Transactions on Ecology and the Environment vol 21, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541 Development of a new high sensitivity monitor for peroxyacetyl nitrate and results from the west-central Mediterranean region D. Zedda, G.W. Keigley, D.W. Joseph and C.W. Spicer Consorzio Ricerche Associate Battelle Columbus Abstract Peroxyacetyl Nitrate (PAN) is a photochemical product of the reaction of organic gases with nitrogen oxides in the atmosphere. It is toxic to plants and animals and it is a severe eye irritant. PAN is widely distributed in the atmosphere with mixing ratios as low as 5 ppt in remote regions and as high as 50 ppb in industrial areas. PAN is intimately involved in the tropospheric ozone cycle. Its thermal characteristics allow it to serve as a reservoir for nitrogen dioxide, storing NO] at cooler times and locations and releasing it under warmer conditions, hours or days later in a different location. This paper will report on a collaborative project to develop new tools for PAN monitoring and calibrating. We have developed a high sensitivity monitor that performs an analysis every 10 minutes. The unit is small, portable, highly sensitive and reliable, and it is meant to be used in the field. The use of a nonradioactive detector eliminates potential difficulties with compliance with radiation regulations in many countries. This paper describes the characteristics of the new instrument, and summarizes ambient monitoring data which shows the relationship between PAN and other pollutants near the coast of Sardinia in the west-central Mediterranean Sea. Transactions on Ecology and the Environment vol 21, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541 80 /*/ Introduction and Background The chemical peroxyacetyl nitrate, also known as PAN, is one of the products of photochemical reactions involving organic gases with nitrogen oxides. PAN is one member of a family of peroxyacyl nitrates formed by these atmospheric reactions. Other members of this family of compounds include, for example, peroxypropionyl nitrate (PPN) and peroxybenzoyl nitrate (PBzN). In the atmosphere, PAN is usually present at much higher concentrations than other peroxyacyl nitrates. PAN is an oxidant and can cause damage to plants and animals in the same manner as other oxidants. It is toxic to animals, and is a severe eye irritant. It is also toxic to plants and can cause severe damage to vegetation. In addition to these direct environmental effects, PAN can serve as a reservoir for nitrogen dioxide, and therefore it can contribute to increased ozone levels at downwind locations. Because the stability of the molecule is very temperature sensitive over the range of ambient air temperatures, PAN can form, and then be transported downwind for many hours when the temperature is cool (e.g. at night or at higher elevations) and then release photochemically active nitrogen dioxide at warmer temperatures (e.g. during the next day or at lower altitudes). This process can allow PAN to be a source of NO] in areas where the emission of NOx is low. It has a variable lifetime ranging from about 30 minutes at ground level in mid latitudes during summer to several months and longer at colder locations. Several studies have been performed to monitor PAN at different latitudes and altitudes in Europe, the United States, and other parts of the world (1-10). PAN is widely distributed in the atmosphere, with mixing ratios in urban areas from <1 ppb (1 ppb = 10"' v/v) to as much as 50 ppb; in rural areas from 0.1 ppb to about 2 ppb, and in very remote regions (over the oceans) from 5 ppt to 100 ppt (ppt = 10"" v/v). To date, PAN has been measured primarily for research purposes. PAN has been measured to assess exposures of plants and/or people to this toxic chemical; to determine its spatial distribution and temporal behavior; as a tracer of photochemical activity; as a surrogate for total photochemical oxidants; and to assist in formulating and evaluating atmospheric chemistry models. It is reported to be a more reliable indicator of photochemical smog than ozone (11,12) because it only originates from photochemical reactions within the troposphere. In the U.S., PAN is not regulated directly, but is discussed with ozone as part of "other photochemical oxidants". It is expected that strategies used to control ozone will also bring about reductions in ambient air concentrations of PAN. There are indications that the Photochemical Assessment Monitoring Station (PAMS) monitoring Transactions on Ecology and the Environment vol 21, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541 Air Pollution 81 network will implement PAN monitoring at some locations in the future. The European Union has set reporting requirements and data exchange protocols for member states for PAN, among other pollutants (13), but monitoring for PAN is not mandatory at this time. In Italy, the Ministero dell'Ambiente has published criteria for locating air monitoring stations in and downwind of urban areas, and has issued directives