Introduction
Formaldehyde (CH2O) is the most abundant carbonyl compound in the
atmosphere, with typical tropospheric concentrations between 0.1 and 10 parts
per billion (ppbv; Grosjean et al., 1996; Fried et al., 2002). Its
sources include secondary photochemical production from biogenic and
anthropogenic volatile organic compounds (VOCs) and primary emissions from
combustion sources. A Bayesian inverse model of satellite columns shows that
the largest global CH2O source is oxidation of isoprene and other
biogenic VOCs, followed by biomass burning and oxidation of anthropogenic
VOCs (Shim et al., 2005). Formaldehyde is lost through
photolysis and reaction with OH, with an atmospheric lifetime of a few
hours. These processes contribute to net ozone production and radical
recycling.
The existing in situ measurement techniques for formaldehyde can be broadly
categorized as wet chemistry, mass spectrometry, and optical spectroscopy.
The wet chemical methods include chromatographic detection of formaldehyde
products with 2,4-dinitrophenylhydrazine (Fung and Grosjean, 1981; Lee
et al., 1998; Spaulding et al., 2002) and fluorimetric detection of
formaldehyde products from Hantzsch (Kelly and Fortune, 1994; Junkermann
and Burger, 2006) or other reactions (Kok et al., 1990; Fan and Dasgupta,
1994). These wet chemical methods are sensitive, but they lack rapid time
response and are susceptible to chemical interferences (Hak et al.,
2005). Formaldehyde has been measured by mass spectrometry using proton
transfer reaction mass spectrometry (PTR-MS; Karl et al., 2003;
Steinbacher et al., 2004; Wisthaler et al., 2008; Warneke et al., 2011).
These measurements are rapid and sensitive but are complicated by strong
humidity dependence (Warneke et al., 2011). Spectroscopic measurements of
formaldehyde exploit its strong absorption features in the infrared or
ultraviolet spectral regions, with detection by Fourier transform infrared
spectroscopy (Yokelson et al., 1996), tunable diode laser
absorption spectroscopy (Fried et al., 1997, 2002),
difference frequency generation absorption spectroscopy
(Weibring et al., 2007), and quantum cascade laser spectroscopy
(Herndon et al., 2007). Formaldehyde has also been
measured spectroscopically by observing its fluorescence near 354 nm
(Mohlmann, 1985; Hottle et al., 2009). Remote sensing instruments for
ground-based (Platt et al., 1979; Heckel et al., 2005) and
satellite-based (Bovensmann et al., 1999; Burrows et al., 1999)
observations use differential optical absorption spectroscopy (DOAS).
Broadband cavity-enhanced absorption spectroscopy (BBCEAS) is a sensitive
technique for direct absorption measurements. Compared to wet chemistry,
mass spectrometry, and fluorescence, a key advantage of this approach is the
inherent accuracy of direct absorption measurements. BBCEAS uses a broadband
light source, typically a light emitting diode (LED) or arc lamp, coupled to
a high-finesse optical cavity (Fiedler et al., 2003). The
light output from the cavity is dispersed with a monochromator and measured
using a multichannel detector. Conceptually, it is similar to a conventional
optical absorption instrument with a multipass cell, but the high-finesse
cavity allows an effective path length of several kilometers in the
ultraviolet spectral region and tens or hundreds of kilometers in the
visible spectral region. Standard spectral fitting methods can be used to
retrieve multiple absorbers because absorption information is acquired
simultaneously for a wide wavelength region (Ball et al., 2004;
Washenfelder et al., 2008). BBCEAS and the related cavity-enhanced DOAS
(CE-DOAS) technique have been successfully used to measure trace gas and
aerosol extinction in the visible spectral range (Ball et al., 2004;
Langridge et al., 2006; Venables et al., 2006; Washenfelder et al., 2008;
Thalman and Volkamer, 2010) and near-ultraviolet spectral range (Gherman
et al., 2008; Langridge et al., 2009; Axson et al., 2011; Kahan et al.,
2012; Washenfelder et al., 2013). However, only a few measurements have been
demonstrated in the ultraviolet spectral region below 340 nm. These include
measurements of O3, O4, and SO2 at 335–375 nm (Chen
and Venables, 2011), measurements of nitrophenols at 320–450 nm
(Chen et al., 2011), and measurements of acetone near 280 nm
(Islam et al., 2013).
