FORSCHUNGSZENTRUM JÜUCH GmbH

Institut für Chemie und Dynamik der Geosphäre 3: Atmosphärische Chemie

A Large Reaction Chamber for Nighttime Atmospheric Chemistry: Design and Characteristics of the Reaction Chamber

Th. F. Mentel A. Wahner

Berichte des Forschungszentrums Jülich ; 3196 ISSN 0944-2952 Institut für Chemie und Dynamik der Geosphäre 3: Atmosphärische Chemie Jül-3196

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A Large Reaction Chamber for Nighttime Atmospheric Chemistry: Design and Characteristics of the Reaction Chamber

Th. F. Mentel A. Wahner

ABSTRACT

A large reaction eh amber for the study of nighttime atmospheric processes in

the planetary boundary layer has been set up. The chamber which is constructed as a

double wall system with gas tight outside walls and chemically inert inside walls has a

volume of 260 m3 and a surface/volume ratio better than 1 rn-I. The mixing ratios of the

reacting trace gases N02, N20 S, and HN03 are measured simultaneously by high

resolution FfIR spectroscopy, 0 3 and NO are monitored by UV absorption or

chemiluminescence. The lifetime of 0 3 and N02 in the chamber with respect to wall

losses were measured to be between 12 and 20 days for 0 3 and between 12.5 and 17

days for N02. An upper limit of <6.3* 10-21 cm3 molecule-1 S·l for the rate coefficient of

the reaction N20 4 + H20 has been determined. The lifetime of HN03 in the re action

chamber is a strong function of the H20 mixing ratio and decreases from 89 h at 8% r. h.

to 6.3 h at 70 % r.h.

1

= INTRODUCTION

Since the mid seventies Smog Chambers evolved as a valuable tool of

atmospheric chemistry for the investigation of the photochemistry in polluted urban

areas. Smog chambers are re action chambers in which defined mixtures of atmospheric

trace gases react and the educt decay as weIl as the product formation are monitored as a

function of time. These experiments enable kinetic studies of atmospheric relevance

under reproducible and controlled conditions. They allow for the independent variation

of single parameters from a set of parameters, which are highly interlaced in the natural

atmosphere.

Several types of smog chambers, ranging from simple Teflon bags, which

utilize natural sun light to induce photochemie al processes, to sophisticated evacuable

eh ambers equipped with solar simulators are described in the literature (e. g. leffries et

al., 1976; Akimoto et al., 1979; Winer et al., 1979; Spieer et al., 1981 and references

therein; Carter et al., 1982; Grosjean, 1984; Leone et al., 1985 and references therein,

Zetzsch, 1987). The chamber volumes range from several hundred liters up to a few

hundred m3• Recent smog eh amber studies are dealing with e. g. the nighttime OH

production from the reactionof 0 3 with alkenes and terpenes (Paulson et al., 1992;

Atkinson et al., 1992; Atkinson and Aschmann, 1993), the chemistry of higher

peroxyacyl nitrates formed from biogenie hydroearbons (Grosjean et al., 1993a;

Grosjean et al., 1993b, Grosjean et al., 1994), the reaction of N03 with aromatic

compounds (Kwok et al., 1994, Atkinson et al., 1994), or the heterogeneous formation

of chlorine and chlorine compounds on NaCI aerosols (Zetzsch et al., 1988; Zetzsch and

Behnke, 1992).

2

= In many cases smog chamber investigations are restricted to high trace gas

concentrations due to the current detection limits of the analytical instrumentation. The

observable concentrations may be representative for polluted urban areas but not for the

large remote parts of the boundary layer or for the free troposphere. Therefore the results

of such studies are not always transferable to low- or non-burdened situations. Another

difficulty of smog chambers originates from wall effects (Carter et al. , 1982; Sakamaki

et al. , 1983; Pitts et al. , 1984). A summary of the efforts to characterize wall processes

in several smog chambers is given by Killus and Whitten (1990). Despite of these

limitations, many of the reaction schemes used in air chemistry modeling are based on

smog chamber studies.

We constructed a large reaction chamber for the study of nighttime atmospheric

processes at ambient pressure and temperature. The chamber is designed to maximize

reproducibility and to minimize potential effects of the reactor walls, in order to provide

long lifetimes of trace gases and aerosols with respect to walilosses. This enables

experiments of several days or longer duration which are necessary to observe slow

atmospheric reactions. The purpose of the chamber is the investigation of dark chemical

processes, including heterogeneous reactions on aerosol surfaces, at mixing ratios which

are typical for low- or non-burdened situations in the planetary boundary layer.

In this paper we will describe the setup and the properties of the new reaction

chamber and the analytical instrumentation. Several types of experiments were

performed to characterize the re action chamber: 1) long term experiments with CO2 and

a mixture of inert gases as tracers to determine the 1eakage rate of the total system and to

quantify typica1 10ss rates. 2) measurements with C02 as an inert tracer to test the

mixing characteristic; 3) lifetime measurements of 0 3 as an examp1e for a reactive trace

3

= gas; 4) measurements of the in eh amber 10ss of N02 and HN03 at three different

humidities.

Due to the current detection limits, the investigations were performed at 1 ppm

initial concentration level. This is a factor of 10-100 more than typical ozone and

nitrogen oxide mixing ratios in the troposphere. In a following paper we will present a

study of the nighttime chemistry of the N02/03 system in the re action eh amber,

including a quantitative interpretation based on model calculations.

EXPERIMENTAL

Construction of the Reaction Chamber

The main features of the reaction chamber are shown in Figure 1 (top). The

dimension of the chamber are 7.14 m by 6.84 m by 5.24 m (l x w x h). The volume is

255.9 m3 and the wall surface 244.2 m2. The surface to volume ratio is better than 1 rn-I.

The chamber is constructed as a double wall system. The outside walls are made out of

aluminum sheets (thickness 1.5 mm). They are fully welded forming agas tight cIosed

system. The inside walls are made of Teflon-FEP film (5 mil) to provide chemically

inert surfaces. The FEP film is heat sealed to a closed bag of about equal size as the

aluminum box. The FEP-bag is freely hanging from the ceiling of the aluminum box and

is pulled towards the floor edges by metal springs. It can be easily exchanged when it is

aged or contaminated.

As all Teflons, FEP is permeable to gases. Therefore, the space between the FEP-bag

and the aluminum box is purged permanently by a stream of clean air to diminish

possible memory effects due to diffusion of trace gases from the FEP bag into that space

4

A) Features of the Reaction Chamber (schematic cut)

11~"'e--- FEP-bag

ceiling cooler

v = 260 m3

S=250m 2

S IV = 1 m-1

• --- aluminum box i [7 x 7 x 5.3 m]

gas purification system for background air

250m3 /h

floor heating

I 0 3 UV-absorption

temperature _ rel. humidity

NO chemilumines­-=-----1 cence

IR-beam

'--------'

double wall purging 1.2 m3 1 h

FTIR

B) Position of the Ports for Measurements on the Front Wall

top floor

ground floor

FIGURE 1 The Reaction Chamber at KFA Jülich.

A) Schematic cut of the reaction chamber with gas purification systems for background air and double

wall purging, conventive rnixing system consisting of a floor heater and a ceiling cooler, and several

analytical instruments. The coupling of the IR beam into the chamber via the retroreflector R is indicated.

