A.Nenes 1,2,3,4, R.J. Weber 1, H.Guo 1, A.Russell 5 with contributions from A.Bougiatioti 1,3 and N.Mihalopoulos 3

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1 NO 3 /NH 3 behavior between gas and aerosol phases Lessons learned from the SE US and implications for model predictions of aerosol and reactive N deposition. A.Nenes 1,2,3,4, R.J. Weber 1, H.Guo 1, A.Russell 5 with contributions from A.Bougiatioti 1,3 and N.Mihalopoulos 3 1 Georgia Tech, School of Earth and Atmospheric Sciences, 2 Georgia Tech, School or Chemical & Biomolecular Engineering, 3 IERSD, National Observatory of Athens, Greece, 4 ICE-HT, Foundation for Research and Technology Hellas, Greece, 5 Georgia Tech, School of Civil & Environmental Engineering

2 Introduction Particle ph: 1. Controls particle phase acid-catalyzed reactions; Isoprene (the largest VOC) acid aqueous phase aerosol strongly promotes aerosol formation through the IEPOX chemical route (Surratt et al., 2007&2010). IEPOX-OA 20% of OA in SE in summer (Xu et al., 2015) 2. Controls acidic and basic gas-particle partitioning of reactive N (and other); Nitric acid and nitrate,, 3. Solubilizes mineral dust and metals; 1-2% Fe mobilized after ph=2 promoted ecosystem nutrient (Meskhidze et al., 2003; Shi et al., 2012 and others) Eastern Med. dust P soluble under acidic aerosol conditions (Nenes et al., 2011)

3 Introduction Example: Fraction of total nitrate that partitions between aerosol/gas Aerosol nitrate is hard to predict in models. Many studies have focused on all possible reasons for biases, except for ph issues. ph drives the fraction of HNO 3 /NO 3 that partitions as aerosol nitrate. Same thing can be said for NH 3 +NH 4 and HCl+Cl This has a profound impact on deposition and cycle of reactive nitrogen (gas phase deposits ~10 faster than aerosol phase nitrate). Theory says: ph ε! ε Fraction of total nitrate in gas/aerosol phase Liquid water content Meskhidze et al., (2003); Weber et al., (2006)

Introduction Particle Acidity sources and evolution in atmosphere: Acidic SO 2 (g) NOx(g) Oxidation H 2 SO 4 (p) HNO 3 (p) formation of (NH 4 ) 2 SO 4, NH 4 HSO 4, NH 4 NO

4 Introduction Particle Acidity sources and evolution in atmosphere: Acidic SO 2 (g) NOx(g) Oxidation H 2 SO 4 (p) HNO 3 (p) formation of (NH 4 ) 2 SO 4, NH 4 HSO 4, NH 4 NO 3, etc. that depends on particle acidity. Basic NH 3 (g) Mineral dust + seasalt (Na, K, Ca, Mg, etc.) The system reacts in counterintuitive ways. Other sources of alkalinity 4

Introduction Historical SO 2 and SO 4 2- trends: In the past twenty years, SO 2 emissions have decreased significantly (-6.2% yr -1, 2000-2010, Hand et al. 2012). SO 4 2- followed SO 2 reduction.

5 Introduction Historical SO 2 and SO 4 2- trends: In the past twenty years, SO 2 emissions have decreased significantly (-6.2% yr -1, , Hand et al. 2012). SO 4 2- followed SO 2 reduction. Scientific questions: 1. Are particles in southeast US becoming neutral as SO 2 emissions go down? 2. Are nitrate particles going to become dominant aerosols in southeast US? (Hidy et al., 2014) (Hand et al., 2012)

6 The acidity paradox Historical Data: SO 4 is going down NH 3 is constant Nitrate is ~ 0 Weber et al. (2016), Nature Geosci.

7 The acidity paradox Historical Data: SO 4 is going down NH 3 is constant Nitrate is ~ 0 (NH 4 ) 2 SO 4 Aerosol response: Should have become more neutralized NH 4 HSO 4 H 2 SO 4 Weber et al. (2016), Nature Geosci.

8 The acidity paradox Historical Data: SO 4 is going down NH 3 is constant Nitrate is ~ 0 (NH 4 ) 2 SO 4 Aerosol response: Should have become more neutralized NH 4 HSO 4 H 2 SO 4 but it s NOT becoming more neutral. In fact it s acidifying. Weber et al. (2016), Nature Geosci.