(a) Schematic of the broadband cavity-enhanced absorption
spectrometer, showing the laser-driven arc lamp, collimating optics, cavity,
and spectrometer. (b) Block diagram showing the flow system to
introduce He, zero air, CH2O, and NO2 to the two instruments.
Despite the distinct advantages of BBCEAS that have made it a powerful
technique for trace gas measurements in the visible spectral region, there
are multiple challenges in the ultraviolet that have precluded the
measurement of atmospheric trace gases or aerosol extinction at ambient
levels. First, LED sources are widely used at visible wavelengths because
they are spectrally powerful, inexpensive, lightweight, and consume little
power. However, LEDs with high optical power are unavailable at wavelengths
shorter than 365 nm. Second, the optical bandpass filters required to reject
out-of-band light from broadband sources are less efficient in the
ultraviolet spectral region. Third, Rayleigh scattering by ambient air
increases inversely with the fourth power of wavelength, λ-4,
and can become a significant loss process that limits light intensity and
optical path length for an optical cavity in the ultraviolet spectral
region. Finally, the thin film coatings used in the fabrication of high
reflectivity cavity mirrors are less efficient in the ultraviolet spectral
region, with greater absorption and scattering losses in the material. To
the extent that these challenges can be overcome, BBCEAS measurements in the
ultraviolet spectral region may be a powerful technique for atmospheric
spectroscopy due to the large number of trace gases with structured
absorption in this region and to the importance of aerosol absorption.
In this work, we report the first laboratory measurements of formaldehyde by
BBCEAS in the ultraviolet spectral region at 315–350 nm. The approach uses
a novel laser-driven Xe plasma as the high-power ultraviolet light source.
This source eliminates some of the issues associated with conventional arc
lamps, including large size, heat dissipation, and instability in the arc
location. Although challenges remain for atmospheric sampling, this approach
brings the CH2O detection limit into the range of ambient
concentrations. The subsequent sections describe instrumentation, data
analysis, and results. The design requirements for ultraviolet BBCEAS to
achieve trace gas detection at background CH2O mixing ratios with 1 min
time resolution are discussed in the conclusions.
Experimental
Description of the BBCEAS instrument
As shown in Fig. 1a, the BBCEAS optical system consists of a laser-driven
arc lamp, collimating optics, optical filters, cavity, and grating
spectrometer with charge-coupled device (CCD) array detector. This is similar to the laboratory
instrument described previously (Washenfelder et al., 2008; Axson et al.,
2011; Kahan et al., 2012), although the light source, collimating optics,
optical filters, and grating spectrometer have been replaced to optimize
measurements from 315 to 350 nm.
We use a broadband light source (EQ-99FC LDLS; Energetiq, Woburn, MA, USA),
consisting of a continuous-wave diode laser at 974 nm that pumps a Xenon
plasma (Islam et al., 2013) with a total electrical power draw of
125 W. The resulting plasma size is less than 100×200 µm,
with spectral output from 170 to 2100 nm. Further details about laser-driven
arc lamps and a comparison to conventional Xenon arc lamps can be found in
Zhu and Blackborow (2011). The light source is air-cooled, but
temperature drift caused the output intensity to vary as much as 10 % in
1 h. To eliminate this drift, we constructed custom temperature control using
water circulation through an attached aluminum plate. Change in lamp
intensity as a function of wavelength is an additional consideration, and we
measured the relative intensity change over 315–350 nm to be less than
0.3 % in 1 h. The light source is purged with a continuous flow of N2
gas to eliminate ozone production in the lamp housing. Inside the housing,
the light is collected using an ellipsoidal reflector and a 600 µm
diameter fiber, resulting in a manufacturer-specified power output of
130 µW nm-1 across the 315–350 nm spectral region. This light is
collimated and coupled into the cavity using an off-axis parabola with 0.36
numerical aperture (RC04SMA-F01; Thorlabs, Newton, NJ, USA). Prior to
entering the cavity, the light passes through two colored glass filters
(Schott Glass WG320 and UG11).
