up to 100 mb (~15 km) is modeled with 16 layers Most vertical layer resolution is in the boundary layer

Photochemical Model The emissions are based on 2002 emission inventories developed for O3/PM2.5 State Implementation Plans (2004 biogenics) Boundary and initial concentrations are monthly averages from a global photochemical model simulation (GEOS-CHEM model) MM5 v3.6.X CAMx 4.30: mechanism 4; CB4+ gas-phase; ISORROPIA inorganic; RADM aqueous phase Snow cover; palmer drought index; BELD3 landuse

Model Performance Metrics Metrics consistent with EPA modeling guidance: –Bias –Error –Fractional Bias –Fractional Error –r 2 Model performance using daily average measurements: SO2, SO4, NH3, NH4, NO3, HNO3 Bias Error Fractional Bias Fractional Error

SO2 SO4 NH3 NH4 HNO3 NO3

NH3 PM2.5 NH4 NHX Fractional Bias (%)

SO2 PM2.5 SO4 Fractional Bias (%) SO2+ PM2.5 SO4

HNO3 PM2.5 NO3 Fractional Bias (%) HNO3+ PM2.5 NO3

Fractional Bias (%) by Site

SO2 HNO3 Monthly average HNO3 and SO2 concentrations at 24 Midwest CASTNET stations (blue dots) CASTNET samples are 7 day averages No cyclone inlet at CASTNET monitor so concentrations should be a little higher CASTNET stations are clustered around the Ohio Valley

The DON (degree of sulfate neutralization) is estimated to determine if sulfate is completely acidic, totally neutralized, or in- between: DON=2 indicates ammonium sulfate; DON=1 indicates ammonium bisulfate; DON=0 indicates particulate sulfuric acid Eliminated data where DON>3 and DON<0 for these plots An indicator of whether PM2.5 nitrate ion formation is limited by the availability of nitric acid or ammonia is the excess ammonia indicator (EA) (Blanchard et al., 2000) Excess Ammonia (µmoles) = [NH3] + [NH4] – 2*[SO4] – [NO3] – [HNO3] When EA<0 then PM2.5 ammonium nitrate formation is ammonia limited When EA>0 then PM2.5 ammonium nitrate formation is nitric acid limited

Model SO4 tends to be fully neutralized much more often than observations Model tends to be more ammonia limited than the observations Model performance for DON (left) and excess ammonia (right)

Remarks PM2.5 SO4, NO3, and NH4 ion performance good Pre-cursor species are well estimated, but not as well as the PM2.5 ions Is HNO3 under-measured, over-predicted, or both? SO2 is systematically over-predicted, but SO4 is well estimated by the model NH3 is under-predicted in the winter Model and observations agree that these sites tend to be limited by HNO3 in PM2.5 ammonium nitrate formation Sulfate is usually fully neutralized in the model which does not always agree well with observation data

Model-Observation and co-located observation error (ug/m 3 ) and fractional error (%) metrics

Wet Deposition (kg/km 2 ) – NADP sites SO 4 = NO 3 - NH 4 +

Quarter 4 Seasonal Average Difference Plot: SO2MOD-BASE Model predictions with SO2 deposition velocity*2 (red) and without (blue) SO2 & SO4 performance metrics SO2SO4