CASTNET Ammonia Special Study (ACCS)
Update: March 2013
The ACCS began in August 2010 and the last sample was collected in October 2011. Final data, including ambient NH3 concentrations and laboratory and field blanks,
have been submitted to EPA. Preliminary results from the AMoN Radiello samplers versus duplicate annular denuders (reference method) from the five sites are described below.
The mean absolute relative percent difference (MARPD) between the passive samplers and the annular denuders from the five sites was 20%. MARPDs between individual
sites ranged from 11% (Palo Duro, TX) to 46% (Connecticut Hill, NY). The left figure below shows the bi-weekly NH3 concentrations as measured by AMoN and the average
concentrations over the 2 one-week denuder sampling periods for each site. Concentrations ranged from less than 0.5 ug/m3 (Connecticut Hill, NY and Rocky Mountain National
Park, CO) to 4.5 ug/m3 (Palo Duro, TX). The figure to the right shows the 2-week average NH3 concentrations from AMoN versus the annular denuders for the 5
ACCS sites. There is good correlation (0.96) between the two sampling methods for the range of concentrations measured during the year sampling period.
The precision of the duplicate denuder systems at the five sites was 10%. Triplicate Radiello passive samplers were deployed at the five AMoN sites during the ten ACCS
sampling periods. The precision of the triplicate samplers was 12% and comparable to previous quality control studies performed by the NADP.
Update: April 2012
Preliminary results from the ACCS were presented at the NADP Total Deposition Science Committee Meeting in Providence, RI (Oct. 2011).
The preliminary results showed AMoN samplers compared well with the URG annular denuders, while the Super SASS mini-parallel plate denuders were likely biased low at
CHE185 and ROM206.
In addition to the ammonia analysis, ion concentrations measured with the CASTNET filter pack were compared with those measured with the Super SASS ion canisters (routinely used in the
Chemical Speciation Network). Preliminary results showed good agreement for gasesous SO2 between the filter pack and the ion canisters, but comparison for the particulates (NO3-
and NH4+) showed the ion canisters was biased low when compared to the filter pack. We believe this bias may be attributable to the cut-point. The Super SASS uses a Teflon cyclone
with a 2.5μm cut-point while the filter pack is an open-faced surface, which likely allows larger particles to collect on the filters.
Final results will be published in two journal articles, one providing the results from the ammonia study and the other showing the results from the ion comparison.
The presentation with the preliminary results from both the ammonia and ion studies can be found on the NADP website here.
Update: November 2011
Ambient ammonia (NH3) concentrations are not routinely measured at CASTNET, Chemical Speciation Network (CSN)
or NCore sites, which results in a significant gap in our understanding of the total nitrogen budget. In some areas, NH3 dry deposition is estimated
to be approximately 30% of the total deposited nitrogen (eg. Rocky Mountain NP). The Ammonia Monitoring Network (AMoN),
a newly formed sub-network within the NADP, measures ambient NH3 concentrations at approximately 50 sites throughout the US.
This study was developed to validate the passive Radiello samplers deployed in the AMoN, to characterize the sulfur and nitrogen species collected on the CASTNET filter pack
and the ion canisters from the Super SASS (used in the CSN), and evaluate a newly developed mini-parallel plate denuder (MPPD) for measuring NH3 in one of the channels of the Super SASS. To accomplish these goals, a five site study (site locations
indicated on the map below) plan was developed. The first sampling period began in August 2010, and the study was completed in September 2011. To stay within budget, two week sample deployments occurred
every 6 weeks for a total of 10 sampling time periods. Each site was collocated with AMoN, which provided a two week NH3 concentration. All AMoN
samplers were prepared, shipped and analyzed by NADPís Central Analytical Laboratory. AMoN data are processed and posted to the NADP
AMoN website routinely. In addition to the passive samplers, each of the five sites also deployed two consecutive 1-week annular denuders systems (ADS) as the reference method.
Three sites deployed two consecutive 1-week MPPDs housed within the Super SASS module and two consecutive 1-week ion canisters. The
MPPDs are being tested for routine use in the CSN for capturing 24-hour NH3 concentrations. The dual ADS systems were analyzed for a full suite of anions and cations for comparison to the
CASTNET filter pack and Super SASS ion measurements. The ion canisters were analyzed by RTI for the non-NH3
analytes. A summary of the study sites are shown in the table below.
Preliminary results show a good comparison between the ADS and AMoN samplers. Comparisons between the MPPDs and the ADS are variable,
but this may be a result of invalid flow at the Rocky Mountain NP (ROM206/406) site.
Updates will be provided here and the final results will be published in peer-reviewed journal articles.
Funding for this study was provided by EPAís Office of Research and Development, Clean Air Markets Division and Office of Air Quality Planning and
Standards (OAQPS). The Super SASS systems were provided by OAQPS.