The optical cavity is formed by two 2.5 cm diameter, 1 m radius of curvature
mirrors (Layertec GmbH, Mellingen, Germany), with manufacturer reported
reflectivity of 0.9995 (1- reflectivity = 500 ppm loss) at their nominal
center wavelength of 330 nm. The mirrors were mounted at either end of a
100 cm long Teflon cell (2.5 cm outside diameter), with gas ports for input,
output, and mirror purge flows. The optical cavity is mechanically
stabilized using 2 cm diameter carbon fiber rods and custom optical mounts.
The light exiting the cavity is imaged by a 2.54 cm diameter F/3.1 lens onto
a 0.5 cm F/2 lens (74-UV; Ocean Optics, Dunedin, FL, USA) that couples the
light into one lead of a custom fiber bundle. There is an additional
shortpass filter to eliminate stray light (XUS0350; Asahi Spectra USA, Inc.,
Torrance, CA, USA).
The optical fiber bundle consists of two leads, with seven 200 µm
diameter fibers each. These are arranged linearly along the input slit axis
of the grating spectrometer, with each lead illuminating a separate region
of the CCD. Spectra were acquired using a 203 mm focal length grating
spectrometer (IsoPlane-160; Princeton Instruments, Trenton, NJ, USA) with an
internal shutter, tunable grating, and manual entrance slit. The astigmatism
of the IsoPlane-160 is less than 100 µm over the 26 mm wide focal
plane. During these experiments, we used a 1200 line mm-1 grating
(300 nm blaze) and a 100 µm entrance slit width. The CCD array is a 16 bit, back-illuminated detector with 2048×512 pixels (PIXIS 2kBUV; Princeton Instruments, Trenton, NJ,
USA), which is thermoelectrically cooled to -70 ∘C to reduce thermal
noise. The PIXIS 2kBUV had negligible time-dependent thermal noise, and its
dark background was dominated by the offset voltage and readout noise.
The two regions of the CCD were used to measure the light output by the
laser-driven arc lamp and the intensity through the optical cavity, as shown
in Fig. 1. This allowed monitoring for drift in the light intensity. Unlike
in previous experiments, the signal from each region was not accumulated in
hardware to generate two spectra. Instead, the signal for the entire CCD was
read out and the two regions were summed in software, which allowed for
greater integration times and protected the shutter assembly from
overheating by reducing its operation frequency. Individual spectra were
acquired with 1.2 or 2.0 s integration time, and averaged for a total data
acquisition time of 30 s or 1 min.
The IsoPlane grating position was set to a center wavelength of 330 nm with
103 nm bandwidth. The optical resolution was determined by fitting narrow
emission lines from a Hg/Ar lamp. The observed line shape was nearly
Gaussian with a full width at half maximum (FWHM) of 0.47 nm at 330 nm.
Cavity ring-down spectrometer for NO2 validation
Concentrations of NO2 were independently measured using cavity ring-down
spectroscopy at 406.4 nm, as described previously (Washenfelder et al.,
2008; Fuchs et al., 2009). Briefly, a continuous-wave diode laser is
modulated at 1.6 kHz and coupled into a high finesse cavity. The output
light is measured with a photomultiplier tube and recorded with a data
acquisition card. Measurements of the laser line shape showed that it was
centered at 406.4 nm and was approximately Gaussian with FWHM of 0.48 nm.
The laser line shape was convolved with a high-resolution NO2 reference
cross section (Vandaele et al., 1998).
