The Modern Student lcrborotoru Chemistry and the Environment
Nitrogen Dioxide and Its Determination in the Atmosphere A Simple Method for Surveying Ambient Pollution Concentrations David Shooter University of Auckland, Auckland, New Zealand
Students are frequently interested in determining pollutant concentrations in the atmosphere. However expensive instruments are often required, making it difficult to incorporate the analyses into a n undergraduate laboratory course. The increasing use of passive-diffusion tube samplers for the measurement of nitrogen dioxide (NO21 and other trace gases