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Environ. Sci. Technol. 2009, 43, 1412–1418

An Automated Analyzer to Measure Surface-Atmosphere Exchange Fluxes of Water Soluble Inorganic Aerosol Compounds and Reactive Trace Gases R I C K M . T H O M A S , * ,†,‡ I V O N N E T R E B S , § ´ OTJES,| PIET A. C. JONGEJAN,| RENE HARRY TEN BRINK,| GAVIN PHILLIPS,† MICHAEL KORTNER,§ FRANZ X. MEIXNER,§ AND EIKO NEMITZ† Centre for Ecology and Hydrology, Edinburgh Research Station, Bush Estate, Penicuik, Midlothian, EH26 0QB, U.K., School of Earth, Atmospheric and Environmental Sciences (SEAES), The University of Manchester, Sackville Street, M60 1QD, Manchester, U.K., and Biogeochemistry Department, Max Planck Institute for Chemistry, P.O. Box 3060, 55020 Mainz, Germany, and Energy Research Centre of The Netherlands (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlands

Received July 14, 2008. Revised manuscript received November 27, 2008. Accepted December 12, 2008.

Here, we present a new automated instrument for semicontinuous gradient measurements of water-soluble reactive trace gas species (NH3, HNO3, HONO, HCl, and SO2) and their related aerosol compounds (NH4+, NO3-, Cl-, SO42-). Gas and aerosol samples are collected simultaneously at two heights using rotating wet-annular denuders and steam-jet aerosol collectors, respectively. Online (real-time) analysis using ion chromatography (IC) for anions and flow injection analysis (FIA) for NH4+ and NH3 provide a half-hourly averaged gas and aerosol gradients within each hour. Through the use of syringe pumps, IC preconcentration columns, and high-quality purified water, the system achieves detection limits (3σdefinition) under field conditions of typically: 136/207, 135/114, 29/ 22, 119/92, and 189/159 ng m-3 for NH3/NH4+, HNO3/NO3-, HONO/ NO2-, HCl/Cl- and SO2/SO42-, respectively. The instrument demonstrates very good linearity and accuracy for liquid and selected gas phase calibrations over typical ambient concentration ranges. As shown by examples from field experiments, the instrument provides sufficient precision (3-9%), even at low ambient concentrations, to resolve vertical gradients and calculate surface-atmosphere exchange fluxes under typical meteorological conditions of the atmospheric surface layer using the aerodynamic gradient technique.

1. Introduction 1.1. Flux Measurement Approaches for Reactive Trace Gases and Aerosols. The atmospheric longevity and transport of inorganic aerosols and associated trace gases alters * Corresponding author e-mail: [email protected]. † Edinburgh Research Station. ‡ The University of Manchester. § Max Planck Institute for Chemistry. | Energy Research Centre of The Netherlands (ECN). 1412

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the magnitude and extent of their impacts on (i) air quality (in relation to human health and visibility); (ii) the Earth’s climate through direct and indirect aerosol effects (relating to the Earth’s albedo and weather patterns); and (iii) ecosystems (through eutrophication and acidification). Theiratmosphericlifetimeisgovernedbysurface-atmosphere exchange fluxes and, for the above effects, a key role is played by anthropogenic inorganic reactive compounds such as ammonia (NH3), nitric acid (HNO3), sulfur dioxide (SO2), and their related aerosol compounds ammonium (NH4+), nitrate (NO3-), and sulfate (SO42-) (1). Fast-response sensors and the eddy-covariance technique can be used to measure inorganic aerosol fluxes (2), but inlet dynamics for sticky gases such as HNO3 and NH3 result in substantial time response problems or poorly quantified aerosol interference effects when using this approach (3, 4). Alternatively, exchange fluxes may be derived from slowresponse measurements of vertical concentration profiles, using variants of the aerodynamic gradient technique (5). For such measurements, particles and gases can be collected in denuders and filter-packs and analyzed for bulk composition (6, 7), but these methods can incur in situ sampling artifacts due to gases being adsorbed or volatilized when using particle prefilters (8, 9), and are labor-intensive (leading to manual-handling related contamination). Resolving vertical concentration gradients of the order 1-20% of the mean ambient concentration is the major challenge when determining surface-atmosphere exchange fluxes using the aerodynamic gradient technique (e.g., ref 10). 1.2. Aerodynamic Gradient Technique. The aerodynamic gradient approach (e.g., refs 5, 11) uses concentration measurements between a minimum of two heights to estimate the local concentration gradient (∂χ/∂z), and calculate the flux (Fχ) by analogy with Fick’s Law:

(

Fχ ) -KH u*,

z - d ∂χ L ∂z

)

