Article pubs.acs.org/est
Measurement of Air-Sea Exchange of Dimethyl Sulfide and Acetone by PTR-MS Coupled with Gradient Flux Technique Hiroshi Tanimoto,*,† Sohiko Kameyama,‡ Toru Iwata,§ Satoshi Inomata,† and Yuko Omori† †
Center for Global Environmental Research, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, 305-8506, Japan. Faculty of Environmental Earth Science, Hokkaido University, North 10 West 5, Kita-ku, Sapporo, 060-0810, Japan. § Graduate School of Environmental and Life Science, Okayama University, 3-1-1 Tsushima-Naka, Kita-ku, Okayama 700-8530, Japan. ‡
ABSTRACT: We developed a new method for in situ measurement of air− sea fluxes of multiple volatile organic compounds (VOCs) by combining proton transfer reaction-mass spectrometry (PTR-MS) and gradient flux (GF) technique. The PTR-MS/GF system was first deployed to determine the air−sea flux of VOCs in the open ocean of the western Pacific, in addition to carbon dioxide and water vapor. Each profiling at seven heights from the ocean surface up to 14 m took 7 min. In total, 34 vertical profiles of VOCs in the marine atmosphere just above the ocean surface were obtained. The vertical gradient observed was significant for dimethyl sulfide (DMS) and acetone with the best-fit curves on quasi-logarithmic relationship. The mean fluxes of DMS and acetone were 5.5 ± 1.5 and 2.7 ± 1.3 μmol/m2/day, respectively. These fluxes are in general in accordance with those reported by previous expeditions.
1. INTRODUCTION Ocean surface waters contain a large amount of organic matter, and serve as one of the largest active reservoirs of organic carbon on the Earth.1 The air−sea exchange of volatile organic compounds (VOCs) dissolved in the surface ocean plays a critical role in the determination of concentrations, variability, and trend of VOCs in the atmosphere, in particular, the marine boundary layer.2,3 Among VOCs emitted from the ocean, substantial attention has been paid to dimethyl sulfide (DMS) for the last decades, since a climate feedback role of the DMS production was first highlighted and has been referred to the Charlson-LovelockAndreae-Warren (CLAW) hypothesis.4 DMS is the main natural source of tropospheric sulfur, and plays a key role in cloud formation and albedo over the global oceans. Oceanic emissions of DMS have large spatial and temporal variations. While DMS is produced from marine microbiological activities in the euphotic zone, the emissions of DMS from the ocean surface to the atmosphere are predominantly controlled by physical factors including wind velocity, temperature and wave action.5 Based on available DMS concentration measurements near the sea surface, a global climatology was extrapolated. The global sea-to-air flux of DMS was then estimated by multiplying the near surface concentrations by a wind speed-dependent gas transfer velocity. However, the relative scarcity of direct air−sea flux measurements remains one of the main sources of uncertainties in the estimates of global DMS missions. Acetone exists ubiquitously in the troposphere.6 Acetone has potential impacts on oxidizing capacity by producing OH radicals and serving as a precursor for peroxyacetyl nitrate, via photolysis in the upper troposphere.7 There are a variety of © 2013 American Chemical Society
sources for acetone, including vegetation, biomass burning, industry, and photo-oxidation of VOCs in the atmosphere, and sinks including oxidation by OH, photolysis, and surface uptake.8 A major uncertainty controlling the atmospheric budget of acetone is the role of the ocean. Previous field observations suggest that the ocean can be either a source or a sink of acetone.9−13 In contrast, a recent modeling study suggests that the ocean is in near-equilibrium with the atmosphere, with the northern hemisphere being a net sink and the tropical oceans being a source.14 Further field observations of acetone are needed to better understand the air−sea exchange of acetone and thereby better constrain the budget of acetone. There is an increasing recognition that the fluxes of these VOCs should be tested by in situ observations. There have been several studies for direct measurements of air−sea fluxes of these VOCs, with DMS having been a main focus. Recently measurements of flux of other organics such as acetone have been increasingly reported. The flux of DMS has long been studied using various micrometeorological methods. In the past decades, detection by gas chromatography/flame photometric detection (GC/FPD) was commonly applied to gradient flux (GF) technique and relaxed eddy accumulation (REA) technique.15,16 Recently, very fast (>1 Hz) detection by online mass spectrometry has been developed and applied to eddy covariance (EC) technique. An atmospheric pressure chemical Received: Revised: Accepted: Published: 526
