amt-14-2429-2021.pdf

Atmos. Meas. Tech., 14, 2429–2439, 2021

https://doi.org/10.5194/amt-14-2429-2021

the Creative Commons Attribution 4.0 License.

Improvements to a laser-induced fluorescence instrument for

measuring SO

– impact on accuracy and precision

Pamela S. Rickly

1,2

, Lu Xu

, John D. Crounse

, Paul O. Wennberg

3,4

, and Andrew W. Rollins

Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, USA

Chemical Sciences Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO 80305, USA

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA

Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125, USA

Correspondence:

Pamela S. Rickly (pamela.rickly@noaa.gov)

Received: 27 October 2020 – Discussion started: 9 November 2020

Revised: 1 February 2021 – Accepted: 12 February 2021 – Published: 26 March 2021

Abstract.

This work describes key improvements made to

the in situ laser-induced fluorescence instrument for mea-

suring sulfur dioxide (SO

) that was originally described by

Rollins et al. (2016). Here, we report measurements of the

fluorescence emission spectrum. These measurements

allow for the determination of the most appropriate bandpass

filters to optimize the fluorescence signal, while reducing the

instrumental background. Because many aromatic species

fluoresce in the same spectral region as SO

, fluorescence

spectra were also measured for naphthalene and anisole to

determine if ambient SO

measurements could be biased in

the presence of such species. Improvement in the laser sys-

tem resulted in better tunability, and a significant reduction in

the 216.9 nm laser linewidth. This increases the online/offline

signal ratio which, in turn, improves the precision and speci-

ficity of the measurement. The effects of these improvements

on the instrumental sensitivity were determined by analyzing

the signal and background of the instrument, using varying

optical bandpass filter ranges and cell pressures and calcu-

lating the resulting limit of detection. As a result, we report

an improvement to the instrumental sensitivity by as much as

50 %.

1 Background

Sulfur dioxide (SO

) is responsible for a number of health

and environmental impacts. Through reaction with the hy-

droxyl radical (OH), SO

produces sulfuric acid, which af-

fects the pH of aqueous particles and leads to acid deposition.

Sulfuric acid also condenses onto organic and black carbon

particles, producing sulfate, which increases the aerosol hy-

groscopicity and influences the accumulation of aerosol liq-

uid water (Fiedler et al., 2011; Carlton et al., 2020). Sul-

furic acid is believed to be the most important source gas

globally for homogeneous nucleation and growth of new

aerosol particles, which may occur primarily in the tropical

upper troposphere (Brock et al., 1995; Dunne et al., 2016;

Williamson et al., 2019). SO

and sulfate particles can be

transported long distances, driving the production of haze

pollution in areas downwind of SO

emissions (Andreae et

al., 1988). Both the direct radiative forcing from aerosol

and the indirect forcing from aerosol–cloud interactions are

important for climate. While both tend to produce an off-

set to greenhouse-gas-induced warming by reducing incom-

ing shortwave radiation, the effect of aerosol–cloud interac-

tions is complicated and produces large uncertainties in cli-

mate models (Finlayson-Pitts and Pitts, 2000; IPCC, 2021).

Changing emissions distributions, coupled with an incom-

plete understanding of the chemistry and microphysics asso-

ciated with sulfur and aerosol formation in the atmosphere,

necessitates further studies which require precise and accu-

rate measurements of SO

throughout the troposphere and

lower stratosphere.

Regulation of anthropogenic emissions has resulted in de-

creased atmospheric SO

concentrations in the United States

and Europe since the 1970s. However, during the early 21st

century, emissions began increasing dramatically in Asia as

a result of increased fossil fuel burning (Smith et al., 2011;

Hoesly et al., 2018). The main source of SO

to the tro-

Published by Copernicus Publications on behalf of the European Geosciences Union.

2430

P. S. Rickly et al.: Improvements to a laser-induced fluorescence instrument for measuring SO

posphere is through direct emission, followed by oxidation

of dimethyl sulfide (DMS). The remaining pathways of SO

formation, H

S, CS

, and carbonyl sulfide (OCS) oxidation,

contribute negligible fluxes (Feinberg et al., 2019). As of

2014, global emission rates of SO

were reported to be ap-

proximately 113 Tg S yr

−

– more than double the flux dur-

ing the 1950s (Hoesly, et al., 2018). Anthropogenic sources

of sulfur, mainly from fossil fuel combustion and smelting,

are the largest global sources of SO

to the atmosphere and,

as of the year 2000, comprised around 67 % of total global

emissions (Feinberg et al., 2019; Lee et al., 2011; Smith

et al., 2011). Biogenic sources make up a small but signif-

icant fraction of SO

emissions, with the majority derived

from marine phytoplankton, in the form of dimethyl sulfide,

which exhibits a global source rate that is approximately

26 % of total global SO

input (Lee et al., 2011; Feinberg

et al., 2019).