B) Positions of the ports on the front wall used in this study for measurements. 1: nephelometer; 2: trace

gas inlet (N02, 0 3); 3: 0 3 UV absorption, hurnidity and temperature; 4: NO chernilurninescence; 5: CN

counter; a,b: inlet and exit of the background air; c: double wall purging; I, 11: pressure safety; FTIR:

CaFrwindow flange for entrance and exit of the IR beam.

5

and vice versa. The air is provided by a purification system which consists of a

palladium catalyst (Huels H54-44) and a molecular sieve stage. The air purification

system supplies a flow of about 20 IImin of hydrocarbon and CO free, dry air, which is

sufficient to exchange the volume between the aluminum box and the PEP-bag (about

10 m3) twice a day. Since the aluminum box is kept at a slight over-pressure by the air

flow, contamination with intruding ambient air is suppressed.

All ports into the chamber are designed in O-ring seal fashion. Whereever

possible the parts inside the re action chamber are made out of Teflon. The ports for

trace gas inlet and gas sampling are located on the front and the back walls of the

reaction chamber (see Figure 1 bottom). Windows opposite to each other on the front

and the back wall enable optical concentration measurements. A dOOf into the PEP-bag

and a second dOOf into the aluminum box are available fOf cleaning and maintenance.

The matrix air for the reaction chamber is furnished by an air purification

system with a maximal capacity of 200 m3/h. This system is equipped with a molecular

sieve stage and an active charcoal filter, each succeeded by particle filters. Bach filter

stage can be bypassed if necessary. The air purification system can be operated in two

modes, either in a flushing mode in which the chamber air is replaced by outside air, or

in a closed loop in which the chamber air is repeatedly circulated through the filter

stages. Optionally, trace gases can be added to the main clean air stream.

The mixing inside the reaction chamber is accelerated by a convection system

(Nolting and Zetzsch, 1987). Four independent hot water loops of 45 m length under the

flOOf and a 160 m long cold water loop on top of the chamber (see Figure 1) allow to

introduce a temperature gradient of -0.2 to -1 Klm, which is sufficient to yield a stable

6

convection. The mixing system typically is switched on about 90 min. before the trace

gas inlet and is switched off after 4 to 6 hours.

Two large bubbIers filled with water, connected to the FEP-bag and the

aluminum box, respectively, are serving as a passive pressure safety system. If the

pressure difference between the chamber and environment is higher than 2 mbar the

bubbIers break through. The bubbIer towards the aluminum box is equipped with a 4

mm diameter bypass for the purging air flux.

Analytical Instruments

The central analytical tool is a high resolution FTIR spectrometer (Bruker

-1 -1 IFS 120HR) with a spectral range from 600 cm to 40000 cm (MIR, NIR, vis, UV) and

-1 a maximum resolution of 0.008 cm . A set of specific instruments is available for the

measurement of NO, 0 3, CO2, condensation nuclei, and the light scattered by aerosols.

Certain parameters such as temperature, relative humidity (back and front wall), ambient

pressure p and the difference pressures ßp between the FEP-bag or the aluminum box

and the outside are monitored permanently. The type, the measurement principle, and

the accuracy of the instruments are listed in Table 1. The output of the analytical

instruments (with exception of the FTIR) is sampled by a data acquisition system. The

maximum time resolution of the data acquisition system for the simultaneous readout of

64 channels is 1 s.

The FfIR spectrometer is used for in-chamber measurements. Therefore one of

the two beams of the spectrometer is designed as parallel beam with a divergence

smaller than 0.35°. The parallel beam is directly coupled into the re action chamber. In

this study, a corner cube retroreflector at the chamber back wall has been used to

7

Table 1 Analytical Instruments ofthe Reaction Chamber at KFA Jülich

Instrument Measured Quantity Accuracy Time Res. Method Calibration Location (Figure 1)

FTIR Spectrometer [N02], [N20 5], rms noise 0.0004 6min. absorption spectroscopy reference spectra 0.7 m above floor,

(Bruker IFS 120HR) [HN03], [CF4], absorbance units path length 15 m, resolu- double pass through

[CH4], [N20], [CO] (see Table 2) tion 0.1 ern-I, 36 scans reaction chamber

Ozone Monitor [03] +/- 1 ppb 1 min. UV absorption stabilized 0 3 port 3

(Ansynco 03 41M) atA= 254 nm permeation source

NO Monitor [NO] +/-0.5 ppb 20s chemiluminescence at NOIN2 gas mixture port 4

(Tee an CLD700AI ppt) A = 320 - 420 nm

Humidity Sensor reI. humidity +/-1% - capacitance vapor press ure above mid front wall; port 3 (Vaisala Type 135Y) temperature +/- 0.2 K - resistance (PtlOO) salt solutions mid back wall

00

Pressure Sensor diff. pressure ßp 0.5% ofthe - membrane capacitor - portI (MKS Baratron 223B) absolute value port 11 Pressure Sensor ambient pressure 0.5% ofthe - membrane capacitor - laboratory (MKS Baratron 122A) absolute value

IR Monitor [C02] +/- 1 ppm 3 min. IR-Absorption CO2 gas mixture port 3

(Maihak Unor)

CN counter submicron particles +/_102 part./cm-3 1 min. light scattering - port 5 (TSI model 3020)

Nephelometer reciprocal visibility - 1 min. scattered light at Freon F 12, F 22 port 1 (self constructed) A= 633 nm

----- L-- ---

II I-m--

provide a double pass of a total of 15 m through the ehamber, 0.85 m above the floor.

The ehamber is separated towards the speetrometer and the retrorefleetor by two pairs of

planparallel CaF2-windows. These windows are replaeeable by windows of different

materials (e.g. KBr, NaCI, UV-quartz), thus all possible wavenumber ranges of the

speetrometer ean be utilized for in-ehamber measurements.

With the setup as deseribed above we were able to measure N02, N20 S, or

HN03 in the ehamber down to mixing ratios of the order of 10 ppb. The adaptation of

the parallel beam to an in-ehamber multirefleetion system will improve these deteetion

limits. A White system of 6 m base length with gold eoated mirrors for maximum 120

passes is already in preparation. The new multirefleetion system will enhanee the

maximum length of the beam path by a faetor of 48.

The mixing ratios of traee gases were determined from absorbanee speetra.

Therefore an 10 single ehannel speetrum was taken through the ehamber before eaeh

experiment (see below). The 10 speetrum is then used to ea1culate absorbanee speetra

from the set of single ehannel speetra reeorded during an experiment. All speetra were

taken in the MIR (5000 - 500 ern-I) at aresolution of 0.1 em-1 with 36 seans per

speetrum. Under these eonditions the maximum time resolution is 6 minutes.

Evaluation of the FTIR Speetra

In order to quantify the mixing ratios of traee gases in the chamber the

absorbanee speetra were fitted by appropriate spectra from a high resolution

(0.125 ern-I) FfIR library (Hanst and Hanst, 1992) in a wavenumber interval whieh was

least perturbed by sharp speetral features of other moleeules. As an example, the

9

vibration al bands and the wavenumber intervals for the NOy species N02, N20s, and

HN03 are listed in Table 2.

