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Differential Photoacoustic (DPAS) Method

for PM Absorption Measurement

Zhenhong Yu

Aerodyne Research, Inc.

zyu@aerodyne.com

Peking University

5/09/2016

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Outline

The Center for Aero-Thermodynamics

(CATD) in Aerodyne

Development of the DPAS technique

Instrument performance

Field measurement on aviation soot

Calibration

Summary

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What does the CATD group do?

Contracting Research:

• Developing a sampling and measurement methodology for ICAO regulation on PM emissions (FAA)

• Evaluating emissions of in-service aircraft (ACRP)

• Characterize volatile PM from aircraft engines (SERDP)

• Investigating emission reduction due to alternative fuels (NASA)

• Working with industrial partners for engine certification (FAA)

Instrument Development:

• Measuring light absorption (UV to near-IR) of PM from either engine emissions or biomass burning for NASA’s Airborne Measurement Program

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Why working on emissions of aircraft engines

• Global aircraft fleet has a market value of $5.2 trillion in 2013

Unit cost: ~$50 million

Unit cost: 13.9 million

COMAC C919

CFM International LEAP-1C engine

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• Aviation consumes 3% of the world’s fossil fuel, of which more than 50% in stratosphere (cruise altitude).

• Current ICAO regulation on emissions only focuses on impact on local air quality via calculating emissions inventory in Landing – Takeoff (LTO) cycle (from ground up to 900 meter).

It is difficult to evaluate emissions of

aircraft engines

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Motivation

Prediction of direct radiative forcing require

parameterization of the optical properties of

light-absorbing carbon emissions with

respect to wavelength

Current parameterizations do not address

the range of variability within sources

Elemental carbon is not a good surrogate

for light absorbing carbon at wavelength

shorter than 800nm.

The light absorbing capacity of

carbonaceous aerosol is not a conservative

property from the point of emissions to

atmosphere

A light absorption measurement technique

at various wavelengths from UV to near-IR

is necessary

A. Laskin et al. Chem. Rev. 2015.

Absorption Ångströgm exponent (AAE) of

black carbon is normally assumed to be -1.0

in the global radiation balance models

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Current Technologies

Filter-based techniques:

• Multi-angle absorption photometer (MAAP)

• Particle soot absorption photometer (PSAP)

• Aethalometer

In situ techniques:

• Micro soot sensor (MSS) (single-cell photoacoustic at 880nm)

• Photo-Acoustic soot spectrometer (PASS-1 at 785nm)

• Single particle soot photometer (SP2)

• Cavity-attenuated phase shift (CAPS) PM SSA monitor

• Cavity ring-down spectroscopy (CRDS)+ scattering measurement

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What is Photoacoustic effect?

l

cf

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One-dimensional

Acoustic Resonators Frequency Quality Factor

~30

Laser radiation

Absorption

Excitation

Non-radiative

relaxation

Heating

Acoustic wave

Expansion

Microphone

Detection

𝐼 = 𝐼0𝜎𝑐𝑙

𝐼 = 𝑄

𝑄 = 𝑊 = ∆𝑃 × 𝑉 = ∆𝑃 × 𝑙 × 𝜋𝑟2

∆𝑃 =𝐼

𝑉=𝐼0𝜎𝑐

𝜋𝑟2

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Why is the correction for absorption of

gas species important?

Light absorption of

gaseous species could

be significant in the UV-

visible region when

evaluating PM

emissions from IC

engines

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DPAS Detection Scheme

Sample Cell

Function

GeneratorDAQ

Laser

Microphone

Reference Cell

Microphone

FFT Analysis

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Current Design

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Major Improvements

Reducing diameter of acoustic cell

Utilizing home-made MEMS

microphone array detectors (x4)

Modulating the 532nm laser via

direct electrical power control

Integrating bandpass filters and

preamplifiers with microphone

array detectors

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Calibration with NO2

Absorption cross section of

(1.45±0.06)10-19 cm2 for NO2 at

532nm from

Osthoff et al. J. Geophys. Res. 2006.

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Allan Analysis

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Instrument Evaluation on Laboratory

Combustor

DPASMSS

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Sampling & Measurement System

N2 diluted

sample

V3/NO

spill

valve

Dekati

DI-1000V2/NO

Control

valve

Aerodyne

DPAS PM

absorption

monitor

AVL Micro

Soot Sensor

(MSS)

AVL MSS pump

N2

SUPPLY

V4/NO

Isolation

valve

HEPA

Filtered

Shop Air

CA CO2

(0-5000 ppm)

HEPA

Pancake

filter

ExhaustP1 T1

P3 T3

2 bar MAX

AVL

Particle

Counter

(APC)

4 Way

Cal Gas + N2

Metal

Filter

N2

heater

P2 T2

Dilution N2

30psig

•½”

Room Air

V9

•¼”

V6

V7Zero

air

HEPA

•3/8”

V5/NC

V1/NC

Supply

valve

•¼”

Gas

measurements

•½”

P4

T4

cyclone

N2 purge

TSI

EEPS

Combustor

exhaust

Compliant with SAE Aerospace

Information Report (AIR) 6241

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Particle Size Distribution

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Comparison between MSS and DPAS

~1ppm NO2

Zeroing

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Differential Results

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Linear Correlation

The expected MAC (532nm) = 7.6±0.6 m2/g according to MAC (633nm) = 6.4±0.5 m2/g

Mulholland & Choi, Proc. 27th Symposium on Combustion 1998

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Under Development

A two-color brown carbon/black carbon

monitor operating at 360nm and 1064nm for

biomass burning studies.

A RGB DPAS instrument that measures

aerosol light absorption at 671nm, 532nm,

and 473nm simultaneously.

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Photoacoustic Signal Calibration

Gas-based methods:

• NO2 at 532nm – W.P. Arnott et al. 2000.

• O3 at 404, 532, and 659 nm – D. Lack et al. 2012.

• O2 at 762nm – G. Tian et al. 2009.

PM-based methods:

• Dried nigrosine dye at 532nm – D. Lack et al. 2006.

• Absorbing polystyrene sphere at 532nm – D. Lack et al. 2012.

Potential Calibration with Metal NPs

• Metal NPs strongly absorb UV-visible light

• Metal NPs can be manufactured uniformly

• Optical properties of metal NPs can be calculated from Mie theory

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Optical Properties of Au-NP

Calculation results validated

against aqueous phase UV-VIS

spectra

Medium effects are important

to consider

Au nanospheres with dp = 130 nm

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Characterization of 130nm Au-NP

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Mono-dispersed particle

size distribution after

removing capping agent

with centrifuging

Good agreement between

extinction measurements and

Mie theory calculations in air

Density=19.6±0.4 g/cm3

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Imaging of Au-NP (130nm)

HRTEM imaging shows that the Au-NPs are uniform

in size with a diameter of 130 nm

D=130.2±1.0nm

Summary

DPAS instrument at 532nm was developed and tested

– The measurement is a turn-key and continuous

operation

– It is capable of providing information on

concentration, mass absorption coefficient (MAC),

absorption Ångströgm exponent (AAE), single

scatter albedo (SSA) in combination with other

measurements.

– The interference from absorption of gaseous

species is subtracted.

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Acknowledgement

Aerodyne Research, Inc.

Dr. Richard Miake-Lye

Dr. Paul Kebabian

Dr. Gregory Magoon

Dr. Jay Peck

James Assif

William Brown

William Rundgren

United Technologies

Research Center (UTRC)

David Liscinsky

Bruce True

Dr. Jacquelynn Garofano