Although global emission rates of SO

have continued to

decrease since the early 21st century, atmospheric sulfur con-

centrations are expected to be affected by continued climate

change and could represent feedback mechanisms within

the climate system. Due to reductions in sulfur deposition,

Hinckley et al. (2020) found that farmers in the United States

need to apply sulfur-containing fertilizer to croplands to en-

hance nitrogen uptake to plants at a rate of 20–300 kg S yr

−

In addition, Kesselmeier et al. (1993) reported that terrestrial

sources of sulfur exhibit behavior similar to monoterpenes

in that they are light and temperature dependent. This sug-

gests that increasing sulfur emissions are likely to occur as

global temperatures continue to rise. Lastly, the effect of cli-

mate warming on the variability in moisture conditions, and

increased land use change, is expected to increase both the

frequency and duration of biomass burning events, which are

expected to further increase sulfur emissions (Westerling et

al., 2006; Heyerdahl et al., 2002). In conjunction, while fos-

sil fuel burning sources of SO

are continuing to decrease, it

is likely that these climate-related and additive sources may

keep SO

emissions from returning to preindustrial levels.

Even small SO

mixing ratios can produce important ef-

fects. Remote regions, including much of the equatorial ma-

rine boundary layer, exhibit mixing ratios of SO

of the or-

der of 100 ppt (parts per trillion). Still, in these regions, the

biogenic SO

may be the primary source of cloud condensa-

tion nuclei. In addition, convective transport from these re-

gions into the tropical tropopause layer can allow these small

sources to reach the lower stratosphere. Sulfate aerosol life-

times in the stratosphere are approximately 100 times that of

aerosol within the lower troposphere, allowing them to per-

sist for 1–2 years (Holton et al., 1995). As a result, sources

of sulfate aerosol and aerosol precursor species reaching the

UT/LS (upper troposphere; lower stratosphere) are dispro-

portionately important for climate compared to short-lived

aerosols in the lower troposphere. However, to date, few

studies have reported measurements of SO

in the UT/LS

(Inn et al., 1981; Georgii and Meixner, 1980; Rollins et

al., 2017, 2018). Understanding in detail the impact that SO

has on the stratosphere is only becoming increasingly im-

portant as discussions of albedo modification by injection of

into the stratosphere are becoming more common (Na-

tional Research Council, 2015).

Despite the potential implications that changing SO

con-

centrations present in the UT/LS and the remote lower tropo-

sphere, few in situ measurements are routinely made in either

of these areas. Most direct measurements have been made

through the use of pulsed fluorescence instruments which

are available commercially; however, this technique tends

to exhibit interferences from other fluorescent species and

limited precision. Most measurements of SO

with parts per

trillion by volume (pptv SO

) precision have been made us-

ing chemical ionization mass spectrometry (CIMS). Many

CIMS SO

ionization chemistry schemes can be sensitive to

variations in ambient water vapor, complicating tropospheric

measurements (Huey et al., 2004; Eger et al., 2019). Opera-

tion of CIMS instruments on unpressurized aircraft capable

of reaching the tropical lower stratosphere (

17 km) is also

challenging from an engineering perspective.

As an alternative, the development of a compact, in situ

laser-induced fluorescence (LIF)-based SO

instrument was

recently reported by Rollins et al. (2016), with a 1

pre-

cision of 2 ppt over a 10 s integration period. That tech-

nique was developed and originally used to quantify SO

the UT/LS region on two NASA WB-57F missions, namely

VIRGAS (Volcano-plume Investigation Readiness and Gas-

phase and Aerosol Sulfur ) in 2015 and POSIDON (Pa-

cific Oxidants, Sulfur, Ice, Dehydration, and cONvection)

in 2016. In the UT/LS ,where potentially interfering fluo-

rescent species (e.g., aromatic compounds) are in negligible

concentrations, the instrument was used in a mode where the

excitation laser was maintained on an SO

resonance, and

the optical background was determined using periodic SO

free zero-air additions. However, it is expected that LIF mea-

surements of SO

in the more complex chemical environ-

ment of the troposphere, and especially in areas where fossil

fuel combustion is occurring and through biomass burning

plumes, might result in interferences from species, including

aromatics, that are also formed during these processes. Prior

to the deployment of the instrument on a NASA Global Hawk

mission in 2017 (HOPE-EPOCH), the LIF instrument was

improved to allow for rapid dithering of the excitation laser

on and off an SO

resonance and to allow for continuous dis-

crimination of SO

from other fluorescent species. It was also

operated this way during the NASA ATom-4 (Atmospheric

Tomography Mission) in 2018 and the NASA/NOAA Fire

Influence on Regional to Global Environments Experiment

– Air Quality (FIREX-AQ) experiment in 2019. Following

ATom-4, it has been investigated how further improvements

to the detection of SO

might be accomplished by quanti-

fying the spectral region of SO

fluorescence for separation

from scattering and identification of fluorescence emissions

Atmos. Meas. Tech., 14, 2429–2439, 2021

https://doi.org/10.5194/amt-14-2429-2021

P. S. Rickly et al.: Improvements to a laser-induced fluorescence instrument for measuring SO

2431

from potentially interfering aromatic species. Here, we report

on these recent improvements and use of the instrument.