July 23, 2013 October 30, 2013 November 12, 2013 November 12, 2013 dx.doi.org/10.1021/es4032562 | Environ. Sci. Technol. 2014, 48, 526−533
Environmental Science & Technology
Article
a gas. Using the Monin-Obukhov (MO) similarity theory, the flux (F) can be expressed as
ionization mass spectrometry (AP-CIMS) has been used for EC technique to determine DMS and acetone fluxes.13,17−20 Although the time resolution of proton transfer reaction-mass spectrometry (PTR-MS) is not high enough to be used for EC, it has been used for the disjunct eddy covariance (DEC) technique to determine the flux of acetone.21 The PTR-MS can be used for EC measurements of VOCs, if only a few ions are measured at a time.22 While the observational evidence of sea-to-air flux of DMS has been accumulated, the measurement of acetone is still challenging and hence very few. It is obviously necessary to accumulate further data for the air−sea flux of these species, or VOCs in general. Field deployable techniques that are independent of existing methods will help provide more insights to constrain the air−sea exchange of VOCs and to improve our knowledge of gas transfer coefficients. Here we present a new methodology to measure the air−sea flux of multiple VOCs by combining PTR-MS with a floating buoy system. This is the first report to determine the air−sea flux of acetone by gradient approach. In addition, this is also the first approach to determine the DMS flux by using gradient approach and online MS. The developed system will enable us to obtain the air−sea flux of DMS and acetone (and other VOCs) from coastal to open oceans.
F=−
κu ∂C̅ * ϕ(z /L) ∂(ln z)
(1)
where u* is the friction velocity, κ is the von Karman constant (assigned a value of 0.4), C is the mixing ratio of the gas, z is the height above the mean sea surface, and ϕ(z/L) is the Monin-Obukhov stability function. The empirical relationships for stability functions have been proposed by many authors.23−25 In this study we used the following relations:23 ⎧(1 − 16z /L)−1/2 :unstable conditions ⎪ ϕ(z /L) = ⎨1 :neutral conditions ⎪ ⎩1 + 5z /L :stable conditions
(2)
where z is the measurement height and L is the MoninObukhov length; u 3T̅ L=− * v κgFv
(3)
where Tv is the virtual temperature, g is the acceleration due to gravity, and Fv is the buoyancy flux;
2. METHOD AND EXPERIMENTS 2.1. Sampling Location. The first field observation with the PTR-MS/GF system was made during a research cruise (KH-10-1) by R/V Hakuho Maru in the western North Pacific Ocean in May 2010 (Figure 1). The ship departed from a port
Fv =
T̅ 1 H + 0.608 E c pρ ρ
(4)
where H and E are the sensible and latent heat flux, respectively, and cp is the specific heat of air at constant pressure, ρ is the air density, T is the air temperature. We calculated u , H, and E from COARE 3.0 algorithm.26 Thus, * the gas flux can be determined based on the observed parameters; the slope of the gradient from the ocean surface, and meteorological data in air and ocean such as wind velocity, ambient temperature, sea surface temperature, and humidity.27 A floating buoy was used as a platform to obtain vertical profiles of VOC concentrations above the ocean surface. As seen in Figure 2, the floating buoy was placed approximately 25 m upwind of the ship by the ship’s boom, to minimize the influence of turbulence due to the existence of the ship and/or contamination from the ship’s facilities/laboratories. A schematic diagram of the PTR-MS/GF system is shown in Figure 3. The buoy was equipped with five sampling ports at elevations of 1, 10, 35, 110, and 245 cm above sea level. The sampling at 1 cm was made as a preliminary testing. Additionally, two sampling inlets were placed at the lower (6.5 m asl.) and upper (14.0 m a.s.l.) decks of the ship. Membrane filters (Advantec-Toyo Ltd., J100A047A) were placed at the inlets to prohibit seawater droplets from being drawn into the sampling tubes. Ambient air sampled at the inlets was continuously drawn at a flow rate of 0.7 sLpm toward a switching unit by 3 mm i.d. PFA-Teflon tube with an approximate length of 30−40 m. All the seven sampling lines were continuously pumped from the inlets to the switching unit by seven individually equipped pumps to avoid ambient air being stagnant in the sampling lines. Sample streams were then switched by automated valves in a switching unit, and only the selected stream was sequentially directed to PTR-MS situated in the ship’s laboratory for quantification of VOCs. No preconcentration or drying was made for PTR-MS. The mixing ratios of H2O and CO2 at each height were measured by a temperature/humidity sensor (Vaisala, model HMP45A) and a
Figure 1. Cruise track (lines) and stations for flux measurements (circles) during the KH-10-1 cruise by R/V Hakuho Maru in the western North Pacific Ocean.
