INCOHERENT BROADBAND CAVITY ENHANCEMENT ABSORPTION SPECTROSCOPY

FOR TRACE GAS DETECTION

Contents

Nitrate – some factsConventional vs Cavity enhanced

absorption spectroscopyIBBCEASExperiment

- Instrumental setup- Calibration

Results from lab measurementsConclusion

Nitrate (NO3-)

Atmospheric NO3- (NO3

-atm) is a mixture of gas

phase nitric acid, produced by the oxidation of NOx, and NO3

- salts formed by heterogeneous reactions involving NOy on aerosol surfaces

NO3- is not present in the sunlit atmosphere,

because it is rapidly destroyed by photolysis.

The chemistry of the night-time atmosphere

• A substantial portion of nitrate found in the soil, stream and ground water is of atmospheric origin

• The global nitrogen cycle has been altered by human activities:– human N2-fixation > 2 x natural fixation (annually)

• NO3- is a substantial source of new N

• The negative impacts of excess N and NO3-

input includes – shifts in biodiversity,– soil acidification and forest decline,– eutrophication of coastal waters and estuaries and – degradation of ground and surface waters

Tracing of atmospheric nitrate deposition in a

complex semiarid ecosystem by University

of California

CONVENTIONAL ABSORPTION SPECTROSCOPY

A laboratory technique used to determine the structure and concentration of a sample based on the amount and wavelengths of light that it absorbs.

It derives concentrations of absorbing species in the atmosphere by using a form of the Beer-Bouguer-Lambert Law.

Beer - Bouguer - Lambert Law

Transmittance, T is calculated as

where I0 – initial light intensity (power P0) I – transmitted intensity (power P) α’ - absorption coefficient of the sample b – path length of light through the

sample

Cavity – Enhanced Absorption Spectroscopy

Sample

High reflectivity plano-concave dielectric mirrors

(typically R > 99.95% in the wavelength of interest)

Detector

Length = b

Advantage of Cavity – Enhanced Absorption Spectroscopy

• The cavity having a length of 1m approximately could create an effective path length of 10km

This increases the sensitivity of the instrument

Iin (1-R)2(1-L) (0th pass)

Iin (1-R)2(1-L)3 R2 (1st pass)

Iin (1-R)2(1-L)2n+1 R2n (nth pass)

Types of Cavity Enhanced Absorption Spectroscopy

1. Cavity Ring Down Spectroscopy (CRDS)2. Integrated Cavity Output Spectroscopy (ICOS)3. Incoherent Broadband Cavity Enhanced

Absorption Spectroscopy (IBBCEAS)

IBBCEAS

Simple, Robust & Economic

High temporal resolution

Portable & Compact

Easy integration into

spectrometers

Applicable to fluids

Applicable over a wide spectral range

Theoretical application to

solids

Theory of IBBCEAS

Iin (1-R)2(1-L) (0)

Iin (1-R)2(1-L)3 R2 (1)

Iin (1-R)2(1-L)2n+1 R2n (n)

+++

Since R<1 and L<1

For an empty resonator with L = 0, Io = Iin (1-R) (1+R)-1

Therefore,

Assuming the losses per pass to be solely due to Beer absorption,

Theory of IBBCEAS

EXPERIMENTAL SETUP

Lab Setup: Transmitter Unit

Lab Setup: Intermediate Unit

Lab Setup: Receiver Unit

Preparation of NO3 In The Laboratory

4 HNO3(aq) + Cu (s) → Cu(NO3)2(aq) + 2 H2O(l) + 2 NO2(g)

NO2 + O3 → NO3 +O2

Using ozoniser, NO2 inside the chamber is made to undergo the following reaction:

Calibration of Mirror Reflectivity ‘R’

Low-loss optic [L(λ) < 0.001, Layertec]

500 550 600 650 700 750

0

2000

4000

6000

8000

10000

Cou

nt

Wavelengh

LLO I0

Transmission without optic (I0)

Transmission with optic (LLO)

Wavelength Calibration585.249

585.103

703.137

703.241

Neon lamp lines (NIST, USA) Measured Neon lamp lines using AvaSpec 3648

Results: NO3 Measurement

Intensity of cavity signal with N2 gas inside the cavity

Intensity of cavity signal with NO3 gas inside the cavity

Results: NO3 Measurement

α' and least squares fit for α’

Results: NO3 Measurement

CONCLUSION A simple IBBCEAS setup to detect trace gases

like NO3 is designed for the In-situ measurement.

The instrument was able to detect NO3 concentration at ppb levels.

The calculated error was about 7% - 9%. Further optimizations are required to improve

sensitive. When the instrument is used in humid

conditions, H2O absorption lines in the spectral range of interest will increase the amount of error.

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