1. The Sensitivity of Aerosol Sulfate to Changes in Nitrogen Oxides and Volatile Organic Compounds Ariel F. Stein Department of Meteorology The Pennsylvania State University

2. Sulfate Is the dominant inorganic constituent of PM 2.5 Reduces visibility because of its hygroscopic characteristics. Hazy summer days. Dry and wet deposition of sulfate: elevates the acidity levels in surface waters and soil. Affects regional and global climates by scattering radiation.

3. HOx cycle

4. S HOx = P HNO3 + 2 P XOOH + P PAN P SO4 = P H2O2 + P SO4G P SO4 = f([NO x ], [VOC]) [VOC] d P SO4 /d[VOC] = [NO x ] d P SO4 /d[NO x ] Sensitivity of SO 4 2- to changes in NO x and VOC abundance P H2O2 =k H2O2 [HO 2 ] 2 P SO4G =k SO4G [SO 2 ][OH]

5. Streamline analysis 14 Jul 1995 1200 UTC

6. Ozone Concentrations

7. MODELS-3 description Community Multiscale Air Quality (CMAQ) model. 3-D Eulerian chemical transport model. Meteorology: Mesoscale Model 5 (MM5) Emissions: MODELS-3 Emissions Processing and Projection System (MEPPS). Chemical mechanism: RADM2 Aerosol model: Regional Particulate Model (RPM) Horizontal resolution: 36 km Vertical layers: 15 Domain: Northeastern USA.

9. NO x and VOC sensitivity definition NO x -sensitive locations { [O 3 ] 35%NOx - [O 3 ] 35%VOC }< 5 ppb (~5%) {{[H 2 O 2 ]+[SO 4 2- ]} 35%NOx - {[H 2 O 2 ]+[SO 4 2- ]} 35%VOC }<0.5 ppb (~5%) ________________________________________________________ VOC-sensitive locations { [O 3 ] 35%NOx - [O 3 ] 35%VOC }> 5 ppb (~5%) {{[H 2 O 2 ]+[SO 4 2- ]} 35%NOx - {[H 2 O 2 ]+[SO 4 2- ]} 35%VOC }>0.5 ppb (~5%)

10. Distribution of ozone sensitivity indicator values

11. Potential sulfate reduction as a function of the non-dimensional indicator.

12. Distribution of {[H 2 O 2 ]+[SO 4 2- ]}/{[HNO 3 ]+[NO 3 - ]} ratios for NO x - and VOC- sensitive regimes

13. Geographical distribution of the VOC-sensitive locations for July 14 at 20:00 UTC. Shaded regions correspond to indicator values lower than 1.

14. Conclusions ­ The species ratios consistently assume different values under NOx_sensitive and VOC_sensitive conditions. ­ Indicator values > 2 ==> NO x sensitivity. ­ Indicator values VOC sensitivity. ­ The value of the indicator at the transition point between NO x _ and VOC_sensitive sulfate remains constant throughout the different runs.