Preparation and delivery of CH2O and NO2
Formaldehyde was prepared by purification of paraformaldehyde, following the
general method described in Meller and Moortgat (2000). Paraformaldehyde was gently
heated in a glass tube connected to a vacuum system. Water from the
paraformaldehyde decomposition was trapped in a cold finger, while
formaldehyde was collected in a 12 L glass bulb. The bulb was subsequently
diluted with N2, for a final concentration of 1–2 ppmv CH2O in
N2. Because formaldehyde readily polymerizes at high
concentrations (Meller and Moortgat, 2000), the diluted
formaldehyde sample was stored in dark conditions at room temperature.
Nitrogen dioxide was prepared by flow dilution from a standard cylinder
containing 15.8 ppmv NO2 in N2 (Scott-Marrin, Inc., Riverside, CA,
USA).
The experimental flow system is shown in Fig. 1b. The BBCEAS and CRDS
instruments were connected in series. Prior measurements under dry conditions have shown negligible losses to Teflon and metal surfaces for
NO2 (Fuchs et al., 2009) and glyoxal (a dialdehyde with greater
Henry's Law constant than formaldehyde; Washenfelder et al., 2008; Min
et al., 2015). A study of formaldehyde sampling artifacts similarly
concluded it is relatively inert to adsorption by Teflon
(Wert et al., 2002). Pressure (626A; MKS Instruments,
Andover, MA, USA) and temperature (K-type thermocouple; Omega Engineering,
Inc., Stamford, CT, USA) were monitored at the exit of the CRDS cavity and
used to calculate number density for the BBCEAS and CRDS. Over the
experimental concentration range, CH2O and NO2 did not measurably
affect mirror cleanliness or reflectivity, so mirror purges were not used
for the BBCEAS instrument. Mirror purges were necessary for the CRDS due to
its existing cell design. Typical flows of 2.0 standard L min-1 (slpm)
of zero air; 0–20 standard cm3 min-1 (sccm) of 15.8 ppmv
NO2; and 0–40 sccm of 1–2 ppmv CH2O were introduced to the cells
and allowed to exhaust to ambient atmospheric pressure.
Operation
During normal laboratory operation, the BBCEAS and CRDS were turned on at
least 2 h prior to beginning measurements. This allowed sufficient time for
the laser-driven arc lamp and its temperature to stabilize. The required
warm up time could be reduced in the future with improved temperature
control. Dark spectra were acquired with identical integration time (1.2 or
2.0 s) as the sample spectra and 5–10 dark spectra were averaged together.
The total acquisition time for each sample spectrum was 1 min, which
included the shutter compensation and CCD readout time, allowing 35 spectra
with 1.2 s integration time to be acquired in 1 min (equivalent to a duty
cycle of 70 %). This duty cycle does not include the intermittent
measurement of dark spectra. He and N2 spectra were measured for 1 min
each to calculate the mirror reflectivity of the BBCEAS channel. During
standard additions, reflectivity measurements were repeated at 15–20 min
intervals, and zero air measurements were repeated between each standard
addition.
Data analysis
Determination of mirror reflectivity and extinction
Extinction in a BBCEAS cavity can be described using an infinite sum for the
light transmission through the cavity (Fiedler et al., 2003)
or a differential equation for light input and output (Washenfelder et al., 2008). These two approaches give an
equivalent expression that relates the optical extinction, α(λ),
to the observed change in transmitted light intensity
through the cavity. If we define the reference spectrum as a cavity filled
with zero air, the expression is
α(λ)=1-R(λ)d+(λ)Rayleigh,ZAIZA(λ)-I(λ)I(λ),
where λ is the wavelength of light, d is the cavity length,
R(λ) is the mirror reflectivity, α(λ)Rayleigh, ZA
is the extinction due to Rayleigh scattering of zero air,
IZA(λ) is the reference spectrum, and I(λ) is the measured
spectrum at each wavelength. If mirror purge volumes were included, the
right-hand side of Eq. (1) would require a multiplicative factor equal to
d/L, where L is the length over which the absorbing sample is present.
Eliminating the mirror purges in the laboratory instrument described here
eliminates a source of uncertainty.
