Chemistry of NOx and SOA:VOC Oxidation by Nitrate Radicals

Andrew Rollins

Cohen research group, department of chemistry

University of California, Berkeley, USA

NOx = NO + NO2

NO2NO

hνO2 O3

O3O2

Τs.s. ~ minutes

OH, O3

SOAAerosolSurface

Area

IPCC AR4

Regional NOx Emission trends

van Aardenne et al., Atmospheric Environment 33 (1999) 633Ð646

Estimates for totalAsian emissions

Measured GöteborgNO2

outline

Motivations Global/Regional changes in NOx:VOC emissions

NOx emissions as control strategy

2 classes of NOx effects on SOA production Product distributions / RO2 chemistry

NO3 + VOC → SOA

Nitrate Radical (NO3)

Isoprene + NO3 SAPHIR experiment Alkyl Nitrate kinetic uptake experiments

SOA NOx Dependence: effects on peroxy radical chemistry

Kroll et al. Environ. Sci. Technol. 2006, 40, 1869-1877Presto et al. Environ. Sci. Technol. 2005, 39, 7046-7054

High NOx and VOCUnexplained / not always observed High NOx and VOC

RO2 + HO2 vs

RO2 + NO

Nitrate Radical (NO3)

52minutes

32

23slow

23

23

x5sec

3

ONNONO

ONONOO

2NONONO

NONO daytime

τhν

Nitrate Radical (NO3)

52minutes

32

23slow

23

23

x5sec

3

ONNONO

ONONOO

2NONONO

NONO daytime

τhν

Nitrate Radical (NO3)

Brown et al 2004

Sunset [NO3]≈10’s ppt

52minutes

32

23slow

23

23

x5sec

3

ONNONO

ONONOO

2NONONO

NONO daytime

τhν

NO3 vs OH and O3 as VOC sinks

Brown et al 2004

VOC kOH kO3 kNO3

Isoprene 102 1.28e-5 0.68

α-pinene 54 8.5e-5 6.2

Limonene 170 2.0e-4 12

Methacrolein 34 1.1e-6 4.4e-3

0.5 x 107 cm-3 = 0.2 ppt OH

20 ppt NO3

Decreased but significant [BVOC] remain at night.

Isoprene emissions increase with temperature and light: ~10% isoprene processed by NO3.

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.

NO3 reactions dominate at night: lower temperatures, decreased boundary layer / increased concentrations.

group Pvap factor

ONO2 6.8 x 10-3

OH 5.7 x 10-3

OOH 2.5 x 10-3

J.H. Kroll, J.H. Seinfeld / Atmospheric Environment 42 (2008) 3593–3624

Jϋlich chamber experiments SAPHIR chamber ~

260 m3. Near Ambient NOx &

VOC Long chamber runs (>

12 hours) NO3 SOA experiments:

Linomene Β-Pinene (high and low

RH) Isoprene (seeded)

Isoprene + NO3

15 hour run Max 10 ppb isoprene,

30 ppb NO2, 60 ppb O3

NH3(SO4)2 seed AMS, SMPS, PTRMS,

GC, TDLIF Many NO3 / N2O5

measurements

Isoprene C5H8

440-6601 TgC / ~13002 TgC total non-methane VOC (biogenic + anthropogenic) ≈ 34 – 50% total carbon.

Two double bonds/ multiple oxidation steps / high reactivity to OH, O3, NO3. Isoprene SOA potential is poorly understood, small yields of SOA (5% by

NO3) could be large Fractions of total global SOA annual production (2-3 TgC / 12-70TgC)4

Early OH and O3 experiments (100s of ppbs isoprene and NOx) concluded Isoprene not an SOA precursor, because 1st 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 NO3 observed with high (80%) yields, increase MW and adding functionality. SOA yields reported at 4.3% - 23.8% (increasing with existing OM).6

1Guenther et al. 20062Goldstein and Galbally 20073Calvert et al. 2000

4Kanakidou et al. 20055Zhang et al. 20076Ng et al. 2008

70-80%3-4%3-4%

Isoprene + NO3 Products

< 10% of isoprene consumed by O3

Chamber Experiment Additions

SOA from:•NO3 + initial oxidation products?•RO2 + RO2 vs RO2 + NO3?

Chamber RO2 fate

RO2 + NO3 not expected to produceLess volatile products than RO2 + RO2

kfit

Modeling Chemistry

NO3

Second generationoxidation produts

Role of secondary chemistry

Initial oxidation products

Secondary oxidation products

2% Yield

Isoprene → X → YNO3 NO3

Isoprene

SOAγ

Role of secondary chemistry

Initial oxidation products

Secondary oxidation products

2 0% Yield10% Yield

Isoprene → X → YNO3 NO3

Isoprene

SOAγ

.Gen.Prod1

SOAγ

st

Importance of NO3 / nighttime oxidation

Apel et al 2002, JGR VOL. 107, NO. D3, 10.1029/2000JD000225

SAPHIR

Ambient

70-80%3-4%3-4%

Aerosol Composition

ObservedSOA

Composition

NO3NO3

NO3

NO3RO2

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 + NO3, 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 NO3 additions lead to aerosol.

2 observations indicate underestimation of aerosol nitrate, or NOx 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 NO2

Measurements of total aerosol bound nitrate mass in:

HNO3

Organic Nitrates

2Δ

2 NOXXNO

TD-LIF Aerosol Organic Nitrate

Coupled to entrained aerosol flow tube for measurement of uptake coefficients

Remove gas phase NOy, pass aerosol

PneumaticNebulizer,(NH4)2SO4

droplets

Diffusion Dryer

NOy Bubbler

Entrained Aerosol Flow Tube

HNO3 on NH3(SO4)2 particles

4

Ak

ω = 34100 cm/sA = 5 x 10-3 cm2/cm3

γ = 0.006

Uptake of synthesized organic nitrates

•Salts•Organic particles

NOx / Aerosol Research Questions

Effects of changing NOx / 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 NO3 oxidation of BVOC’s.

Flow tube uptake measurements of organic nitrates / nitric acid on aerosol surfaces.

Take Home Points

Regulation of NOx emissions is a primary control strategy and we should expect NOx / VOC ratios will change with significant regional differences.

NO3 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