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Harvard Forest Data Archive

HF066

Concentrations and Surface Exchange of Air Pollutants at Harvard Forest EMS Tower since 1990

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Data

Overview

  • Lead: William Munger, Steven Wofsy
  • Investigators: P.S. Bakwin, Allen Goldstein, A.I. Hirsch, Cassandra Horri, Ben Lee
  • Contact: J. William Munger
  • Start date: 1990
  • End date: 2015
  • Status: ongoing
  • Location: Prospect Hill Tract (Harvard Forest)
  • Latitude: +42.537755
  • Longitude: -72.171478
  • Elevation: 340 meter
  • Taxa:
  • Release date: 2018
  • Revisions:
  • EML file: knb-lter-hfr.66.24
  • DOI: digital object identifier
  • Related links:
  • Study type: long-term measurement
  • Research topic: forest-atmosphere exchange
  • LTER core area: inorganic nutrients
  • Keywords: air pollution, atmospheric deposition, carbon monoxide, hydrocarbons, nitrogen compounds, ozone
  • Abstract:

    In North America, anthropogenic activities such as fossil fuel combustion and high-intensity agriculture have increased the inputs of nitrogen oxides in the atmosphere far above natural, biogenic inputs. The effect of this excess N depends on how it is distributed through the environment. If fixed N is deposited as nitrate in forests, it may act as a "fertilizer", stimulating growth and thus enhancing carbon sequestration. But when accumulated deposition exceeds the nutritional needs of the ecosystem, nitrogen saturation may result. Soil fertility declines due to leaching of cations and thus, carbon uptake diminishes. The balance between fertilization and saturation depends on the spatial and temporal extent of nitrogen deposition.

    Measurements of nitrogen oxide concentrations and fluxes made at Harvard Forest are intended to quantify the deposition of nitrogen oxides and to examine the rates for oxidation and deposition of reactive nitrogen that are critical in controlling how far the influence of nitrogen oxide emission sources extends. Measurements made to date indicate that dry deposition of NOy to the Harvard Forest canopy is controlled by advection from source regions, vertical mixing, and chemical reaction. The input is about equally divided between wet and dry deposition depending on the amount of precipitation. Southwesterly winds bring air from the major urban areas along the mid-Atlantic coast, whereas northwesterly wind bring air from less populated regions of northern New England and Canada. As a result, southwesterly winds transport higher concentrations and fluxes of NOx and NOy than northwesterly winds. In the summer, aerodynamically rough forests intercept NOx and emit reactive hydrocarbons that accelerate the oxidation of NOx to rapidly depositing species. As a result, much of the NOx emitted by North America is retained by the region in the summer. This deposition leads to a summertime decrease in reactive nitrogen concentrations and fluxes relative to spring levels.

    In addition to their role as a plant nutrient, nitrogen oxides are a major precursor for photochemical production of tropospheric ozone, a pollutant and greenhouse gas. Measurements at Harvard Forest are used to examine the interannual variability and trends in ozone production and background ozone concentrations.

    The family of nitrogen oxide species is partitioned between active radicals (NOx, NO3), reservoir species (e.g., peroxyacetylnitrate PAN) which can convert back into NO2 and terminal species (HNO3, organic nitrates), which no longer contribute to photochemistry and are efficiently deposited. At low wintertime temperatures, PAN is stable and can be transported to the upper troposphere and remote regions. In the summer, however, the lifetime of PAN is short (few hours) so concentrations may remain low despite abundant photochemical radicals that promote PAN formation. Thus, temperature directly affects the partitioning of nitrogen oxides, which will in turn affect deposition.

    Further measurements resolving key species are needed to distinguish the contributions due to direct NO2 deposition, HNO3 deposition and organic nitrate deposition. A dual Tunable Diode Laser Absorption Spectrometer (TDLAS) for eddy covariance flux measurements of NO2 and concentrations of HNO3 and NO2 has been operational since 1999 and a new CG/ECD for continuous measurement of PAN was installed in the spring of 2000. The combination of HNO3 and NO2 concentrations with existing measurements of O3, NOy, NO, PAN, hydrocarbons, tracers of anthropogenic emissions, and meteorological parameters at the site, provide important new data on the speciation and removal mechanisms for reactive nitrogen in the troposphere and subsequently the photochemistry of ozone in both urban and rural air masses. Simultaneous NOy, NOx, PAN and CO data will allow us to distinguish PAN deposition (loss of NOy) from PAN decomposition (leads to NOx increase, no change in NOy). Because seasonal cycles of PAN loss and formation remain a major uncertainty in understanding atmospheric transport and N deposition, we plan to continue measurements of NOy speciation over several seasonal cycles and climactic variation. The addition of PAN and HNO3 measurements provides a comprehensive analysis of the reactive nitrogen at this site, allowing us to examine the diel and seasonal trends in concentrations to determine their production, deposition, and loss rates.