Calculating extinction from Eq. (1) requires knowledge of the mirror
reflectivity, R(λ). This can be determined by introducing a well-known
extinction into the cavity. Rayleigh scattering is convenient because it
varies slowly with wavelength across the entire region of interest, and
requires only measurements of temperature and pressure. Here, we use
measurements of the Rayleigh extinction for helium and zero air to calculate
R(λ). Based on the recommendation of Thalman et al. (2014), we have
used theoretical Rayleigh scattering cross sections from Sneep and Ubachs (2005;
see Eqs. 11, 12, 23, and 24) for N2 and O2. The
Rayleigh scattering cross section for He is based on an empirical fit to
measurements by Shardanand and Rao (1977),
σRayleigh, He=1.336×10-17×λ-4.1287.
Because the mirror reflectivity is calibrated using well-known
Rayleigh scattering cross sections (uncertainty ± 4 % in the UV),
the derived trace gas absorption or aerosol extinction retains high
accuracy, similar to conventional direct absorption or extinction
instruments.
Spectral retrieval method
The measured extinction in Eq. (1) contains contributions from each of the
absorbers in the cavity:
α(λ)=∑inαi(λ)=∑inσi(λ)Ni=σCH2O(λ)CH2O+σNO2(λ)NO2+…,
where n is the number of absorbers, σi(λ) is the
absorption cross section, and Ni is the number density of the ith
absorber. The number density of each absorber can be determined from the
measured extinction, α(λ), and reference spectra for each
absorber, σi(λ).
(a) Spectral measurements of IHe and
IZA, and the ratio of IZA/IHe.
(b) Cavity loss, 1-R(λ), and effective pathlength, d/(1-R(λ)), determined from the Rayleigh extinction of He and zero air
spectra, as described in the text. (c) Reference spectra for
CH2O (light blue; Meller and Moortgat, 2000) and NO2 (light red;
Vandaele et al., 1998) convolved with a Gaussian lineshape with FWHM 0.47 nm
(CH2O dark blue; NO2 dark red). The CH2O cross sections have
been multiplied by three to put them on the same scale with those of
NO2.
For the spectral retrievals, reference spectra for CH2O
(Meller and Moortgat, 2000) and NO2 (Vandaele et al., 1998)
were convolved with a Gaussian line shape with FWHM 0.47 nm. These reference
spectra were used with DOAS Intelligent System (DOASIS) spectral fitting
software (Kraus, 2006) to retrieve number densities of CH2O and
NO2. The spectra were fit from 315 to 350 nm, and a fourth-order
polynomial was included in each fit to account for drift in the light
intensity and cavity throughput.
Results and discussion
Measured spectra and calculated mirror reflectivity
Figure 2a shows spectra obtained with the absorption cell filled with He and
zero air for 24 spectra with 2.0 s integration time. The spectral intensity
is determined by the product of the laser-driven arc lamp spectrum
(170–2100 nm), the three optical filters, and the transmission of the
cavity mirrors. The intensity is lower near 330–340 nm where the mirror
reflectivity is higher and the mirror transmission is lower. The accumulated
counts at 330 nm were approximately 4.3×106 in 2.0 s. Figure 2b
shows the mirror reflectivity calculated from Eq. (1). The peak
reflectivity is 0.99930±0.00003 (700 ppm loss) at 338 nm and
0.99922±0.00004 at 330 nm. This is slightly lower than the
manufacturer's specifications of 0.9995 at 330 nm. The repeatability of this
result, even after standard procedures for careful mirror cleaning, suggests
that it does not arise from surface deposits on the mirrors. This
discrepancy has been reported previously for BBCEAS instruments and has
been attributed to photons propagating off-axis in the cavity or populating
very high-order transverse modes with greater losses than lower-order modes
(Varma et al., 2009).
For a 100 cm cavity containing 830 hPa of zero air with R=99.930 %
mirrors at 338 nm, the per pass fractional losses are 700 ppm mirror loss
and 73 ppm Rayleigh scattering loss. The measured transmission through a
single mirror is 210 ppm at 338 nm, indicating that absorption and
scattering are 490 ppm and dominate the losses in the cavity. This contrasts
sharply with higher quality mirrors available in the visible spectral
region, where cavity measurements at 455 nm have demonstrated 34 ppm mirror
loss and 19 ppm Rayleigh scattering (Washenfelder et al., 2008).
