The influence of gas-particle partitioning and surface-atmosphere exchange on ammonia during BAQS-Met

Abstract

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 (PM 2.5 ), 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 PM 2.5 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 NH 3 , diurnal profiles and gas-particle partitioning. The measurements were compared with results from A Unified Regional Air-quality Modelling System (AU-RAMS). While the fraction of total ammonia (NH x ≡NH 3 + NH + 4 ) 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 NH x 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.

Role of ambient ammonia in particulate ammonium formation at a rural site in the North China Plain

“…The morning peak of NH 3 was also observed elsewhere and could have resulted from emissions from fertilised soils and plant stomata, evaporation of dew, human sources, and mixing down of ammonia from the residual layer (Trebs et al, 2004;Norman et al, 2009;Bash et al, 2010;Ellis et al, 2011). Figure 3b reveals that the relative humidity (90-89 %) and temperature (21.5-22.1 • C) remained relatively constant before 06:00 but increased later in the morning.…”

“…During the sampling period, the majority of peaks of ammonia over 50 ppb occurred at night, which were attributed to local emissions such as from agri- cultural activity, into a shallow nocturnal boundary layer. It was supposed by Ellis et al (2011) that the downward mixing of air containing higher NH 3 from the residual layer could lead to an increase in surface NH 3 after the break-up of the nocturnal boundary layer.…”

“…Figure 3b reveals that the relative humidity (90-89 %) and temperature (21.5-22.1 • C) remained relatively constant before 06:00 but increased later in the morning. The increasing temperature can heat the Earth's surface and vegetation leaves and reduce the RH, potentially leading to evaporation of NH 3 from soil and plants and volatilisation of ammonium aerosol (Trebs et al, 2004;Norman et al, 2009;Ellis et al, 2011), which may increase NH 3 concentrations in the morning. When the emission was occurring into a shallow boundary layer, NH 3 increase would be more prominent.…”

Nitrogen deposition to the United States: distribution, sources, and processes

“…Horii et al, 2005), and in the 0.1-1.0 cm s −1 range for PAN (Doskey et al, 2004;Turnipseed et al 2006;Wolfe et al, 2009). Biosphere-atmosphere exchange of NO x and NH 3 is bi-directional Lerdau et al, 2000;Ellis et al, 2011), but is treated here as uncoupled emission and deposition processes. The fluxes to the surface are parameterized as uni-directional dry deposition, and the fluxes back to the atmosphere are included as part of natural emissions.…”

Open-path, quantum cascade-laser-based sensor for high-resolution atmospheric ammonia measurements

“…Conventional offline NH 3 sensors (passive filters, denuders and ion chromatography) often have sufficient sensitivity (< 1 ppbv NH 3 ), but have long integration times, lack the requisite selectivity and are labor intensive to analyze (Fehsenfeld et al, 2002;von Bobrutzki et al, 2010;Ellis et al, 2011). Chemical ionization mass spectrometry (CIMS) NH 3 instruments have high time resolution (seconds), selectivity and extremely high sensitivity (< 100 pptv NH 3 ), but require large power consumption (kW) and space (multiple instrument racks) (Nowak et al, 2007;Benson et al, 2010;Sintermann et al, 2011).…”