Aerosol pH Estimation and Organosulfate Detectability from Aerosol Mass Spectrometry Measurements Melinda K. Schueneman1, Benjamin A. Nault1, Pedro Campuzano-Jost1, Duseong S. Jo1,2, Jason C. Schroder1,*, Douglas A. Day1, Alma Hodzic2, Jack E. Dibb3, John Crounse4, Michelle Kim4,

Karl D. Froyd1,5, Brett B. Palm1, Felipe Lopez-Hilfiker6, Ben H. Lee6, Joel A. Thornton6, and Jose L. Jimenez1

[1] Department of Chemistry, and Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado Boulder, Boulder, CO, USA [2] Atmospheric Chemistry Observations and Modeling, National Center for Atmospheric Research, Boulder, CO 80301, USA

[3] Earth Systems Research Center, Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, USA [4] Department of Chemistry, California Institute of Technology, Pasadena, CA, USA[5] NOAA Earth System Research Laboratory (ESRL), Chemical Sciences Division, Boulder, CO, USA [6] Department of Atmospheric Sciences, University of Washington Seattle, Washington 98195, United States

Questions? melinda.schueneman@colorado.edu & jose.jimenez@colorado.edu

Ambient Measurements with the Aerosol Mass Spectrometer

Introduction Airborne Campaigns What Factors Control Sulfate Fragmentation in the AMS? pH Estimation from AMS Data

CU Aircraft High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) [4]

AMS measures non-refractory species such as sulfate, nitrate, chloride, ammonium, and organics

Species thermally decompose and evaporate

Neutral vapors undergo EI ionization to form positive ions

Mass spectrometer measures ion mass

OS cannot produce H2SO4(g) HySOx

+ vs. SOx+ fragments been

proposed as indicators of OS in 2 recent papers

Fragments may be affected by other particle properties

Aboard NASA DC-8 or NSF/NCAR C-130

Atom-1 and 2: remote atmosphere KORUS-AQ: polluted regions DC-3, SEAC4RS, WINTER:

continental regions

Models Used

E-AIM Thermodynamic model (II) to estimate pH [3]

GEOS-Chem v12.1 used to calculate AN and inputs for ISORROPIA pH around the world (for 2006) [5]

Sulfate deconvolution method [1]: estimates OS from the distance inside the triangle below the pure AS point [1,1] Panel A: KORUS-AQ and lab experiments

with ammonium nitrate (AN) and ammonium sulfate (AS) mixtures. fHySOx

+ affected by increasing ANf

Panel B: Lab experiments colored by OA fraction in PM1, some impact on fHySOx

+

Panel C: ATom-1 and ATom-2 have increasing fHySOx

+ with pH Panel D: Campaign averages all outside of

triangular region, impacted by OA, AN, or H2SO4

Submicron aerosols (PM1) impact visibility, health, chemistry, and climate.

PM1 contains species like sulfates, nitrates, organics, ammonium, soot, and more

Campaigns undertaken on NASA DC-8 to measure ambient air

Recent work attempts to deconvolve AMS total sulfate into organosulfates (OS) and inorganic sulfates (IS; e.g. ammonium sulfate) Using measured sulfate fragments [1-2], such as SO+, SO2

+, SO3+, HSO3

+, and H2SO4+

Methods have not been widely tested Separate analytical challenge is quantifying

aerosol pH Thermodynamic models

used widely to calculate pHDirect, in-situ pH

estimation has not been demonstrated, but would be very useful

Lower left: ANMF<0.3, pH<0About half of the free

troposphere (FT) is in this regime

Bottom right: ANMF<0.3, pH>0SOx fragmentation may be usable to estimate OS contribution to total sulfate

Upper right: regions with ANMF>0.3, pH>0AN controls phase of particle, OS and pH cannot be estimated

Upper left: very few pts Very acidic, nitrate present as HNO3(g)Points here only under very low sulfate

pH can be estimated in near real-time in about half of the global atmosphere (when pH < 0). For the rest of the atmosphere, can constrain that pH > 0

Literature methods for quantifying organic vs inorganic sulfate from AMS sulfate fragments are confounded by ammonium nitrate, acidity, and OA

We clarify the chemical regimes where organosulfates OR pH may be estimated (but not both at the same time)

1. Chen, Y., et. al., Environ. Sci. Technol., 2019.2. Song, S., et. al., Atmos. Chem. Phys., 2019.3. Clegg, S. L., et. al., J. Aerosol Sci., 2003.4. DeCarlo, P.F., et al., Anal. Chem., 2006.5. Bey, I., et. al., J. Geophys. Res., 2001.

References

Plot A: Campaign data displayed in four chemical regimes Plot B: GEOS-Chem (v12) results for the troposphere

AcknowledgementsThis work was supported by NASA grants NNX15AH33A & 80NSSC19K0124, and a CIRES IRP project. Thanks to the members of the Jimenez group for supporting my work and studies, and to AMS Users for useful discussions.

A new sulfate ion ratio is used estimate pH: HySOx

+/SOx+

HySOx+/SOx

+ = (H2SO4++HSO3

+) / (SO3++SO2

++SO+)

fH2SO4+=H2SO4

+/(H2SO4++HSO3

++SO3++SO2

+

+SO+)

fHSO3+=HSO3

+/(H2SO4++HSO3

++SO3++SO2

++SO+)

Instrument & campaign-specific calibrations needed

Atmospheric Relevance & Chemical Regimes

In top right, frequency of pH shown for ATom and KORUS-AQ campaigns

Sulfate ion fraction is noisy

Above shows pH & ammonium balance for 6 campaignsAs NH4_bal increases, pH increasesFit NH4_bal < ~0.65 with pH, R2=0.87

NH4_bal. = 0, all sulfate is sulfuric acid NH4_bal ~ 1, ~ pure AS NH4_bal ≠ pH in continental regions, places with

non-constant change in RH and tempLow NH4_bal = ATom, DC-3, SEAC4RSHigher NH4_bal = KORUS-AQ, WINTER

Histograms of NH4_bal show in B-D

Above, ATom and KORUS-AQ data shownpH vs. HySOx

+/SOx+

(calculated without KORUS-AQ)Below pH ~ 0, HySOx/SOx can be used to estimate pH

NH4_bal = (NH4+) / (NO3

-+2*SO42-) [mol/mol]

Ammonium balance is robust; non-AMS users could use this approximation

pH estimation time series for HySOx

+/SOx+ and

NH4_bal for (A) one flight in

ATom-1 (B) an SO2 plume

sampled during WINTER

(C) a scatter plot with linear regression for the NH4_bal predicted pH vs. E-AIM pH

And (D) HySOx

+/SOx+ pH

vs. E-AIM pH

NASA DC-8 Aircraft

Organic and inorganic sulfate species produce specific ion fragments

Conceptual model: Particles with high AN

fraction or high acidity are more liquid, evaporate faster than pure AS

Faster evaporation lower temperatures, less time for thermal decomposition

GEOS-Chem regime data mapped onto the surface for December, January, and February, “DJF”, (A), June, July, and August, “JJA”, (C), and at 400 hPa for DJF (B) and JJA (D)

Conclusions and Important Take-Away Points

Ammonium Balance, NH4_bal, used to estimate pH

pH time series

Ambient Data

GEOS-Chem