1. Southeast US air chemistry: directions for future SEAC 4 RS analyses Tropospheric Chemistry Breakout Group DRIVING QUESTION: How do biogenic and anthropogenic emissions combine to affect atmospheric composition with implications for air quality and climate? - How does the system respond to changing emissions? - How can the changes and responses be monitored from space?

2. Southeast US in summer has the highest isoprene emissions in the world… …and rapidly decreasing NO x emissions: Difference between 2011 and 2005 Apr-Sep OMI NO 2 [Russell et al., 2012] OMI HCHO columns, July 2006

3. The Southeast has a major ozone pollution problem… …and models show very large biases in the region HTAP model intercomparison, 24-h mean ozone [Fiore et al., 2009] EPA [2012] Month (2001) Ozone (ppb) observed models

4. The Southeast also has a major PM 2.5 pollution problem Kim et al. [2015] annual standard

5. AOD has large seasonality over Southeast US …but not PM 2.5 Kim et al. [2015]

6. Some major early results coming out of SEAC 4 RS Identification of high- and low-NO x pathways for VOC oxidation [Crounse et al.] Better understanding of the isoprene oxidation cascade under high- and low-NO x pathways, implications for ozone and aerosol formation [Wolfe et al., Campuzano- Jost et al., Travis et al., Yu et al., Marais et al.] Role of isoprene nitrates as NO x sinks and in the broader NO y budget [Crounse et al., Fisher et al.] Model biases in ozone over Southeast are caused by NO x biases [Travis et al.] AOD seasonal transition over Southeast is driven by both biogenic emissions and photochemistry [Kim et al.] Validation of satellite HCHO measurements [Zhu et al.] Oil and gas influences on VOCs [Blake et al.] Effects of agricultural fires on Southeast US air quality [Liu et al., Simpson et al.] But these and other problems require more work…

7. Problem #1: Constructing NO x /NO y budgets for Southeast compatible with ensemble of observation constraints Can we get good closure for NO y measurements in SEAC 4 RS? Can we reconcile measurements of NO x and of its oxidation products? Can we reconcile NO x emission inventories with observations of NO x concentrations and nitrate wet deposition? What is the role of isoprene nitrates as permanent/reversible sinks for NO x ? What is the role of aerosol uptake/hydrolysis of organic nitrates? NEI cut by 50% wetdep nitrate too low by 50% NEI inventory overestimates NO x Travis et al.

8. Isoprene emission Problem #2: understanding VOC sources and chemistry Can we reconcile the observed products of the isoprene oxidation cascade with current mechanisms? How confidently can we constrain isoprene emissions from HCHO columns? How do the SEAC 4 RS data constrain US methane sources? Methane below 1 km in SEAC 4 RS GOSAT methane, 2009-2011 Fisher et al., Yu et al. NO x emission

9. Problem #3: understanding sulfate formation and properties Is there a missing SO 2 oxidant needed to explain observed sulfate? Why is sulfate aerosol acidic despite excess ammonia available? - Why does ammonium aerosol show the same long-term decreases as sulfate? f=1 f=0.5 Kim et al.

10. Problem #4: Understanding formation of organic aerosol Are different OA measurements consistent? Can we conclude that biogenic sources dominate OA? What are the contributions of isoprene and monoterpenes? Can we derive yields? Can we quantify yield differences between high-NO x and low-NO x VOC oxidation pathways? Kim et al., Marais et al.