Post on 18-Dec-2015
Chemistry of NOx and SOA:VOC Oxidation by Nitrate Radicals
Andrew Rollins
Cohen research group, department of chemistry
University of California, Berkeley, USA
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)
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
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
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.