Factors Controlling the Phase Partitioning ofFactors Controlling the Phase Partitioning of Gaseous and Particulate NHGaseous and Particulate NH33/NH/NH44

++ at the at the mid-Atlantic U.S. Coast: Preliminary Resultsmid-Atlantic U.S. Coast: Preliminary Results

K.M. Russell, J.R. Maben,

W.C. Keene, and J.N. Galloway

University of VirginiaDepartment of Environmental Sciences

AbstractDry deposition of atmospheric NH3(g) and particulate NH4

+ are potentially important but poorly constrained sources of nutrient N for coastal ecosystems in the eastern U.S. Obtaining reliable multiphase data is essential for understanding phase partitioning and removal processes in complex coastal systems. We quantified ambient concentrations of NH3(g) over 2-hour intervals using mist chambers deployed at the mid-Atlantic coast (Lewes, Delaware) from July 31-August 13, 2000. Particulate NH4

+ was sampled in parallel over 12-hour (day/night) intervals with cascade impactors. Results will be interpreted with meteorological data in order to determine the major factors controlling phase partitioning (including relative humidity, aerosol liquid water, and transport). Ultimately, deposition models will be used to assess the importance of NH3(g) and particulate NH4

+ in regional nitrogen budgets.

Importance of Atmospheric NHx in mid-Atlantic Coastal Systems

The study of atmospheric NHx at the mid-Atlantic coast is important for several reasons including:

1. agricultural emissions of NH3(g) are substantial in the region2. long-distance transport of NHx from far away sources is also possible3. partitioning between gas and particle phases poorly understood 4. NHx can serve as a “new” nitrogen source for ecosystemproduction when deposited, potentially contributing to eutrophication

ObjectiveThe objective of this study is to examine the factors controlling the phase partitioning of gaseous and particulate NH3/NH4

+ in the atmosphere at the mid-Atlantic coast and assess consequences for deposition.

MethodsSize-segregated and bulk aerosol samples were collected in parallel from the top of a 10 m sampling tower at Lewes, Delaware, USA during a 2 week intensive July 31-August 13, 2000 (Figure 1). A six-stage Sierra Cascade Impactor (geometric mean particle radii of 5.40, 2.30, 1.10, 0.60, 0.35, and 0.19 m) was used for the sized collection. Aerosol samples were changed at sunrise and sunset. Twenty-four samples (12 daytime, 11 nighttime, and one 24-hour) plus six blanks were collected in total. After collection, the substrates were cut exactly in half and frozen at 0°C in HDPE extraction tubes (placed inside mason jars to prevent gas exchange). The samples were later extracted in 40 mLs deionized H2O and analyzed for a suite of analytes, including NH4

+. Gaseous NH3 was concurrently collected over 2-hour intervals for 16 of the 24 aerosol samples using a mist chamber sampling system. NH3(g) was collected as NH4

+ in deionized H2O. The NH4

+ was immediately analyzed by ion chromatography. Concentrations collected in multiple 2-hour intervals during a single aerosol sample were averaged to provide a representative NH3(g) concentration for each aerosol NH4

+ concentration.

Figure 1: Atmospheric Sampling Tower at Lewes, Delaware

Atmospheric NH3(g) Concentrations•Over the course of the intensive, 2-hour averages ranged from 0.5 nmol/m3 to 262 nmol/m3

•A distinct diurnal cycle was observed with highest concentrations in the late morning (Figure 2). This trend was observed only with westerly winds, however. Lefer et al., 1999 found similar results at a rural continental site with certain wind directions.

Figure 2: Two-hour Average Concentrations of NH3(g) for a 2.5 day continuous sampling period (8/9/00-8/11/00). During this period, westerly winds were prevalent.

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•During the intensive, bulk aerosol NH4+ concentrations ranged from about 3

nmol/m3 to 397 nmol/m3

•Lower concentrations (avg ~14 nmol/m3) were present at the beginning of the study due to easterly flow over the ocean (Figure 3a)•Higher concentrations were present later on in the study (avg ~180 nmol/m3) as flow shifted to westerly (Figure 3b)• There was no evidence of a consistent diurnal cycle for bulk aerosol NH4

+ (Figure 4)

Figure 3: (a) Typical flow path at the beginning of the study (7/31/00-8/3/00). (b) Typical flow path later on in the study when flow changed to westerly.