    CO2 concentration profiles are measured by an infra-red gas analyzer (Licor 6251). Inlet sample flow is maintained at a constant pressure by a pressure control valve upstream of a bypass pump that generates high flow volume to minimize residence time in the inlet line. Sample air for the CO2 analyzer is drawn off and passed through diffusion dryer to remove water vapor down to a low and constant value before passing through the analyzer. Pressure control valve on the instrument exhaust maintains constant pressure in the detector cell. The reference cell of the analyzer is purged with a small flow of calibration gas having a concentration comparable to average ambient concentration. At least three times a day the analyzer is calibrated automatically by replacing sample air with the reference gas to determine the zero offset and with a set of three calibration standards that span the expected range of ambient concentration. A second-order polynomial is fit to the instrument response to standards and used to compute mixing ratio in the samples.

  • Methods:

    Total nitrogen oxides (NOy) is determined by catalytic conversion on a hot gold catalyst and detection using a fast response chemiluminescence analyzer. The NOy catalyst is mounted at 29 m. The analyzer is calibrated by addition of NO and NO2 standards at the catalyst inlet.

    NO and NO2 are determined using a chemiluminescence analyzer with photolytic conversion of NO2 to NO. The analyzer seqeuentially samples from 8 inlet heights to define a concentration profile. Instrument response and photolysis efficiency are determined by addition of calibration standards to the inlet line.

    O3 concentration profiles are measured by a UV-absorbance instrument. O3 fluxes and continuous concentrations at 29 m are determined using a fast-response C2H4-chemiluminescence analyzer.

    CO at 29 m is determined using a modified IR-absorbtion gas-filter correlation analyzer. The sample stream is dried to a dewpoint of less than -20C in a cold trap to eliminate interference by water vapor. The instrument zero is determined by removal of CO in the ambient air with a catalyst. The analyzer is calibrated using a 500 ppb absolute standard.

    C2-C6 hydrocarbons are determined by cryogenic trapping over liquid nitrogen, separation by gas chromatography and detection by FID. Concentrations are computed by ratioing to a neohexane internal standard.

    Peroxyacetylnitrate is determined using a gas chromatograph with electron capture detector.

    NO2 and HNO3 were determined using a two-channel tuneable diode laser absorption spectrometer mounted at the top of a canopy access scaffold tower. NO2 signal was combined with vertical wind velocities to compute an eddy covariance flux of NO2.

  • Use:

    This dataset is released to the public under Creative Commons license CC BY (Attribution). Please keep the designated contact person informed of any plans to use the dataset. Consultation or collaboration with the original investigators is strongly encouraged. Publications and data products that make use of the dataset must include proper acknowledgement.

  • Citation:

    Munger W, Wofsy S. 2018. Concentrations and Surface Exchange of Air Pollutants at Harvard Forest EMS Tower since 1990. Harvard Forest Data Archive: HF066.

Detailed Metadata

hf066-01: CO data

  1. datetime: date and time
  2. seq.day: sequential days beginning on 1/1/1990 (unit: nominalDay )
  3. date: date
  4. year: year
  5. month: month
  6. day: day of month
  7. hour: hour at beginning of interval
  8. co.ppb: CO (ppb), hourly average commencing on the hour (unit: dimensionless / missing value: NA)
  9. sco.ppb: sigma CO (ppb) (unit: dimensionless / missing value: NA)
  10. gc.flag: flag for filled data
    • 1: data has been filled with concentrations measured by FACTS GC
    • 0: data has not been filled

hf066-02: NOy data

  1. datetime: date and time
  2. seq.day: sequential days beginning on 1/1/1990 (EST=UTC-5) (unit: nominalDay )
  3. date: date
  4. year: year
  5. month: month
  6. day: day of month
  7. hour: hour at beginning of interval (EST=UTC-5)
  8. noy.ppt: NOy (ppt) (unit: dimensionless / missing value: NA)
  9. fnoy: NOy flux (unit: micromolePerMeterSquaredPerHour / missing value: NA)
  10. sigmanoy: sigma NOy (unit: dimensionless / missing value: NA)

hf066-03: PAN data

  1. datetime: date and time
  2. year: year
  3. doy: day of year (unit: nominalDay )
  4. hour: hour
  5. seq.day: sequential date (unit: nominalDay )
  6. pan.ppt: peroxyacetylnitrate (PAN) (ppt) (unit: dimensionless / missing value: NA)
  7. sd.pan: standard deviation PAN (ppt) (unit: dimensionless / missing value: NA)
  8. n.obs: number of observations (unit: number / missing value: NA)

