Describing oxidation of organics

Transcription

1 Describing oxidation of organics using the AMS Jesse Kroll, Doug Worsnop, Chuck Kolb, Sean Kessler, Jose Jimenez, Allison Aiken, Pete DeCarlo, Neil Donahue, Kevin Wilson, Jared Smith, Tim Onasch, Manjula Canagaratna, Sally Ng 1) OM/OC, O:C 2) absolute O content 3) oxidation state OM/OC OM/OC: Organic Mass / Organic Carbon = (Organic COrganic O Organic H Organic N ) / Organic C (by mass) allows for chemical information (namely oxygen content) to be derived without detailed chemical measurements, but instead with SMPS, OC/EC Turpin and Lim 2001: Urban (fresh): 1.6 ±0.2 Non urban (aged): 2.1 ±0.2 1

2 OM/OC vs. O/C so which to use? OM/OC OM/OC (pros) wide range of measurements from many groups direct connection (shared vocabulary) between AMS, non AMS communities O:C [Aiken 2008] OM/OC (cons) less chemically specific than elemental ratios For atmospheric aerosols, O:C and OM/OC are essentially equivalent Absolute elemental abundances The absolute number of atoms (or moles) of a given element in particles provide chemical information not obtained by ratios alone Example: atmospheric aging if O:C goes up condensation of SOA? heterogeneous aging? water uptake? / if C content goes up: condensation/soa if C content goes down: volatilization (chemical or physical) if only O content goes up: oxidation if O content goes up, H content goes up x2: water uptake 2

3 Absolute elemental abundances If aerosol contains C, H, O only Mass particle = mass(c) mass(o) mass(h) = 12 mol(c) 16 mol(o) mol(h) = 12 mol(c) 16 (O:C) mol(c) (H:C) mol(c) mol(c) = Mass particle 12 16(O: C) (H: C) mol(o) = mol(c) (O:C) mol(h) = mol(c) (H:C) Mass: org AMS CE or V SMPS ρ Absolute elemental abundances If aerosol contains C, H, O only Mass particle = mass(c) mass(o) mass(h) = 12 mol(c) 16 mol(o) mol(h) = 12 mol(c) 16 (O:C) mol(c) (H:C) mol(c) mol(c) = Mass particle 12 16(O: C) (H: C) mol(o) = mol(c) (O:C) mol(h) = mol(c) (H:C) Mass: org AMS CE or V SMPS ρ Massparticle Massparticle mol(c) (mf ) (H: C) 15 61(mf )

4 Example: laboratory oxidation of alkanes Physical properties of particles Kroll et al., PCCP 2009 Example: laboratory oxidation of alkanes Elemental ratios in particles Kroll et al., PCCP

5 Example: laboratory oxidation of alkanes Elemental abundances in particles increase in O:C not from net addition of oxygen, but from loss of carbon! Similar results in field: (O:C goes up, org constant) DeCarlo et al, ACP 2008 Capes et al, JGR 2008 Dunlea et al, ACPD 2008 Kroll et al., PCCP 2009 Problems with O:C as a metric of oxidation/aging Different functional groups (alcohols vs carbonyls) CH 4 CH 3 OH CH 2 O CO CO 2 O/C Water addition/loss: not an oxidation process! Hydrocarbons (O:C=0) are not equal: alkanes vs alkenes vs aromatics vs PAHs All of these involve changes in H:C, which we tend to ignore. 5

6 Oxidation state of organic carbon In an oxidizing atmosphere, the oxidation state of carbon increases: methane oxidation C 5 carbon chain Measuring oxidation state with the AMS Average carbon oxidation state can be determined from any analytical technique that measures elemental ratios (O:C, H:C) of organics: Ox. State 2 (O:C) (H:C) Exceptions: peroxides (O: 1) heteroatoms (N: 3 5; S: 2 6; etc.) However mostof theseexceptions (organonitrates organosulfates peroxides) However, most of these exceptions (organonitrates, organosulfates, peroxides) involve weakly bound functional groups ( adducts ) The AMS isn t good at measuring adducts!!! Many of these fragments (NO x, SO x ) will show up as inorganic species, and not part of the organic mass spectrum. 6

7 Oxidation state of organic aerosol Ox. State 2 (O:C) (H:C) Ambient (MCMA averages) T0 (morning): 1.0 T0 (afternoon): 0.5 C 130 (city): 0.4 C 130 (outflow): 0.2 C 130 (background): 0.2 Ambient (MCMA factor analysis) Hd Hydrocarbon like (HOA): Biomass burning (BBOA): Oxidized ( fresh, OOA β): Oxidized ( aged, OOA α): Source characterization/laboratory Gas/diesel exhaust: Biomass burning: Secondary organic aerosol: Atmospheric organic aerosol: 2 to 1 Aiken et al., ES&T, 2008 Shilling et al., ACP, 2009 Role of H 2 O ions Impossible to unambiguously apportion H 2 O (and OH and O ) ions among organic, sulfate, and liquid water components Must be done empirically (frag list): for org, tying H 2 O ions to CO 2 prior to 2008: mz18(org) = mz44(org) prior to 2008: mz18(org) mz44(org) Aiken et al 2008: mz18(org) = mz44(org) 7

8 Role of H 2 O ions But: loss or gain of water does not affect oxidation state since the changes in O/C, H/C cancel out: AMS measurements of standards [Aiken 2008] O:C H:C Need to correct both ratios for biases arising from ion fragmentation (fraction of both O, H in ions is smaller than in parent molecules) 8

9 AMS measurements of standards [Aiken 2008] Ox. State 2 (O:C) (H:C) AMS measures oxidation state reasonably well, with no corrections almost no oxidation state bias from AMS fragmentation dehydration reactions (loss of H 2 O) from vaporization/ionization? Alkanes vs alcohols C 3 H 7 C 4 H 9 mass fraction C 3 H 5 C 4 H 7 C 5 H 9 C 5 H 11 C6H13 n-alkane Δ=2, m/z 9

10 Alkanes vs alcohols C 3 H 7 C 4 H 9 mass fraction C 3 H 5 C 4 H 7 C 5 H 9 C 5 H 11 C6H13 n-alkane Δ=2, m/z mass fraction C 3 H 5 C 4 H C 5 H 9 C 6 H 11 1-alcohol Δ=0,2,1 OH m/z Conclusions OM/OC and O:C are essentially equivalent for atmospheric aerosol, so only one of them ( ) needs to be reported Absolute elemental abundances (combining elemental ratios with particle mass) can provide info on gas particle processes Oxidation state of carbon is a good (and unambiguous) measurement of atmospheric oxidation The AMS is very good at measuring carbon oxidation state 10