Measurements of NO2
Using the flow system shown in Fig. 1b, standard additions of NO2
diluted with zero air were introduced to the BBCEAS and CRDS instruments in
series. The calculated extinction for different NO2 concentrations,
with fits determined by DOASIS spectral fitting, are shown in Fig. 3, with
fitting residuals shown in the upper panel. The average fit uncertainty is
360 pptv NO2 in 1 min.
We note that the differential absorption features for NO2 are weaker in
this spectral region compared to the visible region of the spectrum. For
example, the maximum differential NO2 cross section in this region is
24.7 % of the absolute extinction at 348.5 nm (spectral resolution 0.47 nm).
In contrast, the differential cross section is 54.0 % of the absolute
extinction at 448.1 nm, and our previous measurements at 440–470 nm
demonstrate a detection sensitivity (1σ) of 40 pptv in 5 s
(Washenfelder, 2008, 2011; Min et al., 2015). Part of this difference is due to the
spectrally intense LED light source and lower loss mirrors, but part is also
due to the larger differential NO2 cross section in the visible
spectral region.
The correlation plot of the independent NO2 measurements is shown in
Fig. 4. The slope is 1.024±0.027 and the intercept is -0.20±0.32 ppb, with r2=0.998. Despite the weaker differential
absorption features, the ultraviolet BBCEAS instrument accurately quantifies
the NO2 concentrations relative to the more precise CRDS instrument and
is within the combined uncertainty of the Rayleigh scattering mirror
reflectivity calibration and the NO2 absorption cross section (±5.1 %; see Sect. 4.5).
Measured α(λ) for 1 min standard additions and
calculated spectral fits for seven NO2 concentrations from 6 to 39 ppbv.
Two fitting residuals are shown in the upper panel. The average fit
uncertainty for all concentrations shown is 360 pptv NO2.
Correlation plot comparing NO2 measurements from the BBCEAS and
CRDS instruments during 1 min standard additions. The slope is 1.024±0.027 with intercept of -0.20±0.32 ppbv (black line) and r2=0.998.
Measurements of CH2O
The calculated extinction for different CH2O concentrations, with fits
determined by DOASIS, are shown in Fig. 5 with fitting residuals in the
upper panel. The average fit uncertainty is 400 pptv CH2O in 1 min. The
maximum absolute absorption cross section of CH2O at this resolution is
7.6×10-20 cm2 molecule-1 at 326.0 nm. CH2O is
a relatively weak absorber compared to other species, such as glyoxal, whose
peak cross section at 455 nm is 7 times greater. The weak absolute cross
section of CH2O makes it one of the more challenging UV absorbers to
retrieve, despite its strong differential absorption in the ultraviolet
spectral region, where the differential cross section is 91.0 % of the
absolute extinction at 326.0 nm.
Measured α(λ) and calculated spectral fits for
CH2O concentrations from 17 to 65 ppbv. Two fit residuals are shown
in the upper panel. The average fit uncertainty for all concentrations shown
is 400 pptv CH2O.
Correlation plot comparing CH2O measurements from the BBCEAS
with the dilution scaling during 1 min standard additions. The absolute
CH2O concentration is not well known, so the slope is not calculated.
The r2 value is 0.9998 and intercept is -0.6±0.2 ppbv.
The correlation plot for CH2O is shown in Fig. 6. In this case,
CH2O is compared to a scaled concentration based on flow dilution,
rather than to an independent measurement, because the CH2O
concentration in the bulb is not known accurately. For this comparison,
r2=0.9998.
(a) Time series of 30 s measurements of sequential
additions of 9.7 ppbv NO2, 15.6 CH2O, 5.5 ppbv NO2, and 7.5
CH2O during a 17 min period. BBCEAS NO2 and CH2O
concentrations are compared to CRDS NO2 concentrations and calculated
CH2O dilution factors. (b) Measured α(λ) and
calculated spectral fits for 30 s measurements with different CH2O and
NO2 concentrations.