9 Determining aerosol ph: The problem Acidity / ph definition: " #log 1000 )* #log +,- How to determine particle ph: ph cannot be measured for single particles in-situ. ph proxies (ion balance, molar ratios), do not strongly correlate with ph. Ions can be in multiple forms depending on ph and pka. ph depends on LWC, which can vary considerably. )*,+,-./012: Ion balance: NH 4+ /SO 4 2- Molar ratio: (Guo et al., 2015) 9

10 Determining aerosol ph: How we do it (model+obs) Follow the approach of Guo et al. (2015): o Particle ions (SO 2-4, NH 4+, NO 3-, Cl -, Na +, K +, Ca 2+, Mg 2 ); o Gas (NH 3, HNO 3, HCl); o Particle water or total organics & 9:;<= :>?=@AB@C<B<9>; o RH and T; Ions Organics E F* D 2" ISORROPIA-II, with RH and T W i, particle water associated with inorganics W o, particle water associated with organics m dry, ρ p Predicted LWC vs Measured LWC )* ph Fountoukis and Nenes (2007); Lu et al. (2015) PNAS; Guo et al., (2015) ACP; Cerully et al., (2015) ACP

11 The acidity paradox Historical Data: SO 4 is going down NH 3 is constant Nitrate is ~ 0 Aerosol response: Should have become more neutralized It s acidifying. (NH 4 ) 2 SO 4 NH 4 HSO 4 H 2 SO 4 Weber et al. (2016), Nature Geosci.

12 The acidity paradox Historical Data: SO 4 is going down NH 3 is constant Nitrate is ~ 0 Aerosol response: Should have become more neutralized it hasn t. (NH 4 ) 2 SO 4 NH 4 HSO 4 H 2 SO 4 ph calculations: Confirm that this is the case for SE US. Weber et al. (2016), Nature Geosci.

13 SE US: ph is very low despite large reductions in SO 2 Historical Data: SO 4 is going down NH 3 is constant Nitrate is ~ 0 Aerosol response: Should have become more neutralized it hasn t. (NH 4 ) 2 SO 4 NH 4 HSO 4 H 2 SO 4 ph calculations: Confirm that this is the case for SE US.

14 SOAS Data analysis confirms ph calculations Guo et al., ACP, Comparison of predicted vs. observed gasphase NH 3.

15 NH 3 has and probably will remain the same Proof from observation: 3 years (AMoN sites) Summary: In the past, NH 3 has been fairly constant. In the future, NH 3 will probably stay at current level or increase slightly. (Erisman et al., 2008) Weber et al. (2016), Nature Geosci.

16 NH 3 has and probably will remain the same Proof from observation: 3 years (AMoN sites) Summary: In the past, NH 3 has been fairly constant. In the future, NH 3 will probably stay at current level or increase slightly. (Erisman et al., 2008) Proof from mass balance (in the boundary layer): G G1 H IJ # K L I O60220Q/s M # K I! L M N 572 7O8Q2QS GO"Q2010Q/ 871O H IJ :572 "M72O 3 O60220Q/ 871O; K L I :572 "M72O GO"Q2010Q/ KOSQT01U; K L I! :"7810TSO "M72O GO"Q2010Q/ KOSQT01U; M: VQ./G78U S7UO8 60WOG GO"1M Weber et al. (2016), Nature Geosci.

17 NH 3 has and probably will remain the same Proof from observation: 3 years (AMoN sites) Summary: In the past, NH 3 has been fairly constant. In the future, NH 3 will probably stay at current level or increase slightly. (Erisman et al., 2008) Proof from mass balance (in the boundary layer): G G1 H IJ # K L I O60220Q/s M # K I! L M N 572 7O8Q2QS GO"Q2010Q/ 871O H IJ :572 "M72O 3 O60220Q/ 871O; K L I :572 "M72O GO"Q2010Q/ KOSQT01U; K L I! :"7810TSO "M72O GO"Q2010Q/ KOSQT01U; M: VQ./G78U S7UO8 60WOG GO"1M V.1 K L I K L I! (and [NH3 ] is comparable to [NH 4 ]) Weber et al. (2016), Nature Geosci.