stating that authorities must monitor photochemical pollutants, including Og and NC>2, and should monitor an additional group of pollutants including PAN (14). However, PAN monitoring is not now being done on a routine basis. The Ministero dell'Ambiente plans to issue guidance on measure- ment methods for PAN at a future date. PAN measurements have been performed for many years using commercial gas chromatographs that have been modified by the user specifically for PAN analysis. One instrument maker now markets a gas chromatograph specifically for PAN. Using commercial gas chromato- graphs, PAN can be measured in urban areas during the summer without sample pretreatment. However, measurements outside of cities, or in cities in cooler weather, or at higher altitudes, require preconcentration of the air sample to achieve required detection limits. Commercial instruments do not offer this capability. In addition, the commercial chromatographic instruments employ electron capture detectors that make use of radioactive materials. Use of radioactive material is highly restricted in some countries, making instruments with this type of detector less desirable. The program described here is developing and evaluating an improved measurement tool for PAN that will be suitable for commercialization and field use. Design and Characterization of the Instrument The instrument consists of a small portable gas chromatograph, a thermoelectric module for sample preconcentration, a 6-port electrically activated gas sampling valve, a small pump and mass flow controller, and a pulsed discharge electron capture detector (PD-ECD). The column is a 10 m x 0.53 mm fused silica trifluoropropylmethyl polysiloxane crossbond column (Rtx-200 from Restek Corp.). The thermoelectric module is made up of an array of semiconductor couples connected electrically in series and thermally in parallel (15). With a DC potential applied, heat is absorbed at one end of the device, cooling it, while heat is rejected at the other end. The module is built to hold a removable trap containing an absorbent material that collects PAN and PPN at reduced temperature and releases them when Transactions on Ecology and the Environment vol 21, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541 82 Air Pollution the temperature is increased. In practice, we concentrate the sample at -38°C and desorb the sample at 60°C. The pulsed discharge electron capture detector is a non radioactive detector that uses a discharge in helium to produce electrons. The use of this detector eliminates the safety and maintenance problems associated with the use of a conventional electron capture detector with a radioactive source. During the design and optimization of the new instrument, we have evaluated the performance in the laboratory of every component of the system and each operating parameter. These tests have allowed us to select the most advantageous temperatures, column material and length, carrier gas flow rates, and sampling rates and times for ambient air monitoring. However, the instrument is quite flexible, and other choices in operating parameters could be made depending on the monitoring application. One very important characteristic of an air monitoring system is its sampling efficiency. For this instrument, it is desirable to sample at a temperature and flow rate that allow efficient collection of PAN, and to desorb at a temperature and for a time that recovers all of the PAN from the trap with minimal thermal decomposition of the sample. Figure 1 shows the response of the instrument versus the trap temperature. These tests used a sampling rate of 22 mL-min~* and a collection period of 2 minutes. The data show that lower trap temperatures yield maximum response, suggesting that the collection efficiency is enhanced at lower temperatures. The presence of a plateau at the lowest trap temperature examined suggests that the system is collecting PAN at near 100 percent efficiency. Other experiments studied trap desorption temperatures. Maximum response is obtained using a desorption temperature between 60 and 80°C. Other important characteristics of the instrument that we have examined are the effect of sample flow rate on collection efficiency and the linearity of response with respect to sample volume. Figure 2 shows the response of the GC versus the sample volume. The response is linear up to at least 76 mL. Sample flow rate has been examined over the range 5 to 35 mL-min"\ The response is linear throughout this range. Table 1 gives the retention times for PAN, PPN and several other atmospheric constituents under the conditions of our analysis. The PAN peak is well separated from other potentially interfering atmospheric constituents, including carbon tetrachloride, 1,1,1 -trichloroethane, trichloroethylene and tetrachloroethylene. Transactions on Ecology and the Environment vol 21, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541 Air Pollution 83 1.2E+07 8.0E+06 O Q_ W 4.0E+06 O.OE+00 -30 -20 -10 0 10 Trap temperature,°C Figure 1.