(1)

where KH is the eddy diffusivity of sensible heat, dependent on z, the absolute measurement height above the ground surface; d, the zero plane displacement height; u*, the friction velocity; and L, the Monin-Obukhov length, parametrizing atmospheric stability. This approach relies on the prerequisite that fluxes of scalar tracers are constant within a “constant flux layer” that forms over homogeneous surfaces. Adapted from ref 5, the flux can alternatively be expressed as Fχ ) -χ*u*

(2)

Here, u* is now typically calculated by eddy-covariance from sonic anemometry data and χ* is the so-called concentration scale, calculated from the concentrations (χ) measured at heights (z1 and z2) using (modified from ref 12): χ* )

κ[χ(z2) - χ(z1)] z2 - d z2-d z1-d ln - ψΗ + ψΗ z1 - d L L

(

) ( ) ( )

(3)

Here ΨH is the integrated stability function for sensible heat and κ is the empirical von Karman constant (∼0.41). 1.3. Development of Continuous Gradient Systems for Reactive Gases and Aerosols. Appropriate gas samplers that minimize gas-aerosol interconversion artifacts include horizontally aligned rotating wet-annular denuders (WRD, alternatively called WAD) (13), consisting of two concentric 10.1021/es8019403 CCC: $40.75

 2009 American Chemical Society

Published on Web 01/30/2009

FIGURE 1. A schematic representation of the GRAEGOR system (not to scale). See text for details. glass cylinders with an annulus of approximately 1.5 mm. Water soluble trace gases are extracted from a laminar air flow via Brownian diffusion into a film of suitable absorption solution, which is pumped into the annulus to coat the glass walls on rotation. The low mobility of particles prevents them from diffusing into the WRD absorption liquid, but coarse particles and cloud droplets may be captured through gravitational settling (see Section 3). An alternative denuder design is the wet effluent diffusion denuder (WEDD) (14). Wet-chemistry aerosol sampling devices such as the steam-jet aerosol collector (SJAC) (15), the mist chamber system (MCS) (16), and others (17, 18) inject steam into the sample flow to grow particles thus generating a condensate of water soluble aerosol, with sampling times in the order of minutes (e.g., ref 19). Online analysis of outputs from wetchemical samplers by appropriate techniques (e.g., ion chromatography, IC 17, 20) increases precision and accuracy by reducing contamination from manual handling. The AMANDA (ammonia measurement by annular denuder sampling with online analysis) gradient system, developed by the Energy Research Centre of The Netherlands (ECN), comprises of two or three WRDs situated at different sampling heights sharing a single online NH4+ flow injection analyzer (FIA) (21) and has been regarded as the standard for the determination of NH3 exchange fluxes (e.g., ref (22)). In addition to NH3, the WEDD and MCS coupled with appropriate analytical techniques enabled measurement of concentration gradients of either total nitrate and nitrite, or HNO3 and HONO, to produce qualitative indication of flux directions (23-25). A prototype gradient instrument combining the WRD with a SJAC measured individual NH3 and NH4+ gradients above oilseed rape (26); a two-height version of this instrument coupled to an online anion IC system (27) measured gas and aerosol fluxes over a heathland (28) and

improved, single height versions of the instrument were applied to measure concentrations during several studies (29, 30). Based on these systems, and benefiting from further commercial refinements, we present here a robust nextgeneration gradient system for reactive trace gases and aerosols. The GRAEGOR (gradient of aerosol and gases online registrator; ECN, Petten, NL) simultaneously measures a 2-point vertical concentration gradient of gaseous NH3, HNO3, HONO, HCl, and SO2 and related aerosol compounds NH4+, NO3-, Cl-, SO42- in real-time and with sufficient precision and accuracy to allow calculation of surface-atmosphere exchange fluxes under typical ambient concentrations using the aerodynamic gradient technique.