Simultaneous measurements of NO2 and CH2O
The dominant ambient lower atmospheric absorbers in this spectra region are
NO2 and CH2O, and ambient samples will likely contain a mixture of
both. We introduced mixtures of these two species to the BBCEAS and CRDS
cells using the flow system shown in Fig. 1b. A time series with the
sequential introduction of 9.7 ppbv NO2, 15.6 ppbv CH2O, 5.5 ppbv
NO2, and 7.5 ppbv CH2O is shown in Fig. 7. Each point represents
the average of 17 spectra with 1.2 s integration time (30 s total
acquisition time). The decrease in the measured CH2O concentration
during the first addition is due to drift in the output from the pressurized
CH2O bulb. During the second set of additions, the 1σ standard
deviations for NO2 and CH2O are 140 and 210 pptv,
respectively. This represents the detection limit and experimental precision
for these measurements. Increasing the averaging time to 10 min would
improve the detection limits for NO2 and CH2O to 30 and 50 pptv,
respectively, assuming that the averaging follows the square root of the
number of points. Experience from prior ground-based field measurements of
glyoxal and nitrous acid showed that their measurement precision scaled
linearly with the square root of averaging time over 10–60 min periods,
when zeros were acquired at more frequent intervals (Washenfelder et al.,
2011; Young et al., 2012).
Precision and accuracy of the NO2 and CH2O
measurements
An examination of Eq. (1) shows that the precision for extinction, α(λ)min, depends on the smallest measurable difference in light
through the cavity, δImin(λ), which is equal to
(IZA(λ)-I(λ))I(λ)min.
αmin(λ)=1-R(λ)d+(λ)RayleighδImin(λ)
For cavity mirrors with lower reflectivity, R(λ), a smaller value for
δImin(λ) is required to achieve the same detection limit.
As the effective path length becomes shorter, it is necessary to accurately
detect increasingly small changes in intensity, and the drift of the light
source and cavity alignment can become limiting factors in the precision.
Conversely, lower reflectivity mirrors may have larger transmission of the
light source (depending on the balance between transmission, absorption, and
scattering in the mirror coating), increasing the photon count rate and thus
the precision of δImin if the system is shot noise limited.
Based on the acquired counts of 1.0×108 in 1 min, the
calculated 1σαmin at 330 nm would be 7.4×10-10 cm-1 in the shot noise limit. However, achieving this
theoretical value requires a strict cavity stability, with δImin=1.0×10-4. Figure 8 shows an Allan deviation
plot (Allan, 1966) calculated for a 3 h series of spectra, with
a minimum of 5×10-9 cm-1 for 9 s and 6×10-9 cm-1 for 1 min. Temperature-controlling and purging the laser-driven arc lamp have reduced the intensity drift, but Fig. 8 shows that
frequent zeroing will be useful to improve measurement precision.
The absolute accuracy depends on the accuracy of the Rayleigh scattering
cross section for zero air (±4 %), Rayleigh scattering cross
section for helium (negligible contribution), absorption cross section for
NO2 (±3 %; Vandaele et al., 1998), absorption cross section
for CH2O (±5 %; Meller and Moortgat, 2000), sample
pressure (±0.5 %), and temperature (±0.7 %). Summed in
quadrature, the total calculated uncertainty is 5.1 % for NO2 and
6.5 % for CH2O. This expected measurement accuracy is consistent with
the comparison of the NO2 retrievals from the ultraviolet BBCEAS
instrument to the 405 nm CRDS instrument.
Allan deviation plot for zero air measurements acquired by the
BBCEAS instrument, showing the relationship between averaging time and
1σ precision for a single pixel at 330 nm. The dashed line shows the
relationship expected for statistically random noise.
Summary and conclusions
With a 2σ detection limit of 300 pptv CH2O in 1 min, the
current demonstration of this measurement technique is appropriate for
laboratory studies of CH2O, ground-based field measurements in regions
with high CH2O mixing ratios (for example, regions with large emissions
and rapid oxidation of anthropogenic or biogenic VOCs), and ground-based
field measurements of cleaner environments with increased averaging time
(>5 min).
