Measurements of Peroxyacetyl Nitrate (PAN) During the 1997 and 1998 IGAC Aircraft Observations

Jun MAEDA, Hiroshi BANDOW (College of Engineering, Osaka Prefecture University), Ikuo WATANABE (National Institute for Public Health), Kentaro MURANO, and Shiro HATAKEYAMA(National Institute for Environmental Studies)

(1) Introduction

Peroxyacetyl nitrate (PAN) had been known as a harmful substances in terms of photochemical smog in urban areas.Recently this substance has been attracting further attention since studies have indicated that PAN is likely to lead to widespread occurrence of acid deposition and the vicissitude of compounds within the troposphere, because this is an important reservoir of reactive nitrogen compounds1,2).Nevertheless, there have been few observational data of PAN concentrations both in Japan and Western nations.This may be reflection of the difficulty for its quantitative measurement since the PAN concentration is less than 1 ppb except in urban areas.There has been a quite limited data for East Asia, and, therefore, the actual extent of air pollution.
To accumulate observational data, our group had conducted aircraft observations over the Yellow sea and the sea of Japan in fiscal years 1992 and 933,4), over the sea surrounding Yaku Island in 1994 fiscal year5), over the Sea of Japan in 1995 fiscal year5), and over the East China Sea in 1996 fiscal year.Then we conducted aircraft observations over the East China Sea in fiscal years, 1997( 6 days from December 8 to 13, 1997) and 1998( 6 days from February 1 to 6, 1999).
This chapter will report on the results of the observations in fiscal years, 1997 and 1998.

(2) Sampling device loaded on the aircraft

Sampling device is composed of a collecting trap, a thermal-mass-flow controller, a double-stage tank for taking gases in (volume: 1 L), a small vacuum pump for evacuating the tank, a pump for purging sampling line, sensors for monitoring the temperature and pressure inside of the tank, and a small recorder with 6 chopper bars for recording the pressure and temperature in the tank.
A U-shaped Teflon tube (30 cm *1.5 mm I.D.) filled with about 0.2 g of Flusin T (30/60) was installed on a Teflon four-way valve, and used as a concentrating trap tube for PAN. The dry ice used as a coolant is convenient for keeping traps refrigerated, and is easy to handle because the temperature can be maintained simply by supplying additional dry ice once or twice a day. Accordingly, the dry ice was put in the cooling bath of the concentrating trap tubes during the aircraft and ground-based observations.
During the 1997 and 1998 aircraft observations, air was collected into each concentrating trap for 10 minutes at a flow velocity of 20 and 30 mL/min, respectively (Total volumes of collected samples: 200 mL and 300 mL).

(3) Analysis of PANs

On the basis of the conventional method6,7), peroxyocetyl nitrate (PAN) was synthesized by photooxidation of ethyl nitrite. The concentration of PAN was determined by using the extinction coefficient (PAN: 13.9* 10-4, 8))based on the absorbance in the infrared regions (PAN: 8.60 um).
PAN in the ambient sample gases were analyzed by the GC-ECD method. A Teflon tube (100 cm * 2 mm I.D.) filled with 5% PEG400, Chromosorb W, AW-DMCS, and 60/80 mesh was used as a analytical column. The column vessel was kept at room temperature (~ 25 ºC), and nitrogen (about 70 mL/min) was used as a carrier gas. When non-negligible moisture was contained in the samples, the moisture was eliminated using a precut column (identical to the analytical column except for its length of 30 cm) before the samples were introduced into the analytical column.

(4) Observational data

The data obtained in the fiscal years 1997 and 1998 are listed in Table 1 and Table 2, respectively, along with the sampling time (start and/or finishing), the average flight altitude at which the sample air collection was done, and the average concentration of NOx during each air-sampling period for PAN.Please note that the nitrogen oxides concentration for 1998 is abbreviated as NOy in the Table 2, because the air-sampling system for nitrogen oxide analyzer has been modified from the earlier one, in which the NOx-reducing converter was placed after ca. 2m-long sampling tube (one quarter inch o.d. stainless-steel tube).The converter was moved to the position just after the inlet of the sample-tube to minimize the wall-effect of the tube. This modification made us to call the indicated value of the NOx instrument as "NOy" from "NOx" or "NOx*".
In the air masses sampled in December 1997, concentrations of air pollutants were in general low and especially that of PAN was fairly low. This feature can be more easily seen when comparing the ratio of PAN/NOy with the earlier data of this IGAC aircraft observation campaigns. The ratio was less than 0.1 in 1997 observation (Figures 1-3), while the earlier data having been ranging from 0.2 to 0.8.On the other hand, the data obtained on February 4th and 6th in 1999 (Figures 5, 6) show the similar pattern to the previous observation. That is, the ratio of PAN/NOy being higher (from ca. 0.4 to 0.8) in the free troposphere, generally above ca. 1500m in this region, than that at lower altitude, although the ratio at high altitude was low on 2nd (Figure 4). The concentrations of NOy and PAN (and other pollutants as well, which being shown in other chapters of this data book) were extraordinary high in this flight, and the PAN/NOy ratio was not high as that in the typical remote air-mass, suggesting that the polluted plume from an emission source directly hit this flight area at that time.



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