2. Description of the GRAEGOR Instrument The GRAEGOR system (Figure 1) consists of two sample boxes (SB1 and SB2), one detector box, an external air pump, and a computer. Temperature control devices within all boxes minimize temperature fluctuations, and the translucent 1-2 mm ID PTFE tubing liquid transmission lines should be shaded from sunlight. During measurements, solution reservoirs are placed above the sample and detector boxes, and the air pump and eluent storage container below. To ensure accuracy and low detection limits, all reagent solutions are prepared using double deionized (DDI) water with a specific resistance of >18 MΩ · cm. 2.1. Sample Boxes. Each SB continuously removes watersoluble reactive trace gases and aerosols from a 16.7 L min-1 airflow drawn through a WRD and a SJAC, respectively. A critical orifice immediately downstream of the SJAC outlet maintains a constant airflow rate through each SB. The typical use of unheated 5 h. Standards translate to low ambient air concentrations for which the GRAEGOR is designed. The results generally display a very good linear response for the entire concentration range, with an average slope of 0.94 and an intercept of 1.83 µg kg-1 (see the Supporting Information). This off-set from the anticipated zero intercept reflects SO42- contamination of 3-5 µg kg-1 in the DDI solution matrix. 3.2. (II) Gas Standards. (a) NH3. Seven ammonia gas standards (2.4-52.2 µg m-3) were generated under laboratory conditions (see the Supporting Information). A faster response NH3 detector (Nitrolux-100 photoacoustic analyzer, Pranalytica Inc., San Diego, CA) was used in parallel to the GRAEGOR instrument to confirm complete equilibration within the setup. Linear regression for both SBs and the Nitrolux (Figure 2a), show deviations of 3 and >9%, respectively. Additional ambient measurement data of aerosol NH4+ and Cl-, as well as gas compounds SO2 and HCl, are from ECN at Petten and demonstrate the ability of the instrument to achieve high precision because a unique combination of low concentrations, undefined chromatogram peak shapes, and water contamination resulted in the rejection of these species data during SALSA (see the Supporting Information). During SALSA, the conservative measurement uncertainty under field conditions (representing upper limits when the instrument was stable) was determined to be 11% (14% for NH3/NH4+). 3.7. (VI) Detection Limits under Field Conditions. LODs for the GRAEGOR system were quantified from field blank tests by switching off the air pump and sealing the inlets of the sample boxes while solution was running through WRDs and SJACs to simulate normal operation of the instrument. VOL. 43, NO. 5, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. LODs (3σ-Definition) Determined from Blank Solutions ECN 3σ-definition (laboratory conditions) ng m-3 in air NH3/aerosol NH4+ HNO3/aerosol NO3HNO2/aerosol NO2HCl/aerosol ClSO2/aerosol SO42-

38 42 22 14 13

MPIC 3σ-definition (field conditions) ng m-3 in air

43 41 21 15 20

111 79 27 174 123

CEH 3σ-definition (field conditions) ng m-3 in air

117 77 27 169 201

161 191 30 63 254

180 150 16 15 116

TABLE 2. Minimum Surface-Atmosphere Exchange Fluxes for Typical Surface-Layer Concentrations of Compounds Detectable by the GRAEGOR Instrument for Wind Speed Conditions of Approximately 2 m s-1 (u* = 0.2 m s-1), See Text for Details minimum flux Fχ,min (ng m-2 s-1)

compound χ NH3 HNO3 HONO HCl SO2 aerosol aerosol aerosol aerosol

NH4+ NO3ClSO42-

f (∆χmin) typical (daytime) (%) conc. (ng m-3) 3 3 3 3 7 9 9 9 9

2000 1000 100 1000 500 600 400 200 800

unstable neutral stable typical unstable neutral stable conditions conditions conditions (nighttime) conditions conditions conditions z/L ) -0.2 z/L < ( 0.01 z/L ) +0.2 conc. (ng m-3) z/L ) - 0.2 z/L< ( 0.01 z/L ) +0.2 3.28 1.64 0.16 1.64 1.91 2.95 1.97 0.98 3.94

1.83 0.92 0.09 0.92 1.07 1.65 1.10 0.55 2.20

These tests were done either in the laboratory (ECN) or during field experiments (MPIC and CEH) and results are presented in Table 1. LODs were calculated using average WRD blanks plus three standard-deviations (3σ). DDI water quality under field conditions is clearly the major controlling factor for LODs, highlighted by higher and variable field values for HCl/aerosol Cl- and SO2/aerosol SO42-. Although we can provide typical LODs here, these need to be reassessed for each field deployment. 3.8. (VII) Minimum Detectable Surface-Atmosphere Exchange Fluxes. The aerodynamic gradient method relies on well resolved gradients and the fulfillment of demanding micrometeorological requirements of a well developed, steady-state turbulent flow (e.g., ref 11). Both criteria can be limiting: during night-time, when large vertical concentration gradients are easily detectable, turbulence can be low; in contrary, when high turbulence effectively mixes the surface layer concentration gradients are often (very) small, particularly for slowly depositing compounds such as accumulation mode particles. To simplify further discussion, we defined two sampling heights, z1 ) 0.25 m and z2 ) 2.0 m, which may typically be selected for gradient measurements over low vegetation (d ) 0.1m) and ignore surface and quasilaminar boundary layer resistances (32), which would further reduce the flux detection limits (FDL). In order to derive (minimum conservative) surface-atmosphere exchange fluxes which can be measured with the GRAEGOR instrument, eq 1 is reformulated to KH u*,