18 NH 3 has and probably will remain the same Proof from observation: 3 years (AMoN sites) Summary: In the past, NH 3 has been fairly constant. In the future, NH 3 will probably stay at current level or increase slightly. (Erisman et al., 2008) Proof from mass balance (in the boundary layer): MH I J K L I Weber et al. (2016), Nature Geosci. H IJ :572 "M72O 3 O60220Q/ 871O; K L I :572 "M72O GO"Q2010Q/ KOSQT01U; M: VQ./G78U S7UO8 60WOG GO"1M NH 3 concentration (deposition) has and probably will remain the same (as long as the aerosol ph doesn t change)

19 Looking into the future: how will acidity respond? Reference state: average SOAS conditions (RH=75%, T=25 o C) total total For constant total NH 3, R SO4 goes down as SO 4 drops. This is seen in the data too. Weber et al. (2016), Nature Geosci. The ph levels remain insensitive to SO 4 changes in the SE US. Large changes in NH 3 are needed to increase ph 19

3, R SO4 goes down as SO 4 drops. This is seen in the data too.

20 Looking into the future: how will acidity respond? Reference state: average SOAS conditions (RH=75%, T=25 o C) total total For constant total NH 3, R SO4 goes down as SO 4 drops. This is seen in the data too. The ph levels remain insensitive to SO 4 changes in the SE US. Huge changes in NH 3 (which won t happen) are needed to increase ph Weber et al. (2016), Nature Geosci.

21 Some take home messages Findings: Particle ph is low (-0.5 to 1.5) and NH 3 varied little in the SE US. Very low acidity seen in dusty regions too (E.Med; Bougiatioti et al., 2016). Future particle ph may remain low even if SO 4 goes down. ph is insensitive to shifts in NH 3 and SO 4 levels because NH 4 is volatile. You can have very acidic aerosol even if NH 4 /SO 4 > 2. 21

22 Some take home messages Findings: Particle ph is low (-0.5 to 1.5) and NH 3 varied little in the SE US. Very low acidity seen in dusty regions too (E.Med; Bougiatioti et al., 2016). Future particle ph may remain low even if SO 4 goes down. ph is insensitive to shifts in NH 3 and SO 4 levels because NH 4 is volatile. You can have very acidic aerosol even if NH 4 /SO 4 > 2. Implications: Aerosol nitrate, contrary to current belief and policy, may not be a major component of the regional aerosol as sulfate levels drop in the SE US (and other locations of the world). Acid-mediated process may remain unchanged despite S reductions. Mineral dust /seasalt emissions very important modulators of ph. All this has a profound impact on deposition fluxes of reactive nitrogen 22

23 Some take home messages Findings: Particle ph is low (-0.5 to 1.5) and NH 3 varied little in the SE US. Very low acidity seen in dusty regions too (E.Med; Bougiatioti et al., 2016). Future particle ph may remain low even if SO 4 goes down. ph is insensitive to shifts in NH 3 and SO 4 levels because NH 4 is volatile. You can have very acidic aerosol even if NH 4 /SO 4 > 2. Implications: Aerosol nitrate, contrary to current belief and policy, may not be a major component of the regional aerosol as sulfate levels drop in the SE US (and other locations of the world). Acid-mediated process may remain unchanged despite S reductions. Mineral dust /seasalt emissions very important modulators of ph. All this has a profound impact on deposition fluxes of reactive nitrogen Models have never been evaluated for their ability to predict ph, and are important for understanding N deposition (biases). GAW data could be pivotal for addressing this need. 23

24 Acknowledgements THANK YOU! R Health Effects Institute NOAA CPO Award NA10OAR R This publication was made possible by US EPA grant R and R This publication s contents are solely the responsibility of the grantee and do not necessarily represent the official views of the US EPA. Further, US EPA does not endorse the purchase of any commercial products or services mentioned in the publication. 24

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26 SOAS Data analysis confirms ph calculations Guo et al., ACP Guo et al., ACP, The volatilization fraction fluctuates around 0.5. This means that prediction biases in ph would result in appreciable biases in the NH 3(g) fraction. ph is indeed likely for the SE US. Comparison of predicted vs. observed gasphase NH 3. Weber et al. (2016), Nature Geosci.

