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Secondary Organic Aerosol Contributions during CalNex – Bakersfield.
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Secondary Organic Aerosol Contributions during CalNex – Bakersfield 1 U.S. EPA, Office of Research and Development, RTP, NC; 2 Alion Science and Technology, RTP, NC; 3 U.S. EPA, Region 9, San Francisco, CA; 4 Dept Environmental Science and Engineering, University of North Carolina, Chapel Hill, NC T.E. Kleindienst,1 J.H. Offenberg,1 M. Lewandowski,1 I. Piletic,1 M. Jaoui,2 A. Lee,3 C. Bohnenkamp,3 K. Hoag,3 C.L. Rubitschun,4 J.D. Surratt 4
Issues in Particle Chemistry for the Agency The Office of Research and Development (ORD) is the research arm of the U.S. EPA. Research serves to aid regulatory development. Context for our participation in CalNex: • Apportioning method using molecular tracers initially developed to aid and test SOA parameterization in CMAQ; method also included precursors not classically recognized as SOA-producing HCs. • Comparisons between laboratory and ambient aerosol only conducted for samples collected in Eastern U. S.; high biogenic emissions. • High contribution associated with isoprene SOA; at the time of initial development, little appreciation for the detailed mechanism and role of heterogeneous processes in isoprene chemistry (initial collaboration with Seinfeld group at Caltech now continuing with Surratt group at UNC-CH). • EPA Region 9 submitted proposal to leverage ORD research funds through the Regional Applied Research Effort (RARE) program at EPA; requested our participation in CalNex – Bakersfield.
Issues in Particle Chemistry for the Agency Some relevant questions for CalNex measurements: • What are the sources of PM2.5 in the Central Valley as represented by Bakersfield? • How well does the SOA source apportionment technique work with aerosol collected in Western U. S.? • Can we determine the relative importance of anthropogenic and biogenic VOCs to PM2.5 in the Central Valley using apportioning techniques? • What are the relative contributions of primary and secondary organics to PM2.5 in the Central Valley?
Approach for Apportioning SOA • Determine in laboratory samples molecular tracer compounds that appear to be associated with specific precursors. • Examine PM2.5 from field samples to see which compounds are present both in laboratory and ambient atmospheres. • Analyze laboratory samples to determine the ratio of tracer compounds to SOC produced. • Apply the laboratory-based factor to ambient tracer concentration to estimate the contribution of the relevant precursor to organic carbon or mass. • Presently have tracers for SOA from isoprene, monoterpenes ( as α-pinene, aromatics (as toluene), sesquiterpenes (as β-caryophyllene).
I–1 T–3 DHOPA A–2 I–2 I–3 A–4 A–3 C–1 A–5 A–6 Structures of Some SOA Tracers a-Pinene tracers Toluene tracer Pinonic acid Pinic acid Isoprene tracers b-Caryophyllene tracer
Additional Tracers Being Investigated (focus on anthropogenic precursors) • The use of the present technique typically gives for very low anthropogenic contributions to ambient OC simply from a lack of laboratory information on anthropogenic HC precursors. • High MW alkanes have been recently investigated with little success in finding possible SOA molecular tracer candidates. • Recently investigations have been undertake to examine phthalic acid as a possible tracer of the contribution of PAH SOA. • Experiments have been conducted to obtain a mass fractions of phthalic acid for naphthalene and substituted-naphthalene compounds. Upper limit for PAH contributions expressed as naphthalene due to possible primary sources and other factors.