Before the fitting procedure could be perfonned the measured spectra and the

reference had to be matched in resolution, wavenumber position and point density.

Wavenumber corrections of 0.02 cm-1 in the wavenumber range near 1600cm-1 and of

0.04 cm- l near 1300 cm- l improved the spectral residuals, but affected the quantitative

results to less than 2%.

Additional to the reference spectrum of the substance to be determined, a water

reference spectrum and a linear baseline correction were fitted simultaneously to the

measured spectra. We used the least square fitting routine provided by Bruker, which

minimizes the spectral residual ca1culated by the following equation:

residual(v)

v

A(v)

Rx(v)

RH20(V)

fx, fH2o :

sl,of

= A(v) - fx Rx(v) - fH20 RH20(V) - (sl*v + of)

wavenumber

measured absorbance spectrum

reference spectra of substance x to be determined

water reference spectrum

appropriate factors to minimize the residual

baseline slope and offset.

The reference spectra of the substances and of water used in the fitting procedure were

made linear independent of baseline slope and offset. The factor fx multiplied by the

10

..... .....

Table 2 Spectroscopic Determination of NOz, NzOs, and UN03

Molecule N02 N20 5 HN03

Vibrational Mode V3 V3 Vl2 V3

Resolution Quality J-resolved J-resolved rotation al envelope rotational envelope

Determination Intervall 1632.50 - 1629.35 cm-1 1600.25 - 1596.75 cm-1 1258.00 - 1226.00 cm-1 1327.00 - 1324.00 cm-1

Spectral Feature near R-branch maximum near P-branch maximum P,Q,R -branch Q-branch

WaterLines none none 1244.1 cm·\ 1326.1 cm·\

Integral Cross Section 1470 cm-2 atm-1 1470 cm-2 atm-1 853* cm-2 atm-1 834** cm-2 atm-1

of Reference Spectrum

(Hanst and Hanst, 1992)

Detection Limit 20ppb 40ppb 40ppb

- ---- ------- - -

Spectroscopic determination of the trace gases N02, N20 5, and HN03 by in-chamber FfIR absorbance spectroscopy. N02 was

evaluated in two separate wavenumber intervals. The N02 mixing ratio was ca1culated from arithmetic mean od the two results.

* ** Additional experiments indicated that the original cross sections of Hanst and Hanst (1992) had to be divided by 0.6(*) and 0.9(**).

The experiments and the correction procedure will be described in a following paper. For the conclusions in this paper the exact value

of the cross sections is of no importance.

" . .lIL! I 11

i

-optical density of the reference spectra and divided by the path Iength yields the

~ concentration of substance x.

The detection limits of the spectroscopic N02, N20s, and HN03 determination

using the method described above are given in Table 2. They are calculated from the 20'

rms noise (95% confidence level). The rms noise was determined in the wavenumber

range of 2400 - 2600 cm-1 which is free of absorption bands. It inc1udes contributions

from fringing which is due to internal reflection of the 4 CaF2-windows in the beam

path. From 23 arbitrary chosen spectra of all three measurement series the mean rms

noise was 0.0004 absorbance units.

Characterization of the Reaction Chamber

The principle experimental procedure is described in the following. Before

each experiment the reaction chamber was flushed for about 10 hours with non-purified

ambient aif. Since the trace gas concentrations used for the investigations were much

higher than their abundance in the ambient atmosphere, the non-purified outside air was

used as matrix air. If necessary, the water concentration was adjusted either by

circulating the chamber air through the molecular sieve filter or by evaporation of

additional water in the chamber. Otherwise the experiments were performed at ambient

relative humidities (about 50% r.h.). The 10 FrIR spectrum was recorded right before the

trace gas inlet. Then the trace gases were filied into the chamber. A certain time was

allowed for the rnixing process, which was ab out 24 h, if the floor heater was not in use

(inert gases), and ab out 1 h if the convective mixing system was switched on. The

mixing ratios of the trace gases were monitored as function of time by the respective

12

analytical instruments given in Table 1. The frequency of the measurements was

matched to the characteristic time of the observed process.

The leakage rate of the reaction chamber was determined by measurements of

the CO2 lifetime in the chamber (experiment A). The conditions of experiment A are

summarized in Table 3. The floor heater and the aluminum box purging were not in use

("static" experiment). About 70 I STP of CO2 (purity 99.5%) were filled into the

chamber. This raised the CO2 mixing ratio in the chamber to 730 ppm which is weIl

above ambient levels. The decrease of the CO2 mixing ratio inside the chamber at three

ports and the increase in the aluminum box were recorded by a commercial IR monitor

(Maihak Dnor). Since the decay of the CO2 inside the PEP bag was slow, the experiment

was split into two parts. Measurements were performed during the first 80 hours of the

experiment. The chamber was then allowed to stand for 600 h. Thereafter measurements

were taken during another 300 h.

The lifetimes of inert gases in the PEP-bag were measured for a mixture of

CF4, Cf4, N20 and CO (experiment B). The experimental conditions are listed in

Table 3, entree B. The gases were sequentially filled into the chamber at the same

chamber port. The total filling process lasted less than an hour. The double wall purging

was not in use. The mixing ratios of the trace gases in the chamber were monitored for

four weeks simultaneously by in-chamber FTIR spectroscopy.

The mixing in the chamber was characterized by three experiments (e - E)

with CO2 as an inert tracer. The conditions of the experiments C - E are given in

Table 4. A surplus of 100 ppm CO2 was added to the chamber at one corner. The CO2

distribution as a function of time was measured at several points in the chamber by the

IR-monitor. Additionally, the integral CO2 concentration across the chamber was

13

Table 3 Lifetime ofInert Gases in the Reaction Chamber at KFA Jülich

a) Experimental Conditions

Experiment A B

Duration 1000 hours 1 month (80 + 600 + 300 h)

Preparation 10 h flushing with ambient air

Molecule CO2 CF4 C~ N20 CO

Initial Concentration 730ppm 5ppm 11.5 ppm 3.2ppm 1.1 ppm

Measurement Method IR-Absorption in-chamber FTIR

b) Experimental Results

Experiment A B

In-Chamber Loss - 8% 13% 18% 20% after 1 month

Initial ßC02 (first 80 h) :

FEP-Bag (Vch=245 m3) -0.1 05 ppmlh

Double Wall (Vdw=10 m3) +1.61 ppmlh

Total System (Vtot=255 m3) -0.038 ppmlh

"Ce in FEP-Bag 145 d 335 d 190 d 133d 125 d

"Ce in Total System > 1 year

ßC02 : change of the CO2 mixing ratio Veh : volume of the FEP-bag V dw : volume between FEP-bag and aluminum walls Vtot : volume ofthe aluminum box (Veh + Vdw)

"Ce : lIe lifetime

14

Table 4 Mixing Process in the Reaction Chamber at KF A Jülich

a) Experimental Conditions

Experiment C D E

Duration 20h 6h 18.5 h

Preparation lOh flushing with ambient air

Surplus CO2 100ppm

Floor Heater off 30°C 30°C

Vertical Temp. Gradient - - 0.2 Klm - 0.2 Klm

Inlet Corner Right Upper Back Right Upper Back Left Lower Front

Corner Corner Corner

Measurement Points - Left Lower Front - Left Lower Front - Right Upper Back Corner Corner Corner

-Mid Floor

- 1 m Height Front - 2 m Height Front - 2 m Height Front - 2 m Height Front

- 4 m Height Front

- Right Upper Back - Right Upper Back - Left Lower Front Corner Corner Corner

- In-Chamber FTIR - In-Chamber FTIR - In-Chamber FTIR

b) Experimental Results

Experiment C D E

Complete Mixing 8h 4h 4h

Mixing with Exception 4h 0.5 h 0.5 h

of the Inlet Corner

Circulation of C02 Parcel - 3 min. 3min.

