Measuring Concentrations of Atmospheric Trace gases and aerosols
The CEH DELTA (DEnuder for Long-Term Atmospheric sampling) system is a low-cost diffusion denuder system that was originally developed for long-term sampling of ammonia and ammonium (Sutton et al. 2001a), and which has also been tested for long-term sampling of acid gases (HNO3, HONO, HCl, SO2) and aerosols (NO3- NO2-, Cl-, SO42-) (Tang et al. 2008). The system is based around the concept of a single bore glass denuder for sampling trace gases (Ferm 1979). When a laminar air stream passes through the denuder coated on the inside with an acid coating such as citric acid, ammonia is captured by the acid walls (to be later extracted in the laboratory), while aerosols pass through and can be collected by aerosol filters placed downstream of the denuder. Conversely, an alkaline coating (e.g. K2CO3) on the denuders will collect acid gases such as HNO3, HONO, SO2 and HCl. The separation of aerosol from gaseous components is achieved due to the much more rapid diffusion of gaseous species to the tube wall compared with that of particles.
The DELTA system is tuned for monthly sampling, but shorter and longer periods are also possible, depending on ambient concentrations (e.g. 1 week - 3 months). Stable sampling rates of 0.3-0.4 l min-1 are achieved using a piston air pump (or other types of air pump in conjunction with a rotameter), with air volumes being measured by a high sensitivity dry gas meter. The low sampling rate means that short glass denuders (10 - 15 cm in length, optimum length to achiever greater than 99 % capture for the gas of interest) can be used, which allows easy exchange of samples through the post. A simple enclosure is used, which can be mounted easily in the desired location. At some sites, where the interest is in sampling of NH3 / NH4+ only, monitoring may be made via a sampling line in a protected cabin. In this case, the same enclosure is used, but mounted on a desk or wall rather than mounted outside on a pole. When sampling via an inlet tube, the inlet tube should be made of polyethylene (0.5 mm id) and preferably < 2 m long. For sampling HNO3 gas, since it is a sticky gas, sampling must be direct from the atmosphere or loss of HNO3 will incur to inlet lines. The design of the monitoring equipment is shown in Figure 1.
To sample NH3 gas, the glass denuder tubes are coated internally with citric acid, which gives a visible layer of coating on the tubes, allowing a visual QA check on prepared denuder. The post-denuder filters for the analysis of NH4+ aerosol are coated also with citric acid. To establish capture efficiency, the DELTA system uses 2 denuders for every sample. Two borosilicate glass denuders (10 cm long, 10 mm O.D, 6 mm I.D) are connected in series as shown in Figure 2. To enable a fully developed laminar flow, a short 2.3 cm glass tube (not coated) is connected to the front end. The different tube sections are joined by silicone tubing, and provided with two polyurethane caps for sealing before and after sampling. The two parts may be folded over, utilizing a silicone junction, during posting.
Figure 1: Mounting of the DELTA sampling enclosure (left), sampling train for NH3/NH4 (right)
Figure 2: Full sampling train for all species (bottom)
The sampling train used for sampling acid gas and aerosols, plus NH3/NH4+ is also shown in Figure 2. HNO3, SO2 and HCl are removed by the first set of K2CO3 / glycerol coated denuders, and a second set of citric acid coated denuders removes NH3. Two sets of coated filter papers in a 2-stage filter pack at the end of the sampling train collects aerosol components. The first filter is impregnated with K2CO3 / glycerol to sample aerosol NO3-, SO42-, Cl- plus the base cations Na+, Ca2+ and Mg2+, followed by a citric acid coated paper to sample aerosol NH4+ . The use of 2 denuders in series allows for the detection of any breakthrough from the first denuder; the data from the second denuder are used for quality control and quality assurance.
References
Ferm, M., 1979, Method for determination of atmospheric ammonia. Atmos. Environ., 13, 1385-1393
Sutton M.A., Tang Y.S., Miners B. and D. Fowler (2001a) A new diffusion denuder system for long term, regional monitoring of 5atmospheric ammonia and ammonium. Water, Air and Soil Pollution. Focus, 1, 5/6, 145 156.
Tang Y.S, N. van Dijk, M. Anderson, I. Simmons, R. I. Smith, E. Armas-Sanchez, H. Lawrence & M.A. Sutton. (2008) Monitoring of nitric acid, particulate nitrate and other species in the UK - 2007. Interim report under the UK Acid Deposition Monitoring Network to NETCEN/DEFRA.
Ammonia sampling system (Diffusion Tubes and ALPHA samplers)
In the first phase of the NAMN, the performance of 35 mm path length membrane diffusion tube (Figure 3) was improved to permit reliable monthly measurements down to a level of c. 1 µg m-3 (Sutton et al. 2001b). In the second phase of the network, an improved high sensitivity passive method, the CEH ALPHA (Adapted Low-cost Passive High Absorption) sampler (Figure 3) was applied in the network (Tang et al. 2001). Compared with the diffusion tube sampler, the ALPHA method shows an improvement of precision and detection limits by a factor of ~ 20, with a detection limit of c. 0.02 µg m-3 for monthly sampling. In May 2000, the diffusion tube method was replaced by the new improved ALPHA sampler across the NAMN. At 13 sites across the UK NAMN, both ALPHA and active denuder sampling measurements are made in parallel to provide an ongoing calibration and assessment of the passive method against the reference DELTA method.
| DIFFUSION TUBE | |
 |
|
| ALPHA SAMPLER | |
 |
Figure 3: Diagram of a single membrane diffusion tube (top) and a single ALPHA sampler (bottom).
The passive sampling system consists of three replicate ALPHA samplers attached by the use of velcro to an aerodynamically shaped support (upturned frisbee or plant saucer) on a pole or post at about 1.5 m height above ground. Aluminium sheet cut into strips are mounted on top to deter birds from perching. Replicate tubes are used in order to give a more reliable estimation of the air concentration of ammonia.
Paper: Sutton et al 2001 (pdf)
References:
Sutton M.A., Miners B., Tang Y.S., Milford C., Wyers G.P., Duyzer J.H. and Fowler D. (2001b) Comparison of low-cost measurement techniques for long-term monitoring of atmospheric ammonia. J. Env. Monit., 3, 446-453.
Tang, Y.S., Cape, J.N. and Sutton, M.A. (2001b). Development and types of passive samplers for NH3 and NOx. In Proceedings of the International Symposium on Passive Sampling of Gaseous Pollutants in Ecological Research. TheScientificWorld, 1, 513-529.