2 Laser-induced fluorescence

2.1 SO

spectroscopy

Fluorescence occurs during the radiative relaxation of a

molecule after its absorption of a photon. Thus, the LIF sig-

nal is proportional to the molecular absorption cross sec-

tion at the laser wavelength and the quantum yield for flu-

orescence. A study by Manatt and Lane (1993) compiled

numerous measurements of SO

absorption cross sections

over varying wavelengths to find six absorption bands be-

tween 100 and 400 nm. Because strong absorption by oxy-

gen (O

) occurs below 200 nm, the three spectral regions be-

tween 200 and 400 nm are most appropriate for atmospheric

measurements of SO

. Excitation into the

)

state, with

a wavelength region of 170–235 nm, is typically chosen for

fluorescence detection due to the higher absorption cross

section and fluorescence quantum yield compared to longer

wavelengths. Pumping into the B or A bands (

λ >

240 nm),

while useful for absorption measurements, results in neg-

ligible fluorescence. Another consideration is the predisso-

ciation threshold near 218.7 nm (Bludský et al., 2000). As

a result, pumping at wavelengths less than

∼

215 nm re-

sults in negligible fluorescence. Considering the absorption

cross section and fluorescence quantum yields, excitation at

220.6 nm is theorized to produce the maximum detectable

signal using LIF (Rollins et al., 2016).

Due to the limited availability of practical laser technol-

ogy for airborne instrumentation, Rollins et al. (2016) tar-

geted a band head at 216.9 nm through the use of the fifth

harmonic produced from a tunable 1084.5 nm fiber-amplified

diode laser. The detected fluorescence was selected using

a long-pass filter (Thorlabs FGUV5) in combination with

a bandpass filter (Asahi XUV0400), allowing transmission

of the red-shifted fluorescence in the wavelength region of

240–400 nm. A more detailed description of the LIF SO

in-

strument can be found in Rollins et al. (2016). Generally,

while precision sufficient for measurements in the UT/LS

was achieved (2 ppt over 10 s integration period), the de-

tection limit was primarily controlled by the background

from scattered photons, and it was anticipated that, in pol-

luted regions where other fluorescent species exist, the detec-

tion limit might be further degraded due to these additional

sources of background.

2.2 Laser subsystem improvements

The SO

instrument uses a custom-built fiber laser sys-

tem. Pulses from a fiber-coupled tunable diode laser near

1084.5 nm are used to seed a fiber amplifier system. Orig-

inally, the seed laser operated in a gain-switching mode,

where short pulses of current injected into the seed laser gen-

erated the

∼

1 ns optical output pulses. In the present design,

the seed laser is instead operated in a constant current mode,

and its output is modulated using a fiber-coupled electro-

optic modulator (EOM). The EOM can produce pulses of

less than 1 ns full width at half maximum and with an extinc-

tion ratio of 40 dB. While the original design was somewhat

simpler to operate than the present design, gain-switching

of the seed laser significantly broadened the laser spectrum

and eliminated the possibility of tuning the laser wavelength

by modulating the seed laser injection current. Instead, the

wavelength of the system was tuned by adjusting the temper-

ature of the seed laser, which had a settling time of the order

of seconds. The new design is also operated at a laser rep-

etition rate of 200 kHz instead of the 25 kHz of the original

design. This reduces the peak power leading to less spectral

broadening within the fibers and also increases the dynamic

range of the single-photon-counting LIF signal to 200 kHz

rather than 25 kHz. While less UV laser power is achieved

with the higher repetition rate and lower pulse energy, the

overall sensitivity to SO

improves because the SO

spec-

trum can now be fully resolved, which increases the convo-

lution of the SO

spectrum with the laser spectrum.

Output from the fiber amplifier is passed through three

nonlinear crystals (KTP, LBO, and BBO), in the same config-

uration as described by Rollins et al. (2016), to produce the

fifth harmonic of the fiber output at 216.9 nm with, typically,

1 mW of power. Overall, injecting the seed laser at constant

current and operating the amplifier with lower peak powers

results in a significantly narrower laser linewidth. Due to the

narrower laser linewidth, the Doppler-broadened SO

spec-

trum can now be fully resolved (Fig. 1). This significantly in-

creases the effective SO

absorption cross section. The laser

wavelength can also be tuned rapidly to measure on and off

an SO

resonance many times for each second, which elim-

inates spectral interferences from other fluorescent species

(Fig. 2).

2.3 Fluorescence background

Signal on the detector may result from SO

fluorescence or

from a host of other sources. These include Rayleigh, Raman,

or aerosol scattering or fluorescence from the LIF chamber or

windows and red-shifted fluorescence from other gases and

aerosols in the sample. Because the SO

absorption spectrum

has a fine structure, the signal from SO

can selectively be

reduced by more than 1 order of magnitude by tuning the

laser less than 10 pm off an SO

resonance (Fig. 1). All other

sources of photons, however, are expected to have no ap-

preciable structure at this spectral resolution. Therefore, the

signal from SO

can accurately be distinguished from other

photon sources by constantly tuning the laser on and off an

resonance.