Measurement Point - Left Lower Front Right Upper Back

Corner Corner

15

monitored by in-chamber FfIR spectroscopy. As a "cold" reference, experiment C was

performed without floor heating. During the experiments D and E the convective

rnixing system was in operation. This introduced a temperature gradient of -0.2 Klm,

which was determined at five heights.

Several lifetime measurements of 0 3 were performed in the reaction chamber

(experiments F - G). The 0 3 was produced by an electrical discharge ozonizer from pure

O2 (99.98%, AGA-Edelgas) with a conversion rate of approximately I %. An oxygen

flow of 2 Urnin over a time interval of 15 min. resulted in a 0 3 mixing ratio in the

chamber of 1.35 ppm. NOx impurities of the produced 0 3 were not observed. The 0 3

rnixing ratio was monitored by UV absorption (Ansyco 03 41M). The initial conditions

of the 0 3 experiments are summarized in Table 5. The 0 3 lifetime measurements Fand

G were performed before any nitrogen oxides were introduced into the chamber. In

experiment F the double wall purging was not in use. The measurements Hand J were

performed a few days, respectively a few weeks after a previous experiment with

nitrogen oxides.

The losses of N02 in the chamber were measured at three different H20 mixing

ratios (experiments K - M). The experimental conditions are listed in Table 6. The N02

was added as a approximately 1 % rnixture in He (N02 (98%) in He (99.96%), Messer

Griesheim) over 15 rnin. at a flow of 1.331/min. The convective rnixing system was

used as described above. The mixing ratios of N02 and HN03 were monitored

simultaneously by in-chamber FfIR spectroscopy, in addition the NO mixing ratio was

measured by chemiluminescence. In experiment L additional measurements of the

HN02 concentration were performed by means of annular carbonate denuders

(Possanzini et al., 1983; Ferm and Sjödin, 1985; Allegrini et al., 1987).

16

Table 5 Lifetime of 0 3 in the Reaction Chamber at KF A Jülich

a) Experimental Conditions

Experiment F

Duration 14 d

Preparation

Double Wall Purging off

Previous Experiments none

with Nitrogen Oxides

~easurement~ethod

b) Experimental Results

Experiment F

Initial Concentration 8ppm

k1oss(Ü3) 5.8* 10-7 S-1

'tOl 20d

~ean 't03

~ean k1oss(Ü3)

k1oss : overaliloss rate coefficient

't03 : lIe lifetime

G H

25 h 24h

10 h flushing with ambient air

on on

none 3 days before

UV Absorption

G H

1.5 ppm 1.4ppm

6.1*10-7 S-1 1.2* 10-6 S-1

19 d IOd

15 (+1-5) d

7.65*10-7 S-l

17

J

IOd

on

6 weeks before

J

1 ppm

6.7*10-7 S-1

17 d

Table 6 NOz Loss in tbe Reaction Cbamber at KF A Jülicb

a) Experimental Conditions

Experiment K L M

Duration 47h 72h 45 h

Preparation ca. 10 h flushing with ambient air

13 h drying none 11 h evaporation

molecular sieve filter 1.3 I deion. water

Initial M. R. N02 1.36 ppm 1.36 ppm 1.32 ppm

Mean R. H. 3% 48% 72%

Mean Temperature 291 K 294K 292K

Min. / Max. Temp. 289.5/293.0 K 293.2 / 295.2 K 290.7/293.2 K

Mean H20 M. R. 445 ppm 11570ppm 15285 ppm

Min. / Max. H20 M. R. 160/830 ppm 11300/ 11900 ppm 13800/ 15700 ppm

b) Experimental Results

Experiment K L M

N02 Loss after 48 h 15% 11% 12%

k1oss(N02) 9.24*10.7 S-l 6.81*10-7 S-l 7.11*10-7 S-l

'tN02 12.5 d 17 d 16 d

mean kN02 8*10-7 S-l

mean 'tN02 14.5 d

[HN03] below d. 1. 40 ppb below d. 1. 40 ppb below d. 1. 40 ppb

[HN02] not measured increase up to 10 ppb not measured

[NO] const. 10 ppb 2 - 5 ppb 4- 5ppb

upper limit k2 < 2* 10-23 ClIl3 molecule-l S-l

M. R.: mixing ratio; R. H.: relative humidity; k1oss: overallloss rate coefficient;

'tN02: lIe lifetime of N02; kN02: waIlloss rate coefficient, [X]: mixing ratio of trace gas X;

k2: rate coefficient N20 4 + H20 = 2 HN02

18

The wallloss of HN03 was measured at three H20 mixing ratios (experiments

N - P). The experimental eonditions are given in Table 7. The HN03 mixing ratio was

monitored by in-eh amber FTIR speetroseopy. HN03 was ehemically produeed in the

ehamber from N02/031H20. After a eertain time dependent on the H20 mixing ratio,

HN03 is the only observable NOy eomponent in the chamber. The decay of the HN03

from that point on was used to determine the HN03 lifetime in the chamber.

RESULTS

Lifetime of Inert Gases

The results of the C02 lifetime measurements for the "static" experiment A are

shown in Figure 2 and summarized in Table 3. The CO2 mixing ratio inside the FEP-bag

decreases slowly over the whole time interval with approximately 0.1 ppmlh. The

mixing ratio in the aluminum box rises with approximately 1.6 ppmlh in the beginning

of the experiment and approaches slowly the CO2 level inside the FEP-bag at long

times. From the decrease of the CO2 mixing ratio inside the bag and the increase in the

aluminum box during the first 80 hours, each weighted by the respeetive relative

volume, one can calculate the CO2 residenee time in the total system (FEP-bag +

aluminum box) and the lifetime 'Ce inside the FEP-bag due to diffusion or leakage into

the aluminum box. The residenee time of CO2 in the total system is more than one year,

the lifetime 'Ce inside the FEP-bag 150 days.