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Figure 4: Bulk Aerosol NH4+ Concentrations over the Course of the Study

NH3/NH4+ Phase Partitioning

•For days with low NHx concentrations(beginning of the study), NHx is dominated by NH3 (Figure 5)•For days with higher NHx concentrations (later on in the study), NHx is dominated by aerosol NH4

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•Days with higher humidity corresponded with lower NH3(g) concentrations (Figure 6). This is expected because under humid conditions, NH3(g) would condense onto particles more readily.

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Figure 5: Paired NH3(g) and Bulk NH4+ Concentrations for the Same Samples

Figure 6: NH3(g) Concentration as a Function of Relative HumidityThe point in red was left out of the linear regression.

y = -4.4014x + 468.75

R2 = 0.4764

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•Most of the aerosol NH4+ is associated with the fine aerosol fraction (Figure

7). This is expected as atmospheric NH4+ readily reacts with acidic submicron

sulfur aerosols.•The sum of the concentrations in the sized aerosols is in most cases less than the bulk aerosol (Figure 8) indicating losses upon collection. This trend will be re-examined after loss corrections are made using a conservative species such as Na+.

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Figure 8: Comparison of NH4+ Concentrations for Bulk and Sized

Aerosol Fractions for the Same Samples

SummaryIn this study, a preliminary investigation into the factors controlling the phase partitioning of gaseous and particulate NH3/NH4

+ in the atmosphere at the mid-Atlantic coast was performed. The following observations were made:

•A distinct diurnal cycle in NH3(g) concentrations was present with highest concentrations during late morning.

•There was no evidence of a diurnal cycle in aerosol NH4+.

•For days with low NHx concentrations, NHx was dominated by NH3; for days with high NHx concentrations, NHx was dominated by aerosol NH4

+

•Days with higher humidity corresponded with lower NH3(g) concentrations

•Most of the aerosol NH4+ is associated with the fine aerosol fraction

•The sum of the concentrations in the sized aerosols is in most cases less than the bulk aerosol indicating losses upon impaction

Future WorkThe results presented here are part of a larger work in progress. We plan to examine NH3/NH4

+ phase partitioning as a function of pH and aerosol liquid H2O content according to Keene and Savoie (1998) and assess their implications for deposition.

In addition to NHx information, we have also collected a suite of other data including the gas concentrations of HNO3 and SO2 and aerosol concentrations of NO3

-, NO2-, total nitrogen, total water

soluble nitrogen, urea, oxalate, formate, acetate, SO42-, Cl-, Br-,

Ca2+, Mg2+, Na+, and K+. Phase partitioning calculations will be performed for most of these species. Deposition models (e.g. Williams, 1982; Valigura, 1995) will be applied to these data and fluxes to coastal waters will be calculated. The information gained from this larger study will provide a more comprehensive understanding of the speciation and concentration of atmospheric nitrogen species and will enable scientists to more accurately assess ecological implications for deposition to coastal systems.

AcknowledgementsWe wish to thank the USEPA Chesapeake Bay Foundation and the NSF Long-Term Ecological Research Program for providing funding for this project. Tom Church and Joe Scudlark at the University of Delaware were gracious and accomodating hosts for the experiment and Tom Snowden at the University of Miami generously allowed us to borrow a foldover tower. Ishi Buffam, Bonnie Chang, Suzanne Maben, Tracie Mastronicola, and Christy Tyler provided much needed assistance in the field and laboratory.

ReferencesKeene, W.C. and D.L. Savoie. The pH of deliquesced sea-salt aerosol in polluted marine air. Geophysical Research Letters 25 (12): 2181-2184. 1998.

Williams, R.M. A Model for the Dry Deposition of Particles to Natural Water Surfaces. Atmospheric Environment 16 (8): 1933-1938. 1982.

Valigura, R.A. Iterative bulk exchange model for estimating air-water transfer of HNO3. Journal of Geophysical Research 100 (D12): 26,045-26,050. 1995.