The instrumental precision, however, is determined by the

Poisson counting statistics of the sum of the SO

fluores-

cence and background signals which, at low SO

mixing ra-

https://doi.org/10.5194/amt-14-2429-2021

Atmos. Meas. Tech., 14, 2429–2439, 2021

2432

P. S. Rickly et al.: Improvements to a laser-induced fluorescence instrument for measuring SO

Figure 1.

absorption cross section (red; right axis) in compari-

son with laser scans by the LIF SO

instrument (blue and cyan; left

axis). The online wavelength is identified as the largest absorption

cross-section peak.

Figure 2.

measurements, performed at 10 Hz, showing a large

distinction between online and offline signals. Background mea-

surements with zero air show the signal to be negligible in the ab-

sence of SO

tios, is dominated by the non-SO

count rate. Therefore, the

detection limit is determined by these background sources of

photons.

Knowledge of the SO

emission spectrum is key for

choosing detection bandpass filters to maximize the SO

sig-

nal, while minimizing the detection of photons from non-

sources. While the Rayleigh and Raman scatter by N

and O

occur at constant and known wavelengths, red-shifted

fluorescence from other gases and aerosols may vary by com-

pound. Many aromatic species have been reported to have

non-negligible absorption cross sections and fluorescence

quantum yields when pumped near 216.9 nm. Because aro-

matics and SO

are co-emitted during combustion of many

fuels, it is important to understand the affect they may pro-

duce on ambient LIF SO

measurements.

While many aromatics are released during combustion,

two compounds reported as having large emission ratios and

significant absorption cross sections at 216.9 nm are naph-

thalene and anisole (Grosch et al., 2015; Koss et al., 2018;

Mangini et al., 1967; Warneke et al., 2011). Emission ratios

of these compounds are dependent on the type of combus-

tion, fossil fuel, or biomass and the elements involved in

the combustion. Naphthalene is released through both fos-

sil fuel combustion and biomass burning, with the latter pro-

ducing an emission ratio greater than 1 ppb

ppm CO (ppb –

parts per billion; ppm – parts per million; carbon monoxide;

Warneke et al., 2011). Anisole is primarily released through

biomass burning, with an emission ratio reported in combi-

nation with cresol as 1.5 ppb

ppm CO (Koss et al., 2018).

The absorption cross sections reported for these compounds

near 216.9 nm are 2

−

molecule

−

for naphtha-

lene (Grosch et al., 2015) and 2

−

molecule

−

for anisole (Mangini et al., 1967). With the emission ratio

of SO

being similar to these aromatics, in addition to sim-

ilar absorption cross sections, it is anticipated that aromat-

ics could significantly interfere with ambient SO

measure-

ments. To optimize the detection of the fluorescence spectral

region for SO

, we measured the spectral distributions of the

fluorescence emission from SO

, naphthalene, and anisole.

3 Measurement of scattering and fluorescence spectra

The optical bench used for the LIF SO

detection is shown in

Fig. 3. The 216.9 nm laser, approximately 1 mW, enters the

cell perpendicular to the entrance of the sampled air. Sam-

pled air (2500 standard cubic centimeters per minute – sccm)

enters the system through a custom butterfly valve machined

from PEEK (polyether ether ketone) that reduces the pressure

to 170 mbar, which shows minimal change through the sam-

ple cell. The majority of the gas exits the sample cell opposite

to the inlet, while 250 sccm is exhausted through each of the

cell side arms to eliminate dead space in the flow system.

Detection of the fluorescence is then measured orthogonally

to both the sample flow and laser axes. A UV-fused silica

aspheric lens (Edmund Optics; numerical aperture of 0

5) is

used to collect approximately 10 % of the solid angle relative

to the center of the cell. After passing through the measure-

ment cell, the beam passes through a LIF reference cell with

a similar arrangement to the measurement cell. A constant

flow with a mixing ratio near 500 ppb SO

is maintained in

the reference cell. Feedback from the reference cell is used to

ensure that the laser is tuned to the SO

resonance peak and

to quantify any changes in the instrument sensitivity due to

changes in the laser spectrum. The exhausts of the measure-

ment and reference cells are tied together, such that small

perturbations in the system pressure will also be equally ob-

served in the two cells. A National Instruments CompactRIO

(cRIO) computer system is used for controlling all timing re-

quirements. This includes the timing of the seed laser pulses,

amplification, and photon counting and gating.

Atmos. Meas. Tech., 14, 2429–2439, 2021

https://doi.org/10.5194/amt-14-2429-2021

P. S. Rickly et al.: Improvements to a laser-induced fluorescence instrument for measuring SO

2433

Figure 3.

LIF SO

detection optical bench. PMTs (photomultiplier

tubes) are located above the plane of this schematic and oriented to

collect fluorescence from the center of each cell.

For measurements of the fluorescence emission spectra of

and aromatic compounds, the entrance of a round-to-

linear fiber optic bundle (ThorLabs FG105UCA) was placed

at the focal point of the lens where the fluorescence-detecting

photomultiplier tube (PMT) is typically located. The emis-

sion from the linear end of the fiber bundle was focused

by a collimating lens (74-UV; Ocean Insight) onto the en-

trance slit of a scanning monochromator (Acton Research

Corporation; model VM-502). Measurements were made

with the monochromator in the V configuration, with slit

sizes of 4 mm wide, using a Hamamatsu photomultiplier tube

(H12386-210) connected at the exit of the monochromator.