In experiment B, the measurement of the in-chamber lifetime was repeated for

a mixture of CF4, C~, N20, and co. The result is listed in Table 3. After four weeks

19

Table 7 Wall Losses of HN03 in the Reaction Chamber at KFA Jülich

a) Experimental Conditions for HN03 Production

Experiment N 0 P

Total Duration 116 h 96 h 50h

Chamber Preparation ca. lOh flushing with ambient air

13 h drying 1 h drying 11 h evaporation molecular sieve filter molecular sieve filter 1.3 I dei on. water

1. Reactant N02 0 3 N02

[Initial M. R] [1.16 ppm] [1.2 ppm] [1.2 ppm]

2. Reactant 0 3 N02 0 3

[Intial M. R] [1.34ppm] [1.34 ppm] [1.41 ppm]

MeanR H. 7.8% 21.4% 72.2%

Mean Temperature 292K 292K 292K Min./Max. Temperature 290.7/293.7 K 291.1/294.2 K 289.7 / 293.2 K

Mean H20 M. R 1620 ppm 4650ppm 14410ppm

Min./Max H20 M. R 830 / 2390 ppm 4090 / 5050 ppm 13600/ 15680 ppm

HN03 Sole NOy Com- after 56 h after 22 h after 3 h

ponent, Defines t = 0

b) Experimental Results

Experiment N 0 P

Mean H20 M. R 2215 ppm H20 4875ppm H20 14380 ppm H20

[HN03]t=<> 0.521 ppm 0.781 ppm 0.733ppm

kHN03 3.12*10-6 sol 4.37*10-6 sol 4.38*10-5 sol

'tHN03 89h 64h 6.3 h

M. R: mixing ratio; RH.: relative humidity; [HN03]: mixing ratio HN03;

kHN03: wallioss rate coefficient; 'tHN03: l/e lifetime of HN03

20

E 700

c. c. FE P-bag (3 heights) - ... -- - -e- - .. 0 600

_ B _ - -ca - e - - EI ... D)

c >< 500 E aluminum box N 0 (.) 400 .... -------- ambient C02 ....$ ......

...... -~ ~

300

0 250 500 750 1000

duration I h

FIGURE 2 Experiment A: Leakage.

Leakage test of the reaction chamber with CO2 as inert tracer. CO2 mixing ratio as a

function of time in the PEP-bag measured at three heights (dots), in the aluminum box

(squares), and outside the laboratory (diamonds). Fulllines: initial.dC02 in the PEP-bag

and in the aluminum box approximated by linear least square fits with slopes of

-0.105 ppmlh and 1.61 ppmlh, respectively.

21

8% of CF4 , 13% of CH4, 18% of N20, and 20% of CO were lost from inside of the

reaction chamber (PEP-bag). This results in lifetimes 'te of 335, 190, 133, and 125 days,

respectively.

The operation of the double wall. purging, wh ich maintains a large

concentration gradient between bag and aluminum box by removing permeated gases

from the space between the PEP-bag and the aluminum box, reduced the lifetime of COz

in the PEP-bag to 45 days, a factor three lower than determined in the "statie"

experiment A.

Mixing Process

The mixing in the chamber, if the floor heater is not in use, is provided by

natural convection due to small temperature gradients across the chamber

(mostly < 0.3 K), by diffusion and to a certain extent by the advection generated by the

gas flow during the inlet. For this case (Figure 3, upper panel) complete mixing,

indicated by equal CO2 mixing ratios at aIl (three) ports, was observed after eight hours.

But, equal CO2 levels were reached already within four hours at those 2 ports which

were distant from the direct inlet region (compare Table 4). This finding is confirmed by

on-line FTIR measurements, which integrate the COz concentration across the chamber

length, and also show a stable COz concentration after 4 hours.

The temperature gradient of -2 Klm in experiments D and E introduces an

enhanced convection in the chamber. The result of experiment D is shown Figure 3

(boUom panel). The result of experiment E, in which a different inlet port was used, are

comparable to those of experiment D (see Table 4). As can be seen in the Figure 3 and

Table 4, the time for mixing is c1early reduced. Complete mixing, i. e. equal COz

22

E 700 c. C. -0 -CU 600 ... r:n c .~ E 500

N 0 0

400

·1 o 1 2

800

lower front corner

600

400

..0.0 - 004

o : floor heater at 30° C

345

duration I h 6 7 8

FIGURE 3 Experiment C and D: Convective Mixing in the Reaction Chamber.

9

CO2 rnixing ratio measured at three ports in the reaction chamber. Time = 0 is the beginning of the CO2

inlet. Top Panel: "Cold" reference experiment C. After about 8 h equal CO2 levels are measured at all

three ports which indicates complete rnixing. The same CO2 concentration is measured at two of three

ports already after 4 h. The inlet port is deviating. Bottom Panel: Experiment D with floor heating and a

vertical temperature gradient of -0.2 Klm. Complete rnixing, i. e. equal CO2 levels at all three ports is now

observed after 4 h. Equal CO2 concentrations at two of three ports are obtained already after 30 rnin.

Again, the inlet port is deviating. The small inset shows the initial damped oscillation of the CO2

concentration, measured at the right lower front corner, which is furthest away from the inlet corner.

23

mixing ratios at all ports (3 or 7 ports and on-line FTIR), is now obtained within 4 hours

which is a factor of two faster than in the "cold" reference experiment C. However,

homogeneity with exception of the direct inlet corner, i.e. equal CO2 levels at 2 or 6

ports and on-line FTIR is reached already within 30 min., areduction of a factor of 8.

The C02 concentration measured at the corner furthest away from the inlet

corner shows a damped oscillation with a 3 min. time constant as is shown in the inset

of Figure 3. The oscillation flattens out within 20 min. The observation can be repeated,

if inlet and measuring point are exchanged. This indicates that a trace gas parcel initially

needs 3 min. to circulate once through the chamber.

Lifetime of 0 3

The results of the 03 lifetime measurements F - J in the reaction chamber are

demonstrated in Figure 4 and Table 5. Also shown in Figure 4 are linear least square fits

to the logarithmic concentration data as a function of time. The slopes of the fits are

given in Table 5. Since the logarithmic data are fitted weIl by the straight lines, the 0 3

lifetimes are given by the reciprocal slopes of the fits. The experiments Fand G, which

were performed before any experiments with nitrogen oxides, yielded lifetimes 't03

about 20 days, independently, whether the double wall purging system was off or on.

The 0 3 lifetime measurement H, which was conducted shortly after an previous

experiment with nitrogen oxides, yielded a reduced lifetime of 10 days (Figure 4, open

squares). The measurement J, which was performed a few weeks after experiments with

nitrogen oxides, yields again a longer lifetime of 17 days. The mean 0 3 lifetime in the

reaction chamber is about 15(±5) days.

24

10.0 f f f f

5 F: first 120 h of 336 h (no double wall purging)

E a. 3

a. - 2

0 - G ca ... 1.0 - H

I-C)

s:::::

>< E 5

J : first 120 h of 240 h

C"')

0 3

2

0.1 f f f I

o 24 48 72 96 120

duration I h

FIGURE 4 Experiment F - J: 03lifetime measurements in the reaction chamber.

The 0 3 mixing ratio is shown on logarithmic scale as function of time (dots). The

straight full lines are least square fits to the data. During experiment F the double wall

purging was not in use. Experiment Fand G were performed before any nitrogen oxides

were introduced into the chamber. Experiment Hand J were performed three days,

respectively six weeks after previous experiments with nitrogen oxides. The 0 3 lifetimes

calculated from the slope of the fits were 20 d (F), 19 d (G), 12.5 d (H), and 17 d (J).

