# Presentation: 665082

## Measurement of Aerosol Absorption Using Photothermal Interferometry

***Arthur J. Sedlacek, III***
*Atmospheric Sciences Division*
*Brookhaven National Lab*

## What are We Doing and Why? 

***Why?****** ***
*One of the goals of our community is to better quantify the role of aerosol absorption on radiative balance of the atmosphere.*
*Unfortunately existing techniques have significant issues and consensus of the community is that there is a need for more direct and sensitive techniques of** **aerosol absorption.*
***Project Objective***
*Develop improved method for measurement of aerosol absorption using laser interferometry.*

## Current Approaches to Measuring Aerosol Absorption 

*We are all familiar with total extinction:*
** ****	****	****	**** ******ext** = ******scat** +  ******abs
*where *****scat** ***and *****abs** ***include contributions by gases and aerosols*
*1. Measure *****ext *and* ****scat *and take the difference (e.g., Strawa, 2003 **–** CRD)*** **

***Issue:****  difference between 2 large numbers to get a small number*

*2. Deposit particulates onto a filter & measure decrease in transmission of light (e.g.,Bond, 1999; Campbell, 1995)*

***Issue:****  artifact absorption from particulate and/or substrate light scattering can result in over-estimate of true absorption.  Substrate perturbation of particle’s optical properties.*

## Alternative Methods of Measuring Aerosol Absorption 

*It is highly desirable to directly measure aerosol absorption*
*Two such approaches offer this desirable trait:*
***	******Photoacoustic Spectroscopy & Photothermal Interferometry ***

*Both of these techniques rely on the thermal dissipation of spectrally absorbed energy for their respective signals*
***The hallmark of any photothermal-based technique is that it directly measures the aerosol absorption without interference from aerosol scattering  ***

## Photothermal Interferometry for Aerosol Absorption

*	**e.g., shimmering surface or optical mirage effect*

***Photothermal Interferometry for Aerosol Absorption***

_*Interferometry:*_* the study of the interference patterns created by combining two sets of coherent light waves that have a constant phase relation and same polarization.*

*Gladstone-Dale eqn*

*When air is heated the ****optical**** pathlength changes due to the change in the index of refraction *

*Highly sensitive technique for measuring optical pathlength changes.*

**Notes:**
Interferometry is the technique of measuring phase of monochromatic light passing through a heated sample relative to the phase passing through a reference arm.  Any change in the probe arm of the interferometer will result in a change in the power received by the photodetector.

## PTI for Molecular/Aerosol Absorption Through RI Change

*Lin and Campillo (1985) used PTI to measure (NH**4**)**2**SO**4** absorption using a **Mach-Zehnder interferometer*
*Detection sensitivity of 50 x 10**-6** m**-1** (****abs**=1086 cm**-1**) – sensitivity was limited by background water vapor absorption at this wavelength*
*More recently Davis and co-workers (1999) using a modified Jamin interferometer reported a sensitivity of 3.7x10**-8** m**-1** for NH**3** (****abs**=1085 cm**-1**).*

## Controlling Phase Difference with Folded Jamin Interferometer

- Interferometric
- Signal (#1)

- Variable path length by moving modulator unit to designated location on base unit

- Base Plate

- AIMS BEAMSPLITTER BLOCK

*Proposed Modification of Interferomet AIMSTM unit*

- PZT Phase Scanner
- Modulator unit

*© Interferomet Ltd 2005*

- Interferometer
- Laser beam

- Interferometric
- Signal (#2)

*Namba, Rev. Sci. Instr. (1959)*
*Birch, Downs and Ward, J. Phys. E: Sci. Instr. (1988)*
*Moosmüller & Arnott, Opt. Lett. (1996)*

## Advantages of Folded (Double-Pass) Jamin Interferometer

*  **Simple positional alignment*
*  **Common path enables common mode rejection providing increased robustness, stability (mechanical vibrations) and sensitivity*
*  **Interferomet’s fixed and moving retro-reflectors enable feedback to maintain system at quadrature*
*  **Movement of **PZT Phase Scanner** **Modulator unit will enable sample pathlength to be easily changed to accommodate differing opacities*
*  **Compact size (will fold the interferometer laser to further reduce footprint)*
*  **Maximal pathlength for given size (inherent double-pass configuration)*

***Advantages of Folded (Double-Pass) Jamin Interferometer***

## AIMSTM is Designed for Measurements in Industrial Settings

***AIMS****TM**** is Designed for Measurements in Industrial Settings***

*The robust & small packaging will enable quicker initiation of performance tests in the lab as well as rapid deployment in upcoming ASP field campaigns*

*Gas Refractometer configuration**** ***

***Replace gas cell w/aerosol sample chamber & dichroic beamsplitter***

## Research Objectives

*Design of aerosol sampling chamber*
* **goal to keep sampling cell < 315 cc (variable pathlength?)*
* **goal of instrument ~ 7500 cc (50 (l) x 15 (w) x 10 (h))*
* **initial absorption at 532 nm (simply change out DC-BS for other ****s)*
*Performance testing of instrument*
* **susceptibility to mechanical vibrations & thermal stresses*
* **determine nominal detection sensitivity & range*
* **extensive calibration (using gases)*
* **sensitivity of PTI to particle size and composition**	*
*Expectations*
* **dataset containing aerosol absorption as a function of time (10sec)*
* **detection sensitivity on the order of 10**-8** m**-1*
* **robust package suitable for aircraft deployment*