In order to measure the SO

fluorescence spectrum, a flow

of 46 sccm from a 5 ppm SO

gas cylinder (Scott-Marrin,

Inc.) was mixed with 2500 sccm zero air, producing a con-

centration of approximately 90 ppb that was sampled into

the cell. Figure 4a shows the observed spectrum in the pres-

ence (red) and absence (orange) of SO

. The signal observed

at 202–236 nm, in the absence of SO

, is primarily from

Rayleigh scatter of the laser, and the observed width is a mea-

sure of the spectral resolution of the experimental setup. Fig-

ure 4b shows just the SO

fluorescence calculated after the

subtraction of the background.

To verify the calibration and spectral resolution of the

monochromator, a low-pressure, double-bore mercury (Hg)

capillary lamp (Jelight Company Inc.) was positioned in

front of the fiber as a spectral reference. The atomic Hg

emission spectrum, from 200 too 500 nm, was measured us-

ing the monochromator to calibrate the monochromator and

show that the monochromator fiber setup produces a spec-

tral resolution with a full width half max (FWHM) of ap-

proximately 20 nm. Figure 4 shows that the SO

fluorescence

emission spectrum appears to occur in two regions. The main

peak is centered around 302 nm and produces a FWHM value

of 63 nm in our setup. A second peak may exist between

350–360 nm; however, it is not possible to accurately deter-

mine the peak position nor the width in this setup. Using the

monochromator with relatively wide slits was necessary to

achieve enough signal to measure the fluorescence emission

spectrum in our setup, and this somewhat exaggerates the

width of the emission spectrum in Figs. 4–6.

Measurements were similarly made with the aromatic

compounds. Zero air was passed through the headspace

of a vial containing a sample of pure (

99 %) crystalline

naphthalene (Sigma-Aldrich), which has a vapor pressure

of 0.04 mbar at 298 K. This resulted in a naphthalene con-

centration of approximately 3.6 ppm when the flow through

the vial was 100 sccm and the total flow through the instru-

ment was 1300 sccm. Liquid anisole (Sigma-Aldrich), with

a vapor pressure of 5 mbar at 298 K, was similarly used to

produce a concentration of approximately 26 ppm when the

flow through the vial was 10 sccm and the total flow was

2260 sccm (Ambrose et al., 1976). A comparison of the fluo-

rescence spectra of the aromatic compounds to the SO

fluo-

rescence spectrum is shown in Fig. 5.

Anisole produces a similar fluorescence spectrum to the

main SO

emission peak, with a maximum at 304 nm. The

FWHM of 46 nm occurs between 286 and 332 nm, which is

the latter half of the largest SO

fluorescence peak. Simi-

larly, naphthalene produces a fluorescence spectrum peak-

ing at around 340 nm, with a FWHM value of 46 nm, which

slightly overlaps with the tail end of the largest SO

fluores-

cence peak and nearly completely overlaps with the second

region.

Figure 5 shows that aromatic compounds could produce

significant signal in the SO

instrument during measure-

ments in polluted environments. While naphthalene has

a similar absorption cross section and rate of collisional

quenching to SO

, its low-pressure fluorescence quantum

yield has been reported to be 2–3 times larger due to a lower

expected rate of photodissociation as a result of a larger dis-

sociation energy threshold (Hui and Rice, 1973; Martinez et

al., 2004; Reed and Kass, 2000; Suto et al., 1992). However,

the observed naphthalene signal in our experiment is approx-

imately 30 times lower than SO

. Because of its reduced rate

of photodissociation, the fluorescence lifetime of naphtha-

lene (340 ns) is much greater than that of SO

(30 ns; Hui

and Rice, 1973; Martinez et al., 2004). Therefore, the short

counting gate used in this work for SO

detection (25 ns) dis-

criminates the majority of the naphthalene signal from that of

No difference in the fluorescence emission (intensity or

spectral distribution) was observed for the aromatic com-

pounds with the laser tuned on or off the SO

resonance.

This is consistent with expectations based on the literature,

as absorption cross sections for those compounds show no

fine structure comparable to the features in the SO

spectrum

https://doi.org/10.5194/amt-14-2429-2021

Atmos. Meas. Tech., 14, 2429–2439, 2021

2434

P. S. Rickly et al.: Improvements to a laser-induced fluorescence instrument for measuring SO

Figure 4.

Fluorescence spectrum observed from SO

(red) in addition to Rayleigh scattering and background (orange)

(a)

, and the absolute

fluorescence after subtraction of the background

(b)

Figure 5.

Fluorescence spectra of SO

(red), naphthalene (blue),

and anisole (gray) normalized by the calculated concentrations.

(Keller-Rudek et al., 2013). Therefore, these compounds

would only increase the instrument background, resulting in

reduced precision, and would result in unbiased ambient SO

measurements in areas of high aromatic concentrations.