25

N02 Loss in the Reaction Chamber

The results of the experiments K - M are summarized in Table 6. After 2 days

15% of the N02 is lost in the dry case K while 11-12% are lost in the humid cases Land

M. The decay of N02 in the chamber for all the three humidities is shown in Figure 5.

Here, the natural logarithm of the N02 mixing ratio is plotted as a function of time

together with the linear least square fits. The slopes of the fits are given in Table 6. The

fastest loss occurs at low humidity, while the k10ss at high humidities are smaller by a

factor of ab out 0.75. Since the logarithtnic data are fitted reasonably weIl by straight

lines, probably a process of first order in N02 is the dominant loss channel. The lifetime

calculated from the reciprocal slope of the fits are 12.5 days, 17 days, and 16 days,

respectively. The mean loss rate coefficient for all three experiments is 8*10-7 S-1

(±1.2*10-7 S-I), which corresponds to a mean N02lifetime of 14.5 days.

During the N02 decay in the chamber, the HN03 concentration never exceeded

the detection limit of 40 ppb, i.e. it remained at least a factor of 30 smaller than the N02

mixing ratio. In all three experiments non-zero NO mixing ratios were measured (see

Table 6). The NO mixing ratio remained either constant or showed a slight increase

during the experiments. In experiment L the HN02 concentration increased by 10 ppb

over a time period of 3 days. Due to the limitations of the denuder method, the observed

HN02 concentrations have to be regarded as upper limits (Benning 1994).

The rate coefficient for total N02 loss (k1oss) may include los ses to the wall

(kN02) as weIl as chemical los ses in the gas-phase. Since enhanced N02 start

concentrations of about 1.4 ppm are used in this study, one potential loss channel of

N02 could be the formation of N20 4 (Rfl/Rbl) and its subsequent re action with H20

(R2) (England and Corcoran 1974):

26

-E a. a. ----C\I o Z ........ -

0.3

0.2

c 0.1

0.0

-E 0.3 a. a. ----C\I 0.2

0 Z ........ -c 0.1

K: 445 ppm H20

L: 11570 ppm H20

o 12 24 36 48 60

duration I h

FIGURE 5 Experiments K - M: N02 deeay.

72

Deeay rate of N02 in the reaetion eh amber at three humidities. Dots: In([N021/ppm) as a funetion of time.

Straight lines: linear least square fits with the slopes: K: -9.24(±O.16)* 10.7 S·I, L: -6.81(±O.08)* 10.7 sol, M:

-7. l1(±O. 17)* 1007 sol.

27

N02 +N02 +M

N20 4 +H20

=

->

(Rfl/Rb1)

(R2)

For the rate coefficient of (R2) only a upper limit (8.72*10-19 cm3molecule-1s-1 at 292 K)

is known (England and Corcoran 1974).

The expected N02 decay was calculated using the Troe rate coefficient

expressions for kn (2.41*10-14 cm-3 molecule-1 S-1 at 292 K) and kbl (5.49*104 S-1 at

293 K) given by Borrell et al. (1988) and k2 = 9.96*1O-1O*e-6090rr cm3 molecule-ls-1 given

by England and Corcoran (1974). In the calculations we accounted for the walllosses of

HN03 using loss rate coefficients between 3*10-6 S-1 and 5*10-5 S-1 (see below, compare

Table lOb). The wallioss of N02 was set to zero, i. e. we restricted the N02 loss to

(R2). Using these rate coefficients the ca1culated N02 losses for the conditions of the

humid experiments L and M are to large by a factor of 6 compared to those observed.

The calculations predict further a formation of HN03 up to 120 ppb, which is also in

c1ear contradiction to the experimental observations. On the other hand, for the low

humidity experiment K the N0210ss is underpredicted by a factor of 3.

If k2 is varied to reproduce the experimental data for experiment M

(15285 ppm H20 I 292 K), k2 is reduced to 6.7*10-20 cm3 molecule-l S-I. Since it was

assumed that the N02 10ss occurs exc1usively by (R2), the determined k2 may inc1ude a

contribution from N02 walllosses and therefore is an upper limit. With a k2 of 6.7*10-20

cm3 molecule-1 S-1 the loss path via (Rlf/R1b) and (R2) becomes unimportant in

experiment K (445 ppm H20). The overallioss of N02 in the dry case is apparently

28

dominated by wall losses, i.e. k10ss "'" kN02. Since the N02 loss rates in all three

experiments are about equal, it is reasonably to assurne that the k10ss of the experiments

at high humidity, too, represent essentially walllosses. If this is true, k2 has to be much

smaller than 6.7*10-20 cm3 molecule- I S-I. With the experimentally determined kN02

(Table 6) and a k2 of 6.3*10-21 cm3 molecule-I S-1 the experimental data for all three H20

mixing ratios are reproduced within an error of better than 1 %. This new upper limit k2

is smaller by more than 2 orders of magnitude than the value of England and Corcoran

(1974) at 292 K.

Wall Losses ofHN03

The results of the measurements of HN03 losses from the gas-phase N - P are

compiled in Table 7. In Figure 6 the mixing ratios of HN03 are plotted as a function of

time together with linear least square fits to the logarithmic data for all three humidities.

Zero time was chosen as the moment when the mixing ratios of all other observed NOy

species were below their detection limit. The log data are approximated relatively weH

by straight lines, which indicates a first order process as a dominant HN03 loss channel.

However, deviations from a linear behavior in experiment 0 (Figure 6, mid panel) and

even more distinct in the experiment P (Figure 6, bottom panel) indicate that the HN03

loss in the chamber is probably a more complex process. The approximate first order

waHloss rate coefficients kHN03 of HN03 wh ich were calculated from the slope of the

linear fits to the log data, increase non-linearly with increasing H20 concentration

(Table 7).

29

1.00

E N : 2215 ppm H20 c.. c.. 5 -0 - 3 CU :r... C)

2 C

>< E

0.10

1.00

E 0: 4875 ppm H20 c.. c.. 5 -0 - 3 CU :r... C)

2 C

>< E

0.10

1.00

E 5

c.. 3 c.. P: 14380 ppm H20

- 2 0 - 0.10 CU :r... C) C 5 • >< 3 • E 2

0.01

0 12 24 36 48 60 72

duration I h

FIGURE 6 Experiments N - P: HN03 walllosses.

Wall losses of HN03 in the reaction chamber at three humidities The HN03 mixing ratio is plotted on a

logarithmic scale as a function of time (dots). The straight fulllines are least square fits to the data. The

scatter of the lowest three data points in the bottom panel is due to the fact that the mixing ratio is ab out

equal the detection limit of 0.04 ppm. The wall loss rates kHN03 (lifetimes 'tHN03) are N: 3.12*10.6 S·I

(89 h); 0: 4.37*10.6 sol (64 h); P: 4.38*10-5 sol (6.3 h).

30

DISCUSSION

Reaetion Chamber

The reaetion ehamber presented here is one of the larger environmental

eh ambers for atmospherie ehemistry whieh have been deseribed in the literature. Its

surfaee to volume ratio of better than 1 rn-I is even more favorable than that of the

largest ehamber of 312 rn3 (Jeffries et al. , 1976), beeause their ehamber was divided into

to two halves of 156 m3. Sinee the reaetion eh amber is set up for the investigation of

nighttime ehemistry, some of the pitfalls reported for smog eh ambers like photolytieal

production of OH radicals from wall eontaminants or photoenhaneement of wall

reaetions (Akimoto et al. , 1987) are ofno importance.