Although the current precision is not competitive with the best
spectroscopic in situ CH2O instruments, the ultraviolet BBCEAS is
comparable to current ground- and satellite-based DOAS instruments, which
use signal averaging to achieve lower detection limits. State-of-the-art
DOAS instruments report CH2O detection limits of approximately 150 pptv
for data acquisition times of 5–10 min (Warneke et al., 2011), while
CH2O satellite data are typically analyzed as monthly or seasonal
averages (Chance et al., 2000; Wittrock et al., 2006). With
signal averaging, the ultraviolet BBCEAS could be competitive with these
direct absorption instruments, while providing a true in situ measurement.
Previous work has shown that it is possible to construct inlets for
formaldehyde that have minimal sampling artifacts (Wert et
al., 2002). The BBCEAS method may also be useful as a lower-time-resolution
validation for wet chemical, mass spectrometry, and fluorescence methods,
because it is essentially free from the potential interferences or artifacts
of those methods.
Calculated relationship between αmin, mirror
transmission, and the sum of mirror absorption and scattering at 330 nm,
following Eq. (3) and assuming shot noise limited performance. The current
mirrors have absorption and scattering losses of 460 ppm and transmission of
210 ppm (see text), which is the basis for the solid red line. All other
calculations are scaled to this line assuming that the photon count rate
scales linearly with transmission. Note that the optimum performance for the
current set of mirrors would occur for slightly more transmissive
(less reflective) optics, with a minimum at approximately twice
(400 ppm) the measured transmission of 200 ppm.
Ultraviolet BBCEAS measurements would be appropriate for more rapid sampling
of ambient, background CH2O concentrations if its 2σ detection
limit could be improved by approximately a factor of 3 to achieve 100 pptv
in 1 min. This limit would require an increase of the effective path
length or an increase of the optical power at the detector. The first
possibility would require a factor of 9 decrease in absorption and
scattering losses in the mirror coating, with unchanged transmission and
reflectivity. Figure 9 demonstrates the relationship between αmin, mirror transmission, and the sum of mirror absorption and
scattering, calculated from Eq. (3). The second possibility would require a
factor of 9 increase in optical power at the detector, for a system
following the shot noise limit. This would need to be achieved through
improvements in the light source power, geometric light collection, and
optical filtering. More powerful laser-driven light sources or ultraviolet
LEDs may be available in the future to partially satisfy this requirement.
Super-continuum laser light sources are available in the visible spectral
region and offer much higher optical power output and spatial coherence,
allowing them to be more efficiently coupled into an optical cavity. Such
sources are not available in the ultraviolet spectral region but may be in
the future.
The ultraviolet light source and measurements presented here have strong
potential for ambient, high time resolution measurements of aerosol optical
extinction in the ultraviolet spectral region, where brown carbon absorption
is important. Previous measurements have reported aerosol extinction and
retrievals of complex refractive indices from 360 to 420 nm (Washenfelder
et al., 2013; Flores et al., 2014a, b). The ultraviolet
BBCEAS instrument will allow those measurements to be extended to 315 nm.
The 1σ precision of 1.8×10-8 and 6×10-9 cm-1 (1.8 and 0.6 Mm-1) per
min for single pixels (0.05 nm) at 315 and 330 nm determined from the Allan
deviation is appropriate for aerosol extinction measurements even in clean
environments. These values would be further improved by averaging multiple
pixels. Field measurements of dry aerosol extinction and angstrom exponent
at 360–420 nm measured in the rural southeastern USA (Washenfelder et
al., 2015) indicate that the extinction at 315 nm would be 1×10-7–2×10-6 cm-1 (10–200 Mm-1). These values are easily measurable with the current detection
limit, with signal-to-noise ratio of 6–110 (17–300) at 315 nm (330 nm) for 1 min
ground measurements and 6–110 (12–250) for 1s aircraft measurements.