4 Implementation of bandpass filters

In order to achieve the highest sensitivity and lowest limit

of detection, the impact of the fluorescence-detection band-

pass filters was further investigated. With the instrument

in its normal configuration for measuring SO

(PMT pho-

tocathode located at focal point of fluorescence collection

lens), different bandpass filters were used in front of the de-

tection cell PMT to directly measure the SO

fluorescence

signal and quantify the background scatter over a few dis-

crete regions of the spectrum. Figure 6 shows the fluores-

cence spectrum (red) and background (orange) observed us-

ing the monochromator scaled to the count rates observed

with each of the tested filters. It was observed that the back-

ground increases with wavelength until around 300 nm. After

this point, the background slowly decreases, reaching a min-

imum around 400 nm before increasing again near 420 nm.

The boxes and closed circles are indicative of the filter mea-

surements. The heights of the boxes indicate the fluorescence

signal observed, and the width shows the range in which

transmission was achievable with the filter. The closed cir-

Figure 6.

Adjusted SO

fluorescence spectrum (red line) and back-

ground (orange line; top and bottom panels). The filter measure-

ments (boxes and solid circles) show the filter range by the width

of the box, and the height indicates the observed signal. The solid

circles indicate the background observed for the corresponding fil-

ter. Filter 1 is the Semrock 300/SP-25 and the Semrock 244RS-25.

Filter 2 is the SCHOTT UG11 and the Semrock 244RS-25. Filter 3

is the Asahi XUV0400 and the Thorlabs FGUV5. Filter 4 is the

Semrock 280/10-25 and the Semrock 244RS-25.

cles indicate the background observed with the filter of the

corresponding color.

To optimize the SO

detection limit in zero air, the detec-

tion limit was calculated from the scaled SO

fluorescence

and background spectra, using a theoretical bandpass filter,

assuming a low pass of 246 nm and a variable high pass limit,

where 100 % transmission is observed in the pass band and

0 % transmission elsewhere. The detection limit was calcu-

lated with the filter high pass limit at 270 nm and in increas-

ing increments of 10 nm to the full spectrum at 500 nm. Fig-

ure 7 shows the results of this calculation as a function of the

high pass filter limit. As the upper end of the pass band is ini-

tially increased from 250 to 300 nm, the calculated detection

limit rapidly decreases due to significant gains in signal rel-

ative to background here. A minimum in the SO

detection

limit occurs over the range of 350–450 nm, with a theoretical

Atmos. Meas. Tech., 14, 2429–2439, 2021

https://doi.org/10.5194/amt-14-2429-2021

P. S. Rickly et al.: Improvements to a laser-induced fluorescence instrument for measuring SO

2435

Figure 7.

Calculated detection limit of the LIF SO

instrument (red

line), with the addition of 1 ppb naphthalene (blue line), and the

measured detection limit of Asahi XUV0400 bandpass filter (black

marker), with a 1 s integration period.

detection limit as low as 1.8 ppt over a 1 s integration period.

For polluted environments where 1 ppb of naphthalene may

be present, the detection limit would increase by 20 % within

this detection range due to the increased background from

naphthalene fluorescence.

The dielectric bandpass filter that we have used previously

for SO

measurements in the atmosphere (Asahi XUV0400)

efficiently reduces signal from Rayleigh scatter, and we find

that the additional inclusion of the absorptive glass filter

(Thorlabs FGUV5) originally used with this instrument is

no longer necessary. Removal of the FGUV5 filter allows

for greater transmission within the SO

fluorescence spec-

trum range and provides the largest signal-to-noise ratio of

the sampled bandpass filters. However, the transmission is

still limited to 80 %–95 % with the Asahi XUV0400 filter

alone in comparison to the theoretical filter. This results in

a detection limit of approximately 3.4 ppt over a 1 s integra-

tion period, as shown by the marker in Fig. 7. Testing showed

that the detection limit decreases with increasing pressure up

to at least 250 mbar (Fig. 8). Because predissociation lim-

its the SO

fluorescence lifetime to

∼

5 ns, the fluorescence

quantum yield is rather insensitive to pressure in this regime,

and therefore, the signal increases linearly with pressure due

to the increased number density of SO

. While these mea-

surements were performed under dry conditions, ambient

measurements are expected to produce similar results be-

cause the short fluorescence lifetime will also limit the im-

portance of collisional quenching by water vapor (Rollins et

al., 2016). As a result, the detection limit can be reduced by

approximately 15 % by increasing the cell pressure to around

250 mbar. While this is beneficial for measurements in the

lower troposphere, UT/LS measurements will require the cell

pressure to remain less than 100 mbar in order for the instru-

ment to maintain an adequate flow rate.

Figure 8.

Measured LIF SO

detection limit over a pressure range

of 74–277 mbar.

Figure 9.

LIF signal as a result of ozone addition in the range of

0–8000 ppm.