The fully welded metal walls strongly diminish the eontaet of the permeable

PEP-bag with eontaminated ambient air. This is proven by the overalllifetime of CO2 in

the total system of more than one year. The clean air purged double wall system as

realized in our ehamber further reduees the influenee of wall eontamination and memory

effeets.

The reaetion ehamber is equipped with a high resolution FfIR speetrometer. In

eaeh wavenumber range broadband speetra with aresolution better than the pressure

broadening limit of single rotational lines at one atmosphere (in the order of magnitude

of 0.1 em-I in the mid-IR) ean be reeorded. This way e. g. the water speetrum in the mid­

IR will be fully resolved and the speetral windows between the water lines ean be

utilized for simultaneous deteetion of a set of traee gas moleeules.

31

Lifetime of Inert Gases and 0 3

Many of the atmospheric nighttime processes are slow compared to the

photolytical daytime reactions. Therefore, long reaction times are needed to observe

significant throughputs. The observed lifetimes of the inert gases CF4, CI4, CO2, N20,

and CO in the PEP-bag of more than 100 days show that chemical experiments of

typical 7 to 14 days duration will not be dominated by losses due to leakage or

permeation. From permeability data given by the manufacturer for Teflon one can

estimate that permeation through the PEP-film is the main loss mechanism and accounts

for 75% (+/-25%) of the totalloss. The factor 3 difference in the lifetimes for different

molecules, the shortest lifetime for the smallest molecule CO and the longest lifetime

for the biggest molecule CF 4 supports that the main loss mechanism is permeation but

not leakage. The reduction of memory effects by purging the double wall system

shortens the lifetime of inert gases by a factor of three, as measured for C02.

A sensitive test for the performance of smog chambers is the lifetime of 0 3 in

the dark. The lifetime of 0 3 in our reaction chamber varies between 10 and 20 days. The

actuallifetime is depending on the chemical history and wall condition of the chamber.

The mean lifetime of 15 days for 0 3 is significantly shorter than that for inert gases. The

double wall purging is of negligible influence on the 0 3 lifetime. This indicates that

chemicallosses in the gas-phase or on the chamber walls are the major loss channels of

0 3.

The 0 3 loss rates in the dark for several smog chambers are compared in

Table 8. The 0 3 lifetime in our reaction chamber is by far the longest. Even if the 0 3

32

t...l t...l

I

I:"J!L.. 11

Table 8 0 3 Dark Loss Rates in Several Smog Chambers

kloss(03) 1 S-l 't03 /h Initial M. R.I ppm Temperature 1 K Chamber Vol.1 m3 SIV 1 rn-I 't03*SIV 1 h-l rn-I Reference

1.1 * 10-5 - 4.1 * 10-6 25.1 - 68.5 0.1 - 0.2 287 - 295 0.45 8.3 210 - 570 a

5.7(+1-2.3)* 10-6 49 0.34 - 1.45 291 - 299 3.9 3.8 186 b

2.6* 10-5 - 3.2* 1 0-5 8.7 - 13.0 - - 5.8 3.4 30-44 e

1.1 * 10-5 - 2.3* 10-5 12.0 - 25.0 0.043 - 4.83 - 6.07 3.7 44- 93 d

1*10-5 28 - - 17.3 - - e

2.2(+1-1.5)* 10-6 111 0.01 - 1.28 291 - 299 80 1.4 155 b

>4.3*10-6 >65 - - 2 * 156 1.3 >84.5 f

5.8*10-7 - 1.2*10-6 240 - 480 1.4 - 8.0 290 - 296 250 1 240 - 480 this work

mean: 7.65*10-7 360 360

For easier eomparison apart of the values given in the literature were eonverted into suitable units of S-I for rate eoefficients kloss(03) and of h for

l/e lifetimes '1:03. Besides of the initial mixing ratio M. R. and of the temperature, the eh amber volumes and the ehambers volume/surfaee ratio SN

are given. In the 7th table eolumn 't03 (eolumn 2) is multiplied by SN (eolumn 6) to roughly eorreet for the different ehamber sizes.

Referenees: a) Kelly (1982); b) Grosjean (1985); e) Winer et al. (1979); d) Akimoto et al. (1979); e) Spieer (1983); f) Jeffries et al. (1976)

lifetimes are scaled by the smface to volume ratio SN of the chambers, the mean 0 3

lifetime in our re action chamber is by more than a factor of 2 longet than most of those

reported for the other chambers.

N02 Loss in the Reaction Chamber

The lifetimes ofN02 in the chamber of 12.5 to 17 days are comparable to those

found for 0 3, For N02 chemical losses on the walls seems to be the major loss channel.

Like earlier smog eh amber studies we found that the N02 decay is first order in N02.

The data base presented here is to scarce for conclusions on the dependence on water or

temperature. However, in agreement with Sakamaki et al. (1983), we are able to show

that the mechanism for the re action of N02 with H20 via N20 4 proposed by England

and Corcoran (1974) is unimportant. The rate coefficient given by England and

Corcoran (1974) for the reaction of N20 4 with H20 (R2) is more than four orders of

magnitude too large. There are indications that k2 is probably as small as a few times

10-23 cm3 molecule-I S-I. The findings of England and Corcoran (1974) for k2 may be

influenced by heterogeneous reactions on the glass walls of their small reactors.

In agreement with other smog chamber studies (Sakamaki et al. , 1983; Pitts et

al. , 1984; Killus and Whitten, 1990) the decay of N02 was not accompanied by a

significant formation of gas-phase HN03. NO mixing ratios of a few ppb were measured

in all three runs (Table 6), but they originated from impurities in the N02 gas mixture

and partly from the ambient air used to flush the chamber. The maximum NO increase

observed was 3 ppb during 70 hours: A formation of NO of 10 - 50 ppb/h described by

Sakamaki et al. (1983) could not be observed. However, the initial N02 concentrations

in the study of Sakamaki et al. (1983) were 3 to 14 times larger than in our experiments.

34

In the experiment L, in which HN02 was monitored, a slow HN02 formation

with a HN02 production rate 0.15 ppb/h was observed. The production rates of HN02 at

comparable H20 mixing ratios and N02 initial concentrations reported by Sakamaki et

al. (1983) and Pitts et al. (1984) were 6 ppb/h (initial value) and 18 ppb/h, one to two

orders of magnitude faster. The HN02IN02 dark conversion ratio of 0.04 measured in

experiment L is more than a factor of two smaller than the ratios of 0.1 to 0.5 reported

in previous smog chamber studies (Sakamaki et al. , 1983; Pitts et al. , 1984).

Overall, our results are consistent with the re action scheme proposed by Pitts et

al. (1984). They suggest a first order loss process of N02 to the reactor walls, followed

by a slow formation of HN02 and HN03 on the walls. While HN03 sticks to the wall

and does not return to the gas-phase, a part of the formed HN02 returns to the gas­

phase. The amount of HN02 which enters the gas-phase depends on the wall materials

and conditions.

Wall Losses of HN03

The deposition of HN03 on the chamber walls is one of the major loss channels

of nitrogen oxides from the gas-phase in smog chambers. It is assumed frequently that

HN03 once it is deposited or formed on the walls of Teflon chambers does not return to

the gas-phase (Sakamaki et al., 1983; Tuazon et al., 1983; Pitts et al., 1984), although

the opposite has been reported for at least one case (Atkinson et al., 1986).