of Tokyo, first heading southeast. Starting at 30°N, the ship moved toward south along 137°E, and stopped at 2 sites: Stations A (20°N) and B (15°N), where the flux measurements described in the present paper were made. After Station B, the ship moved toward northwest, and went back to the port of Tokyo. This region, where we deployed the PTR-MS/GF system, is characterized to be oligotrophic subtropical ocean. 2.2. PTR-MS/GF System. For a gas emitted from the ocean into the atmosphere, there is a mean gradient in the lower atmosphere, decaying with elevation above the ocean surface. Vertical mixing by turbulent eddies causes upward transport of 527
dx.doi.org/10.1021/es4032562 | Environ. Sci. Technol. 2014, 48, 526−533
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Figure 2. Photos of a floating buoy system used for measuring air−sea fluxes of VOCs by gradient flux technique combined with PTR-MS. A view from the ship (left) and zoom-up view of the buoy (right).
Figure 3. Diagram of the flux observing system, including air sampling aboard a profiling buoy and detection of VOCs by PTR-MS.
between H3O+ and VOCs occur, and a quadrupole mass spectrometer for detection of reagent and product ions. H3O+ ions were produced from pure water vapor at a flow rate of 7.0 sccm by means of a hollow cathode discharge. Sample air was introduced into the drift tube at a flow rate of approximately 75 sccm, and the drift tube pressure and temperature were held at 2.1 mbar and 105 °C, respectively. In the drift tube, VOC in the sample gas was ionized by proton transfer reactions as follows:
nondispersive infrared spectrometer (LICOR, model Li-6262) after dehumidification with magnesium perchlorate, respectively. Fundamental meteorological parameters including wind velocity, wind direction, and temperature and humidity in ambient air were monitored by custom-built sensors (Nippon Electric Instrument, Inc.) equipped on the ship. Surface seawater temperature was measured by a thermometer. We used a commercially available PTR-MS instrument (PTR-MS-hs, IONICON Analytik, Innsbruck, Austria).28,29 The instrument consists of a discharge ion source to produce H3O+ ions, a drift tube in which the proton transfer reactions
H3O+ + VOC → VOC·H+ + H 2O 528
(5)
dx.doi.org/10.1021/es4032562 | Environ. Sci. Technol. 2014, 48, 526−533
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Fractions of the reagent ion (H3O+) and the product ion were extracted through a small orifice into the quadrupole mass spectrometer, where they were detected by a secondary electron multiplier operated in the ion pulse counting mode. The drift voltage was set to 400 V; at that voltage, the field strength of the drift tube, E/N, where E is the electric field strength (V cm−1) and N is the buffer gas number density (molecules cm−3), was 110 Td (Td = 10−17 cm2 V molecule−1). The field strength was set as low as possible to minimize fragmentation of detected VOCs. Mass signals were obtained at 10-s intervals. The ion signal at m/z = 63 was attributed to DMS on the basis of literature evidence.30,31 The ion signal at m/z = 59 was attributed to acetone, but it should be noted that m/z = 59 might have interferences from isomers including propanal and glyoxal,30,31 if they are copresent with acetone. Here no attempts were made to correct for the potential interferences at m/z = 59. The detection sensitivity under dry conditions was determined by dynamic dilution of a gravimetrically prepared gas standard balanced with ultrapure N2 (5 ppm, Japan Fine Products Co., Kawasaki, Japan). The detection sensitivity depended on the humidity of the sample air. The relationship between the humidity and the detection sensitivity for VOCs was determined by means of a procedure described previously.29,32 Briefly, the humidity correction factor (the detection sensitivity relative to that under dry conditions) for DMS and acetone ranges from 1.1 to 1.2. The detection limits for DMS and acetone are 0.06−0.07 ppbv (S/N = 2), with an overall uncertainty being ±20%. In order to obtain reliable data for flux calculations and minimize possible errors, we selected the data with the following standards: (1) data obtained with the wind direction of ±20° relative to the buoy; and (2) the determination coefficient (R2) being more than 0.6, in the gradient (∂C/ ∂(lnz)) of the vertical profiles for each 7-min cycle. For flux calculation, we averaged the data from consecutive 3−4 cycles (21−28 min) that met the above two criteria, resulting in one flux data.
