Heterogeneous Photochemistry of Nitrogen Oxides on Urban Grime

Alyson Baergen and D.J. DonaldsonApril 15, 2014

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Grime

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Grime

- Aesthetics of window soiling- Influence cycling of contaminants–Compartment for partitioning–Facilitate pollutant run off

- Reactive substrate – HNO3

Lombardo, T. et al. Atmos Environ 2005; Diamond, Wu R.W. et al Atmos Environ 2008; M.L. et al. Chemosphere 2001; Baergen A. et al Environ Sci Technol 2013

HNO3 as a NOx Sink

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NO NO2

O2O3

RO2 RO

OH

HNO3

N2O5

O3NO3

NO2

OH

HONO

OHNH4NO3

NH3

Surface sources of HNO3

2NO2 + H2O HNO3+HONO

N2O5 + H2O 2 HNO3

Photochemical Processing

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Surfaces as Photochemical Sources

HNO3

NONO2

HONON2O

Jacobi, H.-W.;et al Environ Sci Technol 2013; Rubasinghege, G.; Grassian, V. H. J Phys Chem A 2009; Zhou, X.; et al. Nature Geosci 2011

Why grime?

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Grime is ubiquitous on

impervious surfaces

Less than 1 micron in thickness

resulting in large surface area to

volume

Nitrate is present within

grime

Lam, B. et al Atmos Environ 2005; Alfaro, S. C. et al. Atmos Environ 2012

Grime Composition

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DecemberOctober

• (1.5±0.2)ng/cm2 of NO3- for samples collected over 4 weeks

• Between 6 and 45% of the extractable ions by mass.

Method: Grime collected directly on ATR-FTIR crystal for 1 week and illuminated with solar simulator

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Nitrate Photolysis on Real Grime

Baergen, A. M.; Donaldson, D. J. Environ Sci Technol 2013

NO3-

ν3 asymmetric stretch

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Nitrate Photolysis on Real Grime

1Zhou, X. et al. J. Geophys Res Lett 2003 Baergen, A. M.; Donaldson, D. J. Environ Sci Technol 2013

J=3x10-2 min-1

Water Glass1 GrimeLifetime

(Adjusted to Summer Sun)

116 days ~13 hours 14 min

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Nitrate Photolysis on Real Grime

GrimeGrime w/ HNO3

Hydrocarbon Grease

Method: Add gas phase HNO3 to grime or film composed of hydrocarbon or grease. Illuminate with

solar simulator and monitor with ATR-FTIR

NO+

(NO, NO2, HONO)

Method: Photochemical Knudsen cell with Electron-Impact Quadropole Mass Spectrometer. Grime collected

on pyrex sample holders for 1 week then exposed to headspace above conc. HNO3

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Gas Phase Products from Grime Photochemistry: EI-MS

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Method: Incoherent Broad Band Cavity Enhanced Spectroscopy to detect gas phase products from illuminated

grime collected for 1 week with additional HNO3

Gas Phase Products from Grime Photochemistry: IBBCEAS

• Production is enhanced when the sample is exposed to high RH

HONO: ~ 3x1011 molec cm-3

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H2O Presence on Real GrimeMethod: Grime collected directly on crystal for Quartz Crystal

Microbalance for 1 week and exposed to increasing then decreasing RH

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Lam, B. et al Atmos Environ 2005; Alfaro, S. C. et al. Atmos Environ 2012; Aubin, D G. et al J.Phys Chem A. 2007; Miet et al Atmo Environ 2009; Monge, M E. et al PNAS 2010; Stemmler, K. et al Atmos Chem Phys 2007;

Sosedova, T. et al Atmos Chem Phys 2007 Zhou, X.; et al . Nature Geosci 2011; Han et al Atmos Environ 2013

Organic Chemistry and NOx/HONO

• Organics could play a role in reducing NO2 to HONO• Photoenhanced on soot, humic acid, polyphenolic compounds• HONO/NOx ratios of 1-5 for HNO3 photolysis on surface

containing organic compounds and moisture• Organics could react with HNO3 or NO2 to form R-ONOx and R-NO2

species which can photolyze• Observed on PAH film and soot

R-NO2 hν R’ + HONOR-ONO hν R’C=O + NO

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Method: Apiezon N vacuum grease exposed to head space of conc. HNO3 and analyzed by ATR-FTIR

Condensed Phase Products from HNO3 and Hydrocarbon Grease

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Method: HNO3-exposed grease was illuminated and analyzed by ATR-FTIR

Condensed Phase Products from HNO3 -Exposed Grease Photochemistry

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Method: Detected products from grease treated with HNO3 with photochemical Knudsen cell putting long pass

filters in the beam of the light source

NO+

Gas Phase Products from HNO3 - Exposed Grease Photochemistry: EI-MS

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Method: IBBCEAS to detect gas phase products from illuminated HNO3-exposed grease

Gas Phase Products from HNO3 - Exposed Grease Photochemistry: IBBCEAS

HONO:~ 2x1012 molec cm-3

NO2: ~1x1012 molec cm-3

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Grime mediates fast NO3- photolysis

Water is present on the film and is important for the chemistry

HONO is a major product of the photochemistry

Grime’s organic fraction of may play a role

Reaction Pathways

R-NO2 hν R’ + HONOR HNO3 R-NO2, RONOx

R-ONO hν R’C=O + NOR-ONOx/R-NO2 hν R’ + NO2

Urban grime may mediate a photochemical pathway for the cycling of nitrogen oxides

back into the atmosphere

Baergen, A. M.; Donaldson, D. J. Photochemical Renoxification of Nitric Acid on Real Urban Grime Environ Sci Technol 2013

Baergen, A. M.; Donaldson, D. J. Formation of Reactive Nitrogen Oxides from the Photochemistry of Urban Grime. In prep

abaergen@chem.utoronto.ca