Chemistry of NO x and SOA: VOC Oxidation by Nitrate Radicals Andrew Rollins Cohen research group, department of chemistry University of California, Berkeley, USA

NO x = NO + NO 2 NO 2 NO hνhν O2O2 O3O3 O3O3 O2O2 Τ s.s. ~ minutes

OH, O 3 SOA Aerosol Surface Area IPCC AR4

Regional NO x Emission trends van Aardenne et al., Atmospheric Environment 33 (1999) 633Ð646 Estimates for total Asian emissions Measured Göteborg NO 2

outline Motivations  Global/Regional changes in NO x :VOC emissions  NO x emissions as control strategy 2 classes of NO x effects on SOA production  Product distributions / RO 2 chemistry  NO 3 + VOC → SOA Nitrate Radical (NO 3 ) Isoprene + NO 3 SAPHIR experiment Alkyl Nitrate kinetic uptake experiments

SOA NO x Dependence: effects on peroxy radical chemistry Kroll et al. Environ. Sci. Technol. 2006, 40, Presto et al. Environ. Sci. Technol. 2005, 39, High NO x and VOC Unexplained / not always observed High NO x and VOC RO 2 + HO 2 vs RO 2 + NO

Nitrate Radical (NO 3 )

Brown et al 2004 Sunset [NO 3 ]≈10’s ppt

NO 3 vs OH and O 3 as VOC sinks Brown et al 2004 VOCk OH k O3 k NO3 Isoprene e α-pinene548.5e-56.2 Limonene1702.0e-412 Methacrolein341.1e-64.4e x 10 7 cm -3 = 0.2 ppt OH 20 ppt NO 3

Decreased but significant [BVOC] remain at night. Isoprene emissions increase with temperature and light: ~10% isoprene processed by NO 3. Products of daytime oxidation persist with high concentrations throughout the night. Blodgett Forest Research Station (Sierra Nevada Mountains, California) Summer 2007 average

Alkene Oxidation by Nitrate Radicals Decrease in vapor pressure of parent molecule upon addition of nitrate group is comparable to products of reaction with OH. NO 3 reactions dominate at night: lower temperatures, decreased boundary layer / increased concentrations. groupP vap factor ONO x OH5.7 x OOH2.5 x J.H. Kroll, J.H. Seinfeld / Atmospheric Environment 42 (2008) 3593–3624

Jϋlich chamber experiments SAPHIR chamber ~ 260 m 3. Near Ambient NO x & VOC Long chamber runs (> 12 hours) NO 3 SOA experiments:  Linomene  Β-Pinene (high and low RH)  Isoprene (seeded)

Isoprene + NO 3 15 hour run Max 10 ppb isoprene, 30 ppb NO 2, 60 ppb O 3 NH 3 (SO 4 ) 2 seed AMS, SMPS, PTRMS, GC, TDLIF Many NO 3 / N 2 O 5 measurements

Isoprene C 5 H TgC / ~ TgC total non-methane VOC (biogenic + anthropogenic) ≈ 34 – 50% total carbon. Two double bonds/ multiple oxidation steps / high reactivity to OH, O 3, NO 3. Isoprene SOA potential is poorly understood, small yields of SOA (5% by NO 3 ) could be large Fractions of total global SOA annual production (2-3 TgC / 12-70TgC) 4 Early OH and O 3 experiments (100s of ppbs isoprene and NO x ) concluded Isoprene not an SOA precursor, because 1 st generation oxidation products of isoprene are too volatile. More recently photochemical experiments demonstrate that Isoprene possibly contributes up to 47% 5 of global SOA, by polymerization and heterogeneous chemistry of initial oxidation products Alkyl Nitrate formation by addition of NO 3 observed with high (80%) yields, increase MW and adding functionality. SOA yields reported at 4.3% % (increasing with existing OM). 6 1 Guenther et al Goldstein and Galbally Calvert et al Kanakidou et al Zhang et al Ng et al. 2008

70-80% 3-4% Isoprene + NO 3 Products

< 10% of isoprene consumed by O 3 Chamber Experiment Additions

SOA from: NO 3 + initial oxidation products? RO 2 + RO 2 vs RO 2 + NO 3 ?

Chamber RO 2 fate RO 2 + NO 3 not expected to produce Less volatile products than RO 2 + RO 2

k fit Modeling Chemistry NO 3 Second generation oxidation produts

Role of secondary chemistry Initial oxidation products Secondary oxidation products 2% Yield Isoprene → X → Y NO 3

Role of secondary chemistry Initial oxidation products Secondary oxidation products 2 0% Yield 10% Yield Isoprene → X → Y NO 3

Importance of NO 3 / nighttime oxidation Apel et al 2002, JGR VOL. 107, NO. D3, /2000JD SAPHIR Ambient

70-80% 3-4% Aerosol Composition Observed SOA Composition NO 3 RO 2 polymerization, decomposition

Aerosol Composition High correlation between AMS nitrate, AMS organic and total alkyl nitrates signals indicates condensation of organic nitrate is responsible for majority of SOA High initial yield of nitrate formation from initial reaction Total mass observed requires SOA by oxidation of one of the organic nitrate products of isoprene + NO 3, not just MVK and MACR.

AMS indicates 15% mass is nitrate mass High yield of nitrates from initial rxn and correlation of nitrate formation with SOA suggest multiple NO 3 additions lead to aerosol. 2 observations indicate underestimation of aerosol nitrate, or NO x release upon SOA condensation

Thermal Dissociation Laser Induced Fluorescence of Aerosol Nitrates 1. Thermal desorption of semivolatiles 2. Thermal dissociation of nitrates: 3. LIF detection of NO 2 Measurements of total aerosol bound nitrate mass in:  HNO 3  Organic Nitrates

TD-LIF Aerosol Organic Nitrate Coupled to entrained aerosol flow tube for measurement of uptake coefficients Remove gas phase NO y, pass aerosol

Pneumatic Nebulizer, (NH 4 ) 2 SO 4 droplets Diffusion Dryer NO y Bubbler Entrained Aerosol Flow Tube

HNO 3 on NH 3 (SO 4 ) 2 particles ω = cm/s A = 5 x cm 2 /cm 3 γ = 0.006

Uptake of synthesized organic nitrates Salts Organic particles

NO x / Aerosol Research Questions Effects of changing NO x / VOC emissions on the total SOA production, and speciation.  Total yield changes?  Aerosol composition? If composition, is CCN affected? Current research:  Chamber SOA and organic nitrate aerosol yields / mechanisms from NO 3 oxidation of BVOC’s.  Flow tube uptake measurements of organic nitrates / nitric acid on aerosol surfaces.

Take Home Points Regulation of NO x emissions is a primary control strategy and we should expect NO x / VOC ratios will change with significant regional differences. NO 3 chemistry important for producing higher MW organics, is active at night when concentrations of primary VOC’s are lower compared to oxidation products providing an increased opportunity for multiple oxidation steps, temperatures are lower. Yields for SOA produced from VOC’s requiring multiple oxidations to achieve low enough vapor pressure for condensation may be underestimated.

Thanks to… Cohen Group  Juliane Fry (Reed College, Oregon)  Ronald Cohen  Paul Wooldridge F.Z. Jϋlich scientists  Astrid Kiendler- Scharr Steve Brown, Hendrik Fuchs, Bill Dubé (NOAA) Sarpong Group (UCB)  Walter Singaram  Massoud Motamed