Field Measurements
Sampling Approach for SOA TracersDuring CalNex – Bakersfield • Tisch 226 L min-1 samplers used for PM2.5 collection • Samples taken onto precleaned quartz filters nominally for 23 h (midnight – 11 p.m.; 300 m3 of air sampled) • Sampling done for nominal periods of 15 May – 30 Jun 2011 • PM samples taken for following components: - Organic carbon mass - Primary molecular tracers (R. Sheesley, Baylor Univ) - Secondary molecular tracers - Carbon-14 fraction (Woods Hole analysis) - Archive filter
Analytical Method for Polar Organic Analysis • React solvent extracted PM2.5 sample with derivatizing agent to formed more volatile compounds able to elute through a GC column (DB-5). • Derivatization performed using N,O-bis-(trimethylsilyl)-trifluoroacetamide (BSTFA) which reacts with both acidic and alcoholic OH groups. • GC-MS analysis performed via positive ion CI w/CH4 as reagent gas. • Derivative forms a series of fragments and adducts that permit molecular weight determination and other structural information. e.g. - • m/z : M – 105 ion indicates an alcoholic OH group present on parent • m/z : M – 117 ion indicates an acidic OH group present • Most SOA molecular tracers do not have standards available and surrogates must be employed. • 5 ion-to-TIC technique can be used to work around coelutions which would otherwise impact the quantification of tracers using the TIC alone.
Total Ion Chromatogram – Bakersfield (4 Jun 10) -P Tr Iso Tr
Concentrations of Isoprene Tracers in Bakersfield Samples
Comparison to Tracer Levels in Detroit, MI (TIC; DEARS Ambassador Bridge Site, 24 Aug 2004; OC = 3.72 g m-3) Biogenic Tracer Concentrations I : Isoprene: 168 ng m-3 A: a-Pinene: 153 ng m-3
Concentrations of Anthropogenic Tracers in Bakersfield Samples
Method for Estimating SOC Source Contributions • Laboratory measurements • Irradiated single component hydrocarbon/NOX mixtures; repeat for other conditions • Identify tracer compounds and determine concentrations as ketopinic acid • Calculate the mass fraction of the tracer compounds to the measured SOC • Apply to field measurements • Measure SOC tracers in ambient PM2.5 • Apply the mass fraction factor to get the SOC for each precursor type • Compare SOC contributions to the measured OC • Assumptions and uncertainties • Assume mass fraction of the tracers is the same in the field as in the laboratory. • Other possible of sources of the tracer compounds may not be known (e.g. phthalic acid). • Standard deviation of the mass fractions from lab experiments range from 30 – 50% • Extrapolations from single hydrocarbon contributions to compound classes. • Measurement of ambient OC and the precursor contribution to OC are independent quantities.
Contributions of Secondary Sources to PM2.5 (Bakersfield, by Organic Carbon – g C m-3)
Contributions of Secondary Sources to PM2.5 (Bakersfield, by Organic Mass – g m-3)
Conclusions/Observations • Using this secondary apportionment method in Bakersfield: • SOC levels ranged from 0.03 – 0.57 g C m-3; average: 0.24 gC m-3 • SOA levels ranged from 0.06 – 1.17 g m-3; average: 0.50 g m-3 • OC levels from 2.1 – 10.2 g C m-3; average: 6.5 gC m-3 (no organic denuder) • SOC from isoprene greater than monoterpenes as α-pinene; no contribution from β-caryophyllene. • As per this method, while biogenics SOC higher than anthropogenics, the absolute values of both are very low compared to the measured OC (ca 5%). • Possible systematic errors of the mass fraction method: • For areas of high biogenic emissions, the method may overpredicts SOC based on a comparison w-SOC estimates of Blanchard et. al. 2008. • For areas of where anthropogenic emissions dominate biogenic emissions, the method may underpredict SOC. • SOC contributions by this method are considerably lower than measured in Eastern U.S during a similar time of year.
Future Work for the CalNex Study • Work for the Pasadena and Bakersfield site. • Finish SOC/SOA analysis for Pasadena. • Finish POC analysis and CMB for primary sources; disposition of UCM. • 14C from Woods Hole; work up results. • Examine other organic acids, hydroxyacids, and polyols from both sites (just started) • Investigate confounding factors in use of phthalic acid for SOA from gas-phase PAHs. • Desired Collaborations: • Compare concentrations between individual tracer compounds and those derived by LC-MS analysis (e.g., α-pinene tracers). • Examine correlations between observed MTs and isoprene-derived organosulfates. • Examine correlations between a variety of tracer compounds and various AMS metrics. • Comparison of common components with other techniques.