27 Why this behavior? NH 3 is semi-volatile, buffers system initially: a lot of (NH 4 ) 2 SO4 but no NH 3(g) at equilibrium: some NH 4 volatilizes. particles become acidic NH 3(g)

28 Why this behavior? NH 3 is semi-volatile, buffers system initially: a lot of (NH 4 ) 2 SO4 no NH 3(g) at equilibrium: some NH 4 volatilizes. particles become acidic NH 3(g) initially: less (NH 4 ) 2 SO4 no NH 3(g) at equilibrium: more NH 4 volatilizes. particles may become a little more acidic NH 3(g)

Predicted LWC vs measured LWC (SOAS) W i : LWC associated with inorganics W o : LWC associated with organics Fraction of organic water Total predicted water (W i + W o ) matches

29 Predicted LWC vs measured LWC (SOAS) W i : LWC associated with inorganics W o : LWC associated with organics Fraction of organic water Total predicted water (W i + W o ) matches measured water very well. LWC diurnal ratio (max/min) is 5. W o was significant, 29-39% of total LWC at all sites. Liquid Water: Predicted vs Measured 29 Guo et al., ACP; Cerully et al., ACP

30 Most of the ph variability is driven by LWC variability Guo et al., ACP; Cerully et al., ACP

31 ph diurnal-ctr ph = 0.94 ± 0.59 Particle Liquid Water Overall, ph is very low: < ph < ph diurnal variation is mainly driven by water variability. Considering water associated with organics (W o ) in ph calculation changes ph by at all sites in the SE US. Guo et al., ACP; Cerully et al., ACP 31

32 Decreasing trend of molar ratio Scenario 5: (close to Scenario 2) 1) Na is increased by factors of 2 and 4. 2) Solution: output NH 3 = 0.1µgm -3 Summary: 1. With more Na +, (NH 4+ -NO 3- )/SO 4 2- decreases faster. SO 4 2- begins to associate with Na ph stays almost constant when SO 4 2- > 0.3µgm -3. However, with more Na +, the transition point (ph starts to increase rapidly) occurs at a higher SO 4 2- level. 3. K +, Ca 2+, Mg 2+ have similar results as Na + if they are internally mixed. 32

33 Determining aerosol ph: The heart of it 1. Solid phase: NaHSO 4, NH 4 HSO 4, Na 2 SO 4, NaCl, (NH 4 ) 2 SO 4, (NH 4 ) 3 H(SO 4 ) 2, NH 4 NO 3, NH 4 Cl, NaNO 3, K 2 SO 4, KHSO 4, KNO 3, KCl, CaSO 4, Ca(NO 3 ) 2, CaCl 2, MgSO 4, MgCl 2, Mg(NO 3 ) 2 Species in bold were introduced in ISORROPIA II (Fountoukis and Nenes, 2007) 2. Liquid phase: Na +, NH 4+, H +, OH -, HSO 4-, SO 4 2-, NO 3-, Cl -, H 2 O, HNO 3(aq),HCl (aq), NH 3(aq), Ca 2+, K +, Mg Gas phase: HNO 3, HCl, NH 3, H 2 O In this study, ISORROPIA-II was run in Forward mode, which calculates equilibrium partitioning given total concentration of species (gas + particle).

34 Low acidities are found everywhere Summertime data (2013) ACSM/WAD (comp.) Nephelometer (LWC) ph analysis Look at average and each airmass type sampled Finokalia Bougiatioti et al., ACPD (2016)

35 Finokalia, Crete ph distributions Airmass type: Mineral dust aerosol (fine) Nitrate formation ph threshold Summary/implications: NH 3 vs SO 4 is like in SE US, aerosol is quite acidic. Most of the time, very low NO 3 levels on fine mode aerosol (Surprise!!).

36 Finokalia, Crete ph distributions Airmass type: Continental aerosol (fine) Nitrate formation ph threshold Summary/implications: NH 3 vs SO 4 is like in SE US, aerosol is quite acidic. Most of the time, low levels of NO 3 on fine mode aerosol.

is very high (vs SO 4 ) and that leads to neutralization of

37 Finokalia, Crete ph distributions Airmass type: Smoke/Biomass burning Nitrate formation ph threshold Summary/implications: NH 3 is very high (vs SO 4 ) and that leads to neutralization of aerosol. Most of the time, a lot (almost all) HNO 3 /NO 3 partitions to aerosol.