In our experiments the loss rate coefficients kHN03 are strong functions of the

H20 mixing ratio. HN03 wall los ses as a function of the water concentration were

reported also for other smog chambers (Table 9). The lifetime of HN03 in our chamber

is larger by a factor of 7 - 16 at low humidities (experiment N) and by a factor of 4 - 5 at

35

Table 9 HN03 Wall Losses in Smog Chambers

H20 M.R. I ppm kHN03 I S-l

92 4.98*10-5

10200 2.31*10"4

20760 3.83*104

0.56 2.2*10-5

14800 1.7*104

1625 3.12*10-6

4620 4.37*10-6

14440 4.38*10-5

r.h. 50% - 55% 2.5*10-5

r.h. 13% - 65% 1.8*104

M. R. : mixing ratio

kHNo3 : wallloss rate eoeficient

'CHN03: l/e lifetime of HN03 SN: surfaee/volume ratio

'CHN031 h Temp./K

5.6

1.2 298

0.73

13 -1.7

89

64 292

6.3

11 291- 299

1.5 -

Chamber Vol. / m3 SN Im-l Reference

5.8 3.4 a

5.8 3.4 b

250 1 this work

3.9 3.8 e

17.3 - d

Referenees: a) Atkinson et al. (1986); b) Tuazon et al. (1983); e) Grosjean (1985); d) Spieer (1983)

36

the high humidities (experiment P) compared to that in the 5.8 m3 chamber (Tuazon et

al., 1983; Atkinson et al., 1986). If the results for the 5.8 m3 chamber are scaled by

surface/volume ratio, these factors are reduced to 2 - 5 and 1.1 - 1.5. While the kHN03

measured in the 5.8 m3 chamber scale almost linearly with the H20 mixing ratio, we

observe an increase of kHN03 in higher order than linear with increasing H20 mixing

ratio. This indicates that we observe an enhanced HN03 loss at high humidities

(experiment P)in our chamber. In experiment P (70% r.h.) we observed the formation

of aerosols from condensation nuc1ei,probably under condensation of water. Therefore

the enhanced HN03 loss at this H20 mixing ratio could be partly due to sticking of

HN03 to the wet aerosol surfaces.

Characteristic Times of the Reaction Chamber and Duration of Chemical Experiments

The performance of the reaction chamber in an chemical experiment is

demonstrated in Figure 7. As an example we chose the conversion of N02 and 0 3 to

N20 S and HN03 at 8 % relative humidity. We are able to provide weIl characterized

data sets for the observed chemical species over aperiod of 120 hours. (The atmospheric

aspects of the N02/03 system and a more detailed discussion including model

calculations will be addressed in a following paper.)

All characteristic times determined and discussed in this paper are summarized

in Table 10. The time scale of the chemical experiment of 120 hours (Table 10, 5th

entree) is c1early shorter than the lifetime of inert and reactive trace gases in the chamber

(Table 10, entrees 1 - 4). It is much longer than the mixing times and the time resolution

37

1.5

• [N205]

E o [HN03]

Q. 1.0 ... Sum[N] Q. - , 0 - '. ca ... C)

~ c >< 0.5 E

.~~&

0.0

o 24 48 72 96 120

duration I h

FIGURE7

Reaction of N02 with 0 3 at a H20 mixing ratio of 1620 ppm and a temperature of 292 K. About 1.2 ppm

N02 were filled into the chamber and equilibrated, then 1.35 ppm of 0 3 were added. Time = 0 is defined

by the addition of 0 3 to the system. Bottom panel: Decay of the mixing ratios of 0 3 (open squares) and

N02 (filled dots) as function of time. Top panel: Formation and decay of N20 S (filled squares) and HN03

(open diamonds), and the decay of the Sum[N02 + 2*N20 S + HN03] (filled triangles) as a function of

time.

38

Table 10 Characteristic Times of the Reaction Chamber at KFA Jülich

Process ! Operation Substance! Quantity Characteristic Time

Standing Time Total System CO2 > 1 year PEP-bag + Aluminum box

Lifetime Inert Gases, CF4, C~, N20, CO2, CO 335 - 125 days Double Wall Purging Off

Lifetime Inert Gases, CO2 45 days Double Wall Purging On

Lifetime Reactive Gases 03, N02 12 -20 days

NOz ! 0 3 Nighttime Chemistry N20 5,HN03 120 hours at 8% r.h. and 1 ppm Initial Level

Wall Deposition, f(r.h.) HN03 6 - 89 hours

Convectional Mixing CO2 20- 30min. Temperature Gradient -0.2 Klm

Time Resolution FTIR N02, N20 5, HN03 5 - 6 min.

Spectral Res. 0.1 cm-1, 36 Scans

Circulation of a Trace Gas Parcel CO2 3 min.

Typical Time Resolution 0 3, NO, CO2 ,CN, Aerosol 1 min. Analytical Instruments

Maximum Time Resolution Data T, p, ~p, r.h. 1 sec. Acquisition for 64 Channels

39

of the analytical instrumentation (Table 10, entries 7 -11). From the comparison of the

time scale of the chemical experiment (Table 10, 5th entry) and of the respective HN03

loss of 89 h (Table 10, 6th entree) it becomes also clear that the removal of NOy via

HN03 from the gas-phase is an important factor which controls the duration of chemical

experiments with nitrogen oxides.

CONCLUSION

A large reaction chamber designed for the investigation of atmospheric

nighttime processes has been constructed. The features of the reaction chamber are a

favorable surface/volume ratio, clean air purged double wall system with inert inside

and gas-tight outside walls, and a non-mechanical mixing system. The long lifetimes

measured for inert gases and for 0 3 and N02 enable chemical experiments of several

days duration. A slow HN02 formation rate and a negligible production of NO and

HN03 as observed during the decay of N02 in humid air are indicative for small wall

effects in our reaction chamber. A new upper limit of the rate coefficient k2 of 2* 10-23

cm3 molecule-1 S-1 for reaction of N20 4 with H20 has been determined.

Long experimental durations and small wall effects are aprerequisite for

successful future experiments at low concentration levels, which are representative for

important parts of the troposphere. A further need of such experiments is the

improvement of the detection limit of the FTIR measurement. The current detection

limit of the order of 10 ppb for nitrogen oxides is restricted by the present maximum

40

available light path of 15 m. An in-chamber multireflection system will provide path

lengths up to 720 m, an improvement of a factor of 48. Studies at low mixing ratios will

also help to determine the minimum mixing ratio, at which the chamber specific

pro ces ses wi11limit the applicability of chamber results to the natural atmosphere.

41

REFERENCES

Akimoto H., Hoshino M., Inoue G., Sakamaki F., Washida N., and Okuda M. (1979)

Design and characterization of the evacuable and bakable photochemical smog

chamber. Environ. Sei. Techn. 13,471-475.

Akimoto H., Takagi H., Sakamaki F. (1987) Photoenhancement of the nitrous acid

formation in the surface reaction of nitrogen dioxide and water vapor: extra radical

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Jül-3196 Februar 1996 ISSN 0944-2952