High aerosol acidity despite declining atmospheric sulfate concentrations:

Transcription

1 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 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 Acidity component sources: Acidic Basic SO 2 (g) NOx(g) VOCs NH 3 (g) Oxidation H 2 SO 4 (p) HNO 3 (p) Org acids (p) Mineral dust, sea salt (Na, K, Ca, Mg, etc.) Formation of (NH 4 ) 2 SO 4, NH 4 HSO 4, NH 4 NO 3, etc. depends on particle ph Fine Mode Mainly in coarse mode (Dp > 1 µm) 2

3 ph Impacts 1. Impacts particle phase acid-catalyzed reactions; Isoprene (the largest VOC) IEPOX-OA IEPOX-OA is 20% of OA in SE in summer (Xu et al., 2015) PM 2.5 mass, optics, 2. Controls acidic and basic gas-particle partitioning; e.g. Nitric acid and nitrate, organic acids Affects nitrogen deposition Solubilizes mineral dust and metals; 1-2% Fe mobilized after 4 days at ph=2 ecosystem nutrient (Meskhidze et al., 2003) Water soluble transition metals adverse health effects (Verma et al., 2014) Fine acidic Coarse neutral

4 Introduction Historical SO 2 and SO 4 2- trends: In the past twenty years, SO 2 emissions have been 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 emission goes down? 2. Are nitrate particles becoming dominant aerosols in southeast US? (Hidy et al., 2014) (Hand et al., 2012)

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

6 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.

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 but it s NOT becoming more neutral. In fact it s acidifying. Weber et al. (2016), Nature Geosci.

8 Determining aerosol ph: The problem Acidity / ph definition: pppp = log 10 HH HH aaaaaa = log 10 LLLLLL 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. HH + aaaaaa, LLLLLL uuuuuuuuuu: μμμμ mm 3 aaaaaa Ion balance: NH 4+ /SO 4 2- Molar ratio: (Guo et al., 2015) 8

9 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 ); o Particle water or total organics & κκ oooooo ; o RH and T; Ions ISORROPIA-II, with RH and T W i, particle water associated with inorganics + HH aaaaaa Organics κκ oooooo W o, particle water associated with organics Predicted LWC vs ph σσ sspp m dry, ρ p Measured LWC Lu et al. (2015) PNAS; Guo et al., (2015) ACP; Cerully et al., (2015) ACP

10 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).

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 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.

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) Weber et al. (2016), Nature Geosci.

17 NH 3 has and probably will remain the same Proof from observation: 3 years (AMoN sites) Proof from mass balance (in the boundary layer): 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) dd[nnnn 3 ] dddd = EE NNNN3 vv dd NNNNN eeeeeeeeeeeeeeees h gggggg NNNN 3 vv NNNN + 4 dd h dddddddddddddddddddd rrrrrrrr NNNN 4 + aaaaaaaaaaaaaa EE NNNN3 : gggggg ppppppppp NNNN 3 eeeeeeeeeeeeeeee rrrrrrrr; vv dd NNNNN : gggggg ppppppppp dddddddddddddddddddd vvvvvvvvvvvvvvvv; vv dd NNNN 4 + : pppppppppppppppp ppppppppp dddddddddddddddddddd vvvvvvvvvvvvvvvv; h: bbbbbbbbbbbbbbbb llllllllll mmmmmmmmmm ddddddddd Weber et al. (2016), Nature Geosci.

18 NH 3 has and probably will remain the same Proof from observation: 3 years (AMoN sites) Proof from mass balance (in the boundary layer): 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) dd[nnnn 3 ] dddd = EE NNNN3 vv dd NNNNN eeeeeeeeeeeeeeees h gggggg NNNN 3 vv NNNN + 4 dd h dddddddddddddddddddd rrrrrrrr NNNN 4 + aaaaaaaaaaaaaa EE NNNN3 : gggggg ppppppppp NNNN 3 eeeeeeeeeeeeeeee rrrrrrrr; vv dd NNNNN : gggggg ppppppppp dddddddddddddddddddd vvvvvvvvvvvvvvvv; vv dd NNNN 4 + : pppppppppppppppp ppppppppp dddddddddddddddddddd vvvvvvvvvvvvvvvv; h: bbbbbbbbbbbbbbbb llllllllll mmmmmmmmmm ddddddddd bbbbbb vv dd NNNNN vv dd NNNN 4 + Weber et al. (2016), Nature Geosci.

19 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): NNNN 3 hee NNNN 3 vv dd NNNNN EE NNNN3 : gggggg ppppppppp NNNN 3 eeeeeeeeeeeeeeee rrrrrrrr; vv dd NNNNN : gggggg ppppppppp dddddddddddddddddddd vvvvvvvvvvvvvvvv; h: bbbbbbbbbbbbbbbb llllllllll mmmmmmmmmm ddddddddd EE NNNN3 increased slightly (~10%) during the last decade globally. (Erisman et al., 2008) NH 3 has and probably will remain the same Weber et al. (2016), Nature Geosci.

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. 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 20

21 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 (wont happen) are needed to increase ph Weber et al. (2016), Nature Geosci.

22 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)

23 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)

24 Low ph is everywhere (WINTER Campaign) Guo et al. (2016) ph = 0.7 ± 1.3 (median = 0.7, 10% percentile= -1, 90% percentile= 2.15) WINTER: aerosol ph was low; similar to SE

25 Low ph is everywhere Summertime data (2013) ACSM/WAD (comp.) Nephelometer (LWC) ph analysis Look at average and each airmass type sampled Finokalia Bougiatioti et al., ACP (2016)

26 Finokalia, Crete ph distributions Airmass type: Continental aerosol (fine) Bougiatioti et al., ACP (2016) 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.

27 Finokalia, Crete ph distributions Airmass type: Mineral dust aerosol (fine) Bougiatioti et al., ACP (2016) 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!!).

28 Finokalia, Crete ph distributions Airmass type: Smoke/Biomass burning Bougiatioti et al., ACP (2016) 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.

29 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 HHHHHH3 = HH + +KK HH RRRR [HH + ] HH + +KK aa LLLLLL10 14 ε HHHHHH3 Fraction of total nitrate in gas/aerosol phase Liquid water content Meskhidze et al., (2003); Guo et al., in press

30 PM 1 ph and the observed partitioning of nitrate and ammonium follow S-curves Nitric Acid - Nitrate Ammonia - Ammonium East US No NH 3 data in WINTER Winter Summer

31 PM 1 ph and the observed partitioning of nitrate and ammonium follow S-curves Nitric Acid - Nitrate Ammonia - Ammonium East US No NH 3 data in WINTER Winter Summer

32 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.

33 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

34 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) not known. How the total nitrate (gas+particle) is over/underpredicted not often evaluated.

35 ph: A unique tool for understanding model behavior [NO 3 ] from shifs in ph Response in nitrate to ph biases: Approach: Take monthly average [HNO 3(g) +NO 3 ]. Perturb ph ± 0.5 units from monthly average. ph ε NO3 (ph) for each curve Nitrate response then is: [NO 3 ] = [HNO 3(g) +NO 3 ] avg ε NO3 (ph) 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. Regional responses can be larger or smaller to a ph error. ph or ε NO3 (ph) has never been evaluated in models!

36 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 throughout, even in dust (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. 36

37 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 throughout, even in dust (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. 37

38 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 throughout, even in dust (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. 38

39 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. 39