Calibrated Chemical Amplifier for Atmospheric RO, Measurements Donald R. Hastie,’ Michael Weissenmayer, John P. Burrows,* and Geoffrey W. Harris Air Chemistry Department, Max-Planck Institute for Chemistry, Saarstrasse 23, Postfach 3060,0-6500 Mainz, Germany
An experlmentai and madding study of an amblent RO, (HO HO, RO -t-R02) detector is presented. As descrlbed prevkurly, the detector utiilzes chmkai ampllflcatlon of the radical concentration through a chain reaction involving NO and CO to produce NO2. Modifications reported here overcome a PAN and PNA hterferanceto produce a detector that k a factor of -25 low sensitive to these interferencesthan the conventional dedgn, has improved rejection of artHact signals, and can have varlablelength inlets wlthout requiring a controlled nitrogen flow. A modd of the chemical ampHfier chemistry, whkh includes chemkai and wail loss of radicals, showed that slmpilfled calculations greatly overestimate the chain length. llw varlatbn of the chain length wtlh wail loss rates, radical concentration, reaction time, and radical type has been investigated. The wide variation In reported chain lengths has been attributed to a chain length dependence on radical concentration and inadequacies associated with one of the calibration techniques. Absolute radkal calibration of tho Instrument was performed by using the thermal decomposition of PAN as a source of known concentrations of M measurements over a M a y peroxyacetyl radkak. A period show a dlwnai variation of RO, radical concentrations and that daytime maximum concentrations of 3 pptv are readily dkcernibie.
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INTRODUCTION The chemistry of the troposphere is dominated by the reactions of gas-phase free radicals. The hydroxyl radical (HO), formed following the photolysis of ozone (OJ (I), is the most important tropospheric gas-phase oxidant and is responsible for initiating the tropospheric oxidation of virtually all gases emitted into the atmosphere (2). Reaction of HO with hydrocarbons or CO leads to the production of hydroperoxy (HO,), organic oxy (RO), and organic peroxy (RO,) radicals. In NO,-rich environments, the reactions of these RO, radicals (HO HOz RO + R02)combine in photochemical cycles to produce O3(3). Where there is insufficient NO,, O3 is removed via its reaction with the peroxy radicals, and RO, thus plays a critical role in determining the oxidative capacity of the atmosphere. In addition, free-radical reactions are important in heterogeneous processes (4), while the self-reactions of peroxy radicals lead to the formation of peroxides, which may be taken up by clouds where they act as important oxidants. For example, approximately half of the oxidation of SO2 to sulfate (SO4”) may proceed via aqueous-phase oxidation by hydrogen peroxide (5). Despite their central importance to tropospheric chemistry, there have been few measurements of RO, concentrations because of the difficulties associated with measuring the very
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*Author to whom corres ondence should be addressed.
Permanent address: &emistry Department and Centre for Atmos heric Chemistry, York University, 4700 Keele St., North York, 8ntan0, Canada M3J 1P3. 0003-2700/91/0363-2048$02.50/0
small concentrations present in the troposphere. Calculations
suggest daytime HO concentrations are in the range (1-40) X lo5 or 0.004-0.16 pptv (parts per trillion by volume) and those of H 0 2 are (1-50)X lo7 or 0.4-20 pptv (e.g., ref 6). A number of specific laser-induced fluorescence (LIF) methods for HO measurement have been developed (7-10) as has a long path laser absorption method (11-13). A chemical conversion technique in which HO reacts with “CO to produce 14C02,which can be collected and measured by using radioactive counting techniques, has also been used (14-1 6 ) . There have also been some spectroscopicmeasurements of tropsopheric H 0 2 concentrations. The low-pressureLIF HO instrument (7)was used to monitor HOz concentrations by conversion of HOz to HO through the addition of excess NO. Milhelcic et al. (17-19)have used matrix isolation-electron paramagnetic resonance spectroscopy (MIESR) for the measurement of H02 and speciated R 0 2 radicals. An alternative strategy for the measurement of RO, was developed by Cantrell and Stedman (20)and Cantrell et al. (21),who proposed a chemical amplifier system to measure the sum of HO, RO, H02, and R 0 2 radicals. The chemical amplifier utilizes a chain reaction involving NO and CO to produce a large number of NO2 molecules for each radical. HO,RO, HOz, and R02 radicala can all be measured by this technique since the chain can be initiated at any of the reactions (1)-(5).