38 ph: A unique tool for understanding model behavior Case study: aerosol nitrate simulations Aerosol nitrate is hard to predict. Many studies have focused on all possible reasons for biases, except for ph issues. ph has a profound impact on the fraction of HNO 3 /NO 3 that partitions as aerosol nitrate. With a simple analysis of model output we can determine how ph errors predispose models to nitrate prediction bias. Theory says: ph ε! ε Fraction of total nitrate in gas/aerosol phase Liquid water content Meskhidze et al., (2003); Weber et al., (2006)

ph: A unique tool for understanding model behavior Apply theory to understand &

as a function of the ph ph Nitric Acid Depending where you are on the nitrate

39 ph: A unique tool for understanding model behavior Apply theory to understand & analyze simulations ph field ε NO3 Aerosol Nitrate Approach: Use a model (CMAQ) over a simulation period. Obtain HNO 3(g), NO 3, ph for each grid point Plot ε NO3 = [NO 3 ] / [HNO 3 +NO 3 ] as a function of the ph ph Nitric Acid Depending where you are on the nitrate response function, ε NO3, you can be insensitive or very sensitive to ph biases. Observations and ph analysis can indicate the degree of ph bias in simulations.

ph: A unique tool for understanding model behavior Analyze

$average Monthly average partitioning fraction follows the$ sigmoidal shape with ph.

40 ph: A unique tool for understanding model behavior Analyze CMAQ simulations from Domain average Sept ε NO3 ε NO3 Dec CMAQ (ph field) July 10 average Monthly average partitioning fraction follows the sigmoidal shape with ph. Nitrate formation region (ε NO3 )shifts to lower ph as temperature drops. The sensitive regions to ph bias shift to lower ph as well. ph

41 ph: A unique tool for understanding model behavior ε NO3 Monthly average ph value Nitrate formation region (ε NO3 )shifts to lower ph as temperature drops. The sensitive regions to ph bias shift to lower ph as well. Winter ph levels are higher than summertime ph ph Conclusion: Whether predictions of nitrate are sensitive to ph biases depends on If you are in the sensitive region of nitrate formation (worse in wintertime) The magnitude of ph bias (ph obsare important) not known. How the totalnitrate (gas+particle) is over/underpredicted emissions & other processes.

42 ph: A unique tool for understanding model behavior Response in nitrate to ph biases: Approach: Take monthly average [HNO 3(g) +NO 3 ]. Perturb ph ±0.5 units from monthly average. Nitrate changes in response as: [NO 3 ]= [HNO 3(g) +NO 3 ] ε NO3 (ph) We can apply this on a cell-by-cell basis, or other semi-volatile acidic gases. Conclusion: Summertime shifts in ph lead to minor impacts on nitrate levels. Wintertime nitrate levels can be quite sensitive to ph shifts, affecting levels up to 30% of the total nitrate (gas+aerosol) ~ 1 µg m -3 aerosol nitrate. We still don t know how model ph compares to actual measurements. The bias may be larger than 0.5 ph units. We absolutely need to address this!

43 Some take home messages Findings: Particle ph is low (-0.5 to 1.5) and NH 3 varied little in the SE US. Very low acidity seen in dusty regions too (E.Med; Bougiatioti et al., 2016). Future particle ph may remain low even if SO 4 goes down. ph is insensitive to shifts in NH 3 and SO 4 levels because NH 4 is volatile. You can have very acidic aerosol even if NH 4 /SO 4 > 2. Implications: ph proxies used for decades do not work well and should be avoided. Aerosol nitrate, contrary to current belief and policy, may not be a major component of the regional aerosol as sulfate levels drop. Acid-mediated process may continue to remain unchanged. Mineral dust (land use change)/seasalt emissions very important. Models have never been evaluated for their ability to predict ph and presents a unique opportunity for understanding predictive biases. 43

44 ph: A unique tool for understanding model behavior Conclusion: Whether predictions of nitrate are sensitive to ph biases depends on If you are in the sensitive region of nitrate formation (wintertime) The magnitude of ph bias (ph obsare important) not known. How the totalnitrate (gas+particle) is over/underpredicted emissions & other processes.

45 WINTER Nitrate response function The essence of acidity impacts on partitioning Guo et al., in prep.

High aerosol acidity despite declining atmospheric sulfate concentrations:

High aerosol acidity despite declining atmospheric sulfate concentrations: Lessons from observations and implications for models. A.Nenes 1,2,3,4, R.J. Weber 1, H.Guo 1, A.Russell 5, A.Bougiatioti 1,3