5 Effect of ozone

Typical mixing ratios of ozone that can be encountered dur-

ing stratospheric sampling are not anticipated to affect the

LIF signal. However, due to the relatively high quantum

yield for SO

photolysis at 216.9 nm, forming primarily sul-

fur monoxide (SO), the possibility of significantly enhancing

the LIF signal through the chemiluminescent reaction of SO

with ozone was investigated using higher ozone mixing ra-

tios (Hui and Rice, 1972; Okabe, 1971; Ryerson et al., 1994).

Figure 9 shows the observed LIF signal in the presence of

significant additions of ozone to the inlet. At ozone additions

up to 2000 ppm, small increases in LIF signal were observed

with increasing O

(2 % LIF signal

ppm O

). This further

demonstrates that, at stratospheric O

mixing ratios acces-

sible by aircraft (

5 ppm), the signal changes by

10 %.

At O

above 2000 ppm, significant decreases in the LIF sig-

nal were observed. We attribute these decreases to photolysis

of O

at 216.9 nm, followed by destruction of SO

by fast

reaction (2

−

molecule

−

) with the atomic

oxygen produced by O

photolysis (Sander et al., 2011).

https://doi.org/10.5194/amt-14-2429-2021

Atmos. Meas. Tech., 14, 2429–2439, 2021

2436

P. S. Rickly et al.: Improvements to a laser-induced fluorescence instrument for measuring SO

Figure 10.

Closed circles indicate the linearized count rate divided

by measured laser power observed during calibration. Dashed line

represents the fit to the calibration data, indicating a sensitivity of

26 cps mW

−

ppt

−

and background of 1000 cps mW

−

6 Field performance

With the instrument configured with the typical bandpass fil-

ter (Asahi XUV0400), and at a pressure of 170 mbar, cali-

brations were performed to assess the impact of the improve-

ments on the precision of the SO

measurement. Calibra-

tions of the instrument are performed in which a mixture

of zero air and SO

standard are introduced to the instru-

ment, with a mixing ratio range of around 1.6–8 ppb SO

This mixture is comprised of a flow of 1–5 sccm of 5 ppm

, with 3000 sccm zero air which has passed through a

KMnO

scrubber, removing any SO

that may be present in

the zero air.

An example of the new calibration is shown in Fig. 10.

Typical sensitivity is 26 counts per second (CPS) per ppt for

and 1000 CPS of background. Thus, the background is

a photon count rate equivalent to 38 ppt of SO

. In our pre-

vious work (Rollins et al., 2016), we reported an in-flight

background of 480 CPS and sensitivity of 4.1 CPS per ppt or

a background equivalent to 117 ppt of SO

. Therefore, the

signal relative to background has increased threefold, which

we attribute primarily to the narrower laser linewidth in the

new configuration. With the new signal level, the 1

detec-

tion limit for 1 Hz measurements is 3.4 ppt. For 10 s of inte-

gration time, the detection limit would be 1.1 ppt, which is

nearly half of what we previously stated for 10 s of integra-

tion.

During the NASA ATom-4 field campaign, measurements

of SO

were acquired by both the LIF instrument and the

California Institute of Technology chemical ionization mass

spectrometer (CIT CIMS) instrument. ATom-4 sampled pri-

marily pristine air masses with a limited number of mea-

surements of ship emissions, biomass burning plumes, and

volcanic emissions. This allowed for the first in situ com-

parison between the current LIF technique and another SO

measurement method. CIT CIMS uses fluoride ion transfer

chemistry from CF

−

reagent ion (e.g., SO

−

→

−

O), followed by mass spectral analysis us-

ing a compact time-of-flight mass spectrometer (CToF) with

typical mass resolution of m

1200. The precision of

CIMS measurements degrades with increasing water vapor

concentration because of the rising interference of the formic

acid signal (CH

−

) which has a mass that

differs from SO

by only 0.054 Da. In the marine boundary

layer, when water vapor was

ppm, the CIMS SO

precision (1

standard deviation over a 1 s integration pe-

riod) is larger than 130 ppt, a value greater than the typical

concentration (

100 pptv) as reported by the LIF in-

strument. Therefore, CIMS measurements, when water va-

por was

ppm, are excluded from the comparison.

Figure 11a shows an orthogonal regression of both measure-

ments from ATom-4 at 10 s time resolution. Overall, an ex-

cellent correlation was observed between the two instruments

(

99). Furthermore, measurements during FIREX-AQ

acquired during smoke plume penetration again show excel-

lent correlation between the CIT CIMS and SO

LIF instru-

ments (Fig. 11b). This indicates that aromatic compounds are

not biasing the SO

LIF measurements. The CIMS instru-

ment reported 12 % and 10 % lower SO

than LIF during the

ATom-4 and FIREX-AQ missions, respectively. While these

are within the combined uncertainties of the measurements,

it indicates a systematic calibration error with one or both of

the instruments that has not been resolved at this time.