€302+ NO
+
NO2 + HO
+ CO + H + C02 H + 02 + M HO2 + M RO, + NO RO + NO2 RO + 02 R”C0R’ + HO2 HO
-
+
-.+
(1) (2)
(3) (4) (5)
The number of NO2 molecules produced per radical is dependent on the rates of reactions (1)-(5)and on the reactions that remove radicals from the system. Regularly replacing the CO with N2 turns the amplifier chemistry on and off, yielding a modulated NOz signal, which is a measure of the radical concentration. A specific NOz detedor is required and the chemiluminescent reaction of NO2 with luminol (22, 23) was used for this purpose (20,21). Clearly the actual number of NO2 molecules produced per radical depends on experimental conditions such as NO and CO concentrations,on the reaction time, and on the processes that remove radicals from the system. Cantrell et al. (21) investigated their experimental arrangement and considered the reaction of HO with NO to be the most significant radical loss process:
HO + NO
+ M-
HONO
+M
(6)
They estimated the NO2 molecules produced per radical, or the chain length (CL), to be as high as 1500. However, ex@ 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63, NO. 18, SEPTEMBER 15, 1991
2049
NO2 DETECTOR - COMMON TO 1A & 1B
RADICAL DETECTOR FIG. 1A
FROM CHEMICAL
G ' H r1-L
NO2 Linearisation Flow
I
=NO A[
-
REACTOR
I
LUMINOL SOLUTION NO 2 DETECTOR
NO 2 DETECTOR
AIR PUMP
c
SIGNAL OUT. TO PUMP AIR
IM
FIG. 1B
REACTION CHAMBER
= C O /,3WAY VALVE
f
\ Heater 16 cm x 114' B
0.9m ( t = 0.22 sec ) NO2 Linearisation Flow 3.2 m ( t = 0.78 sec )
i
Q DETECTOR
Figure 1. Schematic diagrams of the radical detectors used in this study. (A) The conventional amplifier according to ref 21; (B) the final configuration used in this study.
periments to determine CL have yielded chain lengths varying from 40 to 1100 (21, 24). In this paper, a new experimental and theoretical study of the chemical amplifier based RO, detector is reported. A number of problems with the "conventional" RO, detector are identified, and a modified detector that overcomes these problems is described. In particular, it is shown that the conventional RO, detector suffers interferences from ambient concentrations of peroxyacetyl nitrate (PAN - CH3C03N02) and pernitric acid (PNA - H02NOJ. Modification of our RO, detector enabled these interferences to be reduced by a factor of approximately 25 compared to a conventional detector. A procedure suitable for field calibration of the RO, detector using radicals generated from the thermal decomposition of PAN has been developed. This procedure overcomes many of the difficulties previously encountered in calibrating the detector. The discrepancies between the calibrations reported in the literature are shown to be due to a combination of a nonlinearity in the amplifier and an inappropriate method of radical concentration determination. The end product of this study is a calibrated radical detector suitable for the measurement of ambient RO, radical concentrations in the troposphere.
EXPERIMENTAL SECTION Instrumentation. The RO, detector consists of a reactor, in which the chemical amplification takes place, and an analyzer to determine the NO2 produced in the reactor. Several designs of reactor were used in this work. Initially a conventional reactor similar to that described by Cantrell et al. (21),as shown in Figure lA, was constructed. This was found to have many shortcomings and was extensively modified: our final configuration is shown in Figure 1B. For the diagnostic and development studies, the NO and CO mixing ratios in the amplifier were 2 ppmv and 4%, respectively. In the conventional RO, detector, a radical zero was obtained by
&--Photomultiplier
Figure 2. Schematic diagram of the NOPdetector used in this study. The inset shows details of the detector cell.
replacing the flow of CO with an equal flow of N2,thereby suppressing the chain mechanism. In our version, a zero was obtained simply by moving the point at which the CO was added closer to the NO2 analyzer, which resulted in the removal of the radicals by reaction with NO before amplification could occur (see below). The residence time in the reactor was 6.7 s for the conventional system and 1.1s for the final configuration. In the latter system, the first 16 cm of the inlet/reactor was of quartz and could be heated to >200 "C as necessary. The remainder of the reactor consisted of 4.1 m of 0.36-cm4.d. (1/4-in.0.d.) FEP Teflon tubing and of the volume inside the NO2 detector. The luminol NO2 detector used is shown in Figure 2. It is similar to instruments described elsewhere (22,23,25) but uses a larger reaction surface. Air was drawn across a 7.5 X 5-cm glass fiber filter paper wetted with a solution containing luminol, and the chemiluminescence from the reaction of NO2 was detected by an EM1 6255 (50-mm-diameterend-looking)photomultiplier (26). The air flow rate was 2.4 L min-', and the luminol reagent was delivered at a flow rate of 0.1 mL min-'. The photomultiplier was connected to a nanommeter and the output displayed on a Linseis chart recorder. The luminol detector is inherently nonlinear below a few ppbv, the exact onset of this nonlinearity being solution dependent (25-27). To ensure that all measurements were made in the luminol instrument's linear region, NO2from a permeation device was continuously added to the air stream entering the detector, yielding a constant baseline offset correspondingto 20 ppbv NO* With this configuration, the noise equivalent detection limit for a 1-sanalog time constant was 50 pptv NO2and was linear in the range of concentrations encountered in this study (