Figure 4. Time-series of DMS and acetone in ambient air sampled at seven heights from 1 cm to 14 m above the ocean surface. The raw data obtained at 3 s intervals (dots) and the data selected for flux calculation (open squares) are shown. The DMS and acetone data are corrected for the humidity dependence of the PTR-MS detection sensitivity.
COARE) 3.0 gas transfer model.26 In Figure 6, we find a clear correspondence between the variations in fluxes derived by the GF approach and the NOAA/COARE model. Based on the reduced-major-axis (RMA) regression, there is a high correlation (r2 = 0.66) between the two data sets with the regression line being generally close to a 1:1 correspondence (slope = 0.87 ± 0.14, intercept = 5.5 ± 3.6). The sensitivity of eddy diffusivity (and hence flux) to different stability functions was also examined. The eddy diffusivity calculated by eq 2 for unstable conditions was in agreement with those based on COARE 2.5 and Granchev et al. (2000)33 in the range of ±2.5%. These results support the validity of the GF approach used in this study. It is known that the difference in CO2 partial pressure between the atmosphere and ocean in this period of year in this area is small.34 This can explain the insignificant gradient for CO2. The gradients for DMS and acetone indicate that both of these have an oceanic source. This feature will be elaborated more in the following section. It should be noted that the data of DMS and acetone at the lowest height (1 cm asl.) were sometimes not on the quasi-logarithmic least-squares fits but higher than the predicted mixing ratios from the fit. Although we examined the possibility of contamination in filters/tubes, memory effects in sampling lines, and artificial production in inlet filters, none of these was identified. Since the lifetime of DMS and acetone is thought to be long enough (i.e., hours for DMS and a month for acetone in the lower troposphere),12 the gradient in the lowest layer of the marine boundary layer is not significantly altered by photochemical oxidation. For DMS and acetone, data at these heights do have substantial overlaps, but the mixing ratios at 1 cm still seem to have a positive bias for DMS and acetone, perhaps due to greater influence from the ocean surface. In addition to a positive bias, the associated large variability suggests that precise air samplings and measurements
3. RESULTS AND DISCUSSION Figure 4 shows the time series of the mixing ratios of DMS and acetone detected by PTR-MS during the measurements at Station A. It took 50 s to perform the ion monitoring at targeted mass numbers at each sampling height. The sampling lines were sequentially switched to the next height in predetermined random order at seven heights in total. This procedure resulted in total 7 min for each cycle. In this case we saw higher mixing ratios at lower heights for both DMS and acetone. The data for the last 30 s at each height were used to derive the vertical gradient and to calculate air−sea fluxes for each observation. Figure 5 illustrates the vertical gradient for absolute humidity, CO2, DMS, and acetone at Station A (20°N) during nighttime. In general, quasi-logarithmic relationships are seen within the range of measurement uncertainties for all the species except CO2. The absolute humidity at 1 cm was apart from the gradient curve and close to the one at 10 cm, suggesting that the vertical motion of the floating buoy relative to the air−sea surface was approximately ±5 cm. Hence we adapted the uncertainty in the height determination to ±5 cm. We compared the latent heat (water vapor) fluxes determined by the gradient flux method and those calculated by the National Oceanic and Atmospheric Administration/ Coupled Ocean Atmosphere Response Experiment (NOAA/ 529
dx.doi.org/10.1021/es4032562 | Environ. Sci. Technol. 2014, 48, 526−533
Environmental Science & Technology
Article
Figure 6. Comparison of latent heat (water vapor) fluxes observed by the gradient flux method and calculated by NOAA/COARE 3.0 gas transfer model. The best-fit line obtained by the reduced-major-axis (RMA) regression (y = (0.87 ± 0.14) x + (5.5 ± 3.6), r2 = 0.66, N = 34) and y = x lines are indicated by solid lines and dashed lines, respectively.
provinces.5 Our observation is also in good agreement with the numbers by prediction of the sea-to-air DMS flux to be