7 Summary

Rollins et al. (2016) reported a new, in situ method for mea-

suring SO

in the UT/LS using LIF. Here, we report improve-

ments to the technique that allow for measurements in pol-

luted areas containing other fluorescent species and an over-

all reduction in the detection limit. This was accomplished

by limiting non-SO

fluorescence background sources and

by improvements in the linewidth and tunability of the laser

system. Similar to SO

, aromatic species are largely emit-

ted during combustion processes, many of which have large

absorption cross sections near 216.9 nm and significant flu-

orescence quantum yields. To determine the effect of these

compounds on measuring SO

, the fluorescence spectra of

, and two aromatic compounds, naphthalene and anisole,

were measured. While strong overlap was exhibited in the

fluorescence spectra of these aromatic species with SO

, the

excitation spectrum of SO

has a fine structure near 216.9 nm

while the excitation spectra of the aromatics is relatively in-

variant near 216.9 nm. Therefore, these compounds will only

increase the background, slightly reducing the precision of

the instrument, but will not result in biased SO

measure-

ments. Similar consequences for the LIF SO

measurements

are expected from other aromatic species, since those species

generally do not have a fine structure in their excitation spec-

tra.

Atmos. Meas. Tech., 14, 2429–2439, 2021

https://doi.org/10.5194/amt-14-2429-2021

P. S. Rickly et al.: Improvements to a laser-induced fluorescence instrument for measuring SO

2437

Figure 11.

Correlation between CIT CIMS and LIF SO

during

(a)

all 12 of the NASA ATom-4 flights and

(b)

one FIREX-AQ flight on

3 August 2019. The dashed line represents the 1

1 ratio, and the solid line represents the orthogonal fit. The 10 s averaged data are used in

the ATom-4 comparison to improve the signal-to-noise ratio, and 1 s averaged data are used in the FIREX-AQ comparison.

The SO

fluorescence spectrum was also used to deter-

mine the bandpass filter application that would best limit the

amount of scatter observed and provide the best limit of de-

tection. Using a theoretical bandpass filter, with transmission

beginning at 246 nm, the ending transmission range, in which

the lowest detection limit is expected, is 350–450 nm. This is

expected to result in a detection limit of 1.8 ppt. Using the

Asahi XUV0400 bandpass filter, we are able to reach a de-

tection limit of 3.4 ppt.

Improvements in the laser system are responsible for sig-

nificant performance improvements over our previous work.

Here, we reduced the laser linewidth, which increased the

LIF signal by nearly a factor of 3. The laser wavelength is

now controlled by current tuning of the laser, which can be

performed rapidly, allowing for measurements of online and

offline fluorescence signals many times in a second. This

eliminates the possibility of spectral interferences from other

species such as aromatics. Increasing the laser repetition rate

from 25 to 200 kHz also greatly increased the dynamic range.

While it is shown that increasing the cell pressure to

around 250 mbar would reduce the limit of detection by ap-

proximately 15 %, this higher cell pressure can only be used

in the lower troposphere. Measurements in the UT/LS will

require the cell pressure to be operated near 40–50 mbar. It

was also demonstrated that increased ozone concentrations

observed in the UT/LS would not significantly influence LIF

measurements. The resulting production of O(

D) does

not become large enough to decrease SO

within the sam-

pling cell until reaching ozone concentrations greater than

4000 ppm, more than 2 orders of magnitude greater than

stratospheric ozone concentrations.

The culmination of these changes to the LIF SO

instru-

ment has resulted in an increased instrumental sensitivity and

lower limit of detection. Calibrations suggest that the instru-

mental sensitivity has improved by approximately 3 times,

relative to the background, from that reported in Rollins et

al. (2016). Comparison with measurements performed by

the CIT CIMS instrument during the NASA ATom-4 and

FIREX-AQ campaigns demonstrated good agreement. These

results suggest that the LIF SO

instrument is highly suitable

for measurements in both polluted and pristine environments.

Data availability.

The data collected for FIREX-AQ are available

from the NASA/NOAA FIREX-AQ data archive at https://www-air.

larc.nasa.gov/cgi-bin/ArcView/firexaq (last access: 23 April 2019,

NASA/NOAA, 2019). The data collected for ATom-4 are avail-

able from the NASA ESPO data archive at https://espoarchive.nasa.

gov/archive/browse/atom/id14 (last access: 23 April 2019, NASA,

2019).

Author contributions.

The research was designed by PSR and

AWR. Measurements were taken by PSR, AWR, LX, JDC, and

POW. The paper was written by PSR, with contributions from all

coauthors.

Competing interests.

The authors declare that they have no conflict

of interest.

Acknowledgements.

We would like to thank the NASA DC-8 crew

and management team for their support during ATom-4 and FIREX-

AQ integration and flights. We thank Michelle Kim and Hannah

Allen for operating CIT’s CIMS instrument during ATom-4.

Financial support.

This research was funded by the ATom investi-

gation, under NASA’s Earth Venture program, and the FIREX-AQ

investigation, under NASA’s Atmospheric Compostion: Upper At-

mospheric Composition Observations program CE8 (CIT grant nos.

NNX15AG61A and 80NSSC18K0660, respectively).

Review statement.

This paper was edited by Dwayne Heard and re-

viewed by two anonymous referees.

https://doi.org/10.5194/amt-14-2429-2021

Atmos. Meas. Tech., 14, 2429–2439, 2021