hf066-04: NO profile data

  1. datetime: date and time
  2. year: year
  3. doy: decimal day of year (unit: nominalDay / missing value: NA)
  4. no.29m: NO concentration at 29m (ppt) (unit: dimensionless / missing value: NA)
  5. no.24.1m: NO concentration at 24.1m (ppt) (unit: dimensionless / missing value: NA)
  6. no.18.3m: NO concentration at 18.3m (ppt) (unit: dimensionless / missing value: NA)
  7. no.12.7m: NO concentration at 12.7m (ppt) (unit: dimensionless / missing value: NA)
  8. no.7.5m: NO concentration at 7.5m (ppt) (unit: dimensionless / missing value: NA)
  9. no.4.5m: NO concentration at 4.5m (ppt) (unit: dimensionless / missing value: NA)
  10. no.0.8m: NO concentration at 0.8m (ppt) (unit: dimensionless / missing value: NA)
  11. no.0.3m: NO concentration at 0.3m (ppt) (unit: dimensionless / missing value: NA)

hf066-05: NO2 profile data

  1. datetime: date and time
  2. year: year
  3. doy: decimal day of year (unit: nominalDay / missing value: NA)
  4. no2.29m: NO2 concentration at 29m (ppt) (unit: dimensionless / missing value: NA)
  5. no2.24.1m: NO2 concentration at 24.1m (ppt) (unit: dimensionless / missing value: NA)
  6. no2.18.3m: NO2 concentration at 18.3m (ppt) (unit: dimensionless / missing value: NA)
  7. no2.12.7m: NO2 concentration at 12.7m (ppt) (unit: dimensionless / missing value: NA)
  8. no2.7.5m: NO2 concentration at 7.5m (ppt) (unit: dimensionless / missing value: NA)
  9. no2.4.5m: NO2 concentration at 4.5m (ppt) (unit: dimensionless / missing value: NA)
  10. no2.0.8m: NO2 concentration at 0.8m (ppt) (unit: dimensionless / missing value: NA)
  11. no2.0.3m: NO2 concentration at 0.3m (ppt) (unit: dimensionless / missing value: NA)

hf066-06: O3 data

  1. datetime: date and time
  2. seq.day: sequential days beginning on 1/1/1990 (unit: nominalDay )
  3. date: date
  4. year: year
  5. month: month
  6. day: day of month
  7. hour: hour at beginning of interval
  8. o3: O3 (ppb) hourly averages from the continuous O3 analyzer commencing on the hour (unit: dimensionless / missing value: NA)

hf066-07: O3 profile data

  1. datetime: date and time
  2. year: year
  3. seq.day: start time for observation in sequential fractional days starting at January, 1 1990 (unit: nominalDay / missing value: NA)
  4. doy: start time for observation in fractional day of current year (unit: nominalDay / missing value: NA)
  5. o3.29m: ozone concentration (ppb) at 29m above ground (unit: nmole-O3 per mole-air) (unit: dimensionless / missing value: NA)
  6. o3.24.1m: ozone concentration (ppb) at 24.1 m above ground (unit: nmole-O3 per mole-air) (unit: dimensionless / missing value: NA)
  7. o3.18.3m: ozone concentration (ppb) at 18.3 m above ground (unit: nmole-O3 per mole-air) (unit: dimensionless / missing value: NA)
  8. o3.12.7m: ozone concentration (ppb) at 12.7 m above ground (unit: nmole-O3 per mole-air) (unit: dimensionless / missing value: NA)
  9. o3.7.5m: ozone concentration (ppb) at 7.5 m above ground (unit: nmole-O3 per mole-air) (unit: dimensionless / missing value: NA)
  10. o3.4.5m: ozone concentration (ppb) at 4.5 m above ground (unit: nmole-O3 per mole-air) (unit: dimensionless / missing value: NA)
  11. o3.0.8m: ozone concentration (ppb) at 0.8 m above ground (unit: nmole-O3 per mole-air) (unit: dimensionless / missing value: NA)
  12. o3.0.3m: ozone concentration (ppb) at 0.3 m above ground (unit: nmole-O3 per mole-air) (unit: dimensionless / missing value: NA)

hf066-08: CO2 profile data

  1. datetime: date and time (EST)
  2. seq.day: sequential days beginning on 1/1/1990 (EST) (unit: nominalDay )
  3. doy: decimal day of year (EST) (unit: nominalDay / missing value: NA)
  4. co2.29m: CO2 concentration at 29m (ppm) (unit: dimensionless / missing value: NA)
  5. co2.24.1m: CO2 concentration at 24.1m (ppm) (unit: dimensionless / missing value: NA)
  6. co2.18.3m: CO2 concentration at 18.3m (ppm) (unit: dimensionless / missing value: NA)
  7. co2.12.7m: CO2 concentration at 12.7m (ppm) (unit: dimensionless / missing value: NA)
  8. co2.7.5m: CO2 concentration at 7.5m (ppm) (unit: dimensionless / missing value: NA)
  9. co2.4.5m: CO2 concentration at 4.5m (ppm) (unit: dimensionless / missing value: NA)
  10. co2.0.8m: CO2 concentration at 0.8m (ppm) (unit: dimensionless / missing value: NA)
  11. co2.0.3m: CO2 concentration at 0.3m (ppm) (unit: dimensionless / missing value: NA)