The influence of gas-particle partitioning and surface-atmosphere exchange on ammonia during BAQS-Met R. A. Ellis1, J. G. Murphy1, M. Z. Markovic1, T. C. VandenBoer1, P. A. Makar2, J. Brook2, and C. Mihele2 1Department of Chemistry, University of Toronto, 80 St. George St., Toronto, ON, M6P 2S1, Canada 2Environment Canada, Science and Technology Branch, Air Quality Research Division, 4905 Dufferin St., Toronto, ON, M3H 5T4, Canada
Abstract. The Border Air Quality and Meteorology study (BAQS-Met) was an intensive
field campaign conducted in Southwestern Ontario during the summer of 2007.
The focus of BAQS-Met was determining the causes of the formation of ozone
and fine particulate matter (PM2.5), and of the regional significance
of trans-boundary transport and lake breeze circulations on that formation.
Fast (1 Hz) measurements of ammonia were acquired using a Quantum Cascade
Laser Tunable Infrared Differential Absorption Spectrometer (QC-TILDAS) at
the Harrow supersite. Measurements of PM2.5 ammonium, sulfate and
nitrate were made using an Ambient Ion Monitor Ion Chromatograph (AIM-IC)
with hourly time resolution. The median mixing ratio of ammonia was 2.5 ppb,
with occasional high spikes at night resulting from local emissions.
Measurements were used to assess major local emissions of NH3, diurnal
profiles and gas-particle partitioning. The measurements were compared with
results from A Unified Regional Air-quality Modelling System (AURAMS). While
the fraction of total ammonia (NHx≡NH3 + NH4+)
observed in the gas phase peaks between 0.1 and 0.8, AURAMS tended to
predict fractions of either less than 0.05 or greater than 0.8. The model
frequently predicted acidic aerosol, in contrast with observations wherein
NHx almost always exceeded the observed equivalents of sulfate. One
explanation for our observations is that the net flux of ammonia from the
land surface to the atmosphere increases when aerosol sulfate is present,
effectively buffering the mixing ratio of gas phase ammonia, a process not
included in the model. A simple representation of an offline bi-directional
flux parameterization using the ISORROPIA thermodynamic model was successful
at reducing the population of zero gas fraction points, but not the higher
gas fraction points.
Citation: Ellis, R. A., Murphy, J. G., Markovic, M. Z., VandenBoer, T. C., Makar, P. A., Brook, J., and Mihele, C.: The influence of gas-particle partitioning and surface-atmosphere exchange on ammonia during BAQS-Met, Atmos. Chem. Phys., 11, 133-145, doi:10.5194/acp-11-133-2011, 2011.