(

)

(

)

z2 - d z 1 - d , L L Fχ,min ) ∆z z2 - d z 1 - d f (∆χmin) χambient ∆χmin ) -Vtr u*, , L L 100

(5)

where derivatives have been substituted by corresponding differences, the ratio KH/∆z, having the dimension of a velocity (m s-1), is usually called transfer velocity Vtr (between z1 and z2), and the minimum detectable concentration 1416

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0.62 0.31 0.03 0.31 0.36 0.56 0.37 0.19 0.75

500 100 800 100 200 1200 1000 500 1200

0.82 0.16 1.31 0.16 0.77 5.91 4.92 2.46 5.91

0.46 0.09 0.73 0.09 0.43 3.30 2.75 1.37 3.30

0.16 0.03 0.25 0.03 0.15 1.12 0.93 0.47 1.12

difference ∆χmin is determined by the ambient concentration χ and the percentage precision f (∆χmin), individually determined using precision data. Multiplication of Vtr with a typical (as, e.g., measured during SALSA) ambient concentration of compound χ and its precision (f (∆χmin)/100) results in individual minimum detectable surface-atmosphere exchange fluxes and are shown in Table 2 for u* ) 0.2 m s-1. The use of the Fχ,min (Table 2) concept has not been used for similar gradient studies, but it highlights the importance of the field precision, particularly when kH and χ are both high, in order to obtain statistically significant fluxes (see the Supporting Information). 3.9. (VIII) Applicability for the Measurement of SurfaceAtmosphere Exchange Fluxes. Results from SALSA for NH3 and HNO3 are shown in Figure 3a (z1 ) 0.5 m, z2 ) 2.0 m), and from GRAFE in Figure 3b (z1 ) 0.15 m, z2 ) 2.03 m) for HNO3 and aerosol NO3-. Bars above Figure 3b represent the fertilized field fetch (FFF) and the gaps represent unfertilized control fetch. Error bars indicate the flux accuracy estimated by applying a Gaussian error propagation method (30, 33). During SALSA, NH3 and HNO3 featured a net deposition flux during daytime (Figure 6a, September 12 and 16, 2005) when westerly flows prevailed. On September 13, northern and northeastern flows were predominant, and caused disturbed profiles and very small gradients of both species. Ammonia fluxes showed a net emission peak during the morning of September 13 (Figure 6a). Particle fluxes typically depend on particle size, composition, surface morphology, and turbulence; during the SALSA field campaign, the combination of these factors resulted in the aerosol concentration gradient at the two heights being always below the instrument precision and therefore aerosol fluxes below the FDL (Table 2). This was not the case at the GRAFE site, where aerosol Fχ > Fχ,min for 32-58% of the time (gaseous species were resolvable between 48-97% of the time). Combined with diurnal observations of HNO3 concentration maxima and deposition peaks between 15:00 and 17:00, increasing aerosol NO3- emission

fluxes with increasing HNO3 deposition fluxes above the fertilized plot under high NH3 emission (not shown) were apparent. These observations can be explained by shifts in the NH3-HNO3-NH4NO3 equilibrium and will be discussed in a separate paper. These results of I-VIII demonstrate that GRAEGOR is capable of measuring concentrations with sufficient precision to resolve vertical gradients of water-soluble reactive compounds selectively for gas and aerosol phase at the hourly time-scale; thus allowing for the determination of surface-atmosphere exchange fluxes. The high sensitivity offered by the GRAEGOR (particularly the IC unit) not only allows for the resolution of small concentration gradients, but also demands careful field handling of DDI to avoid contamination errors. Further improvements in the FDLs are possible through advanced processing techniques such as (a) identification of potential small biases between channels through periodic side-byside operation of sampling boxes, (b) quantification of instrument background levels due to periodic switching off of the air pump, and (c) block averaging of one-hour fluxes.

Acknowledgments We gratefully acknowledge financial support by UK Department for Environment, Food and Rural Affairs through projects AQ02505 (Acid Deposition Processes) and AQ02521 (Operation and Management of the EMEP Supersite at Auchencorth Moss); the German Science Foundation (DFG contract ME2100/1-1 SALSA); the Max Planck Society; and the Dutch Ministry of Environmental Affairs. We are thankful for logistic support by the German MeteorologicalService(MeteorologicalObservatoryHohenpeibenβerg). We are indebted to Peter Graf for allowing measurements on his grassland and to Veronika Wolff for her help during the field experiment. We express gratitude to the University of Edinburgh for the use of their grassland and Lawrence Hodgson-Jones for facilitating the GRAFE measurements.

Supporting Information Available Additional information. This material is available free of charge via the Internet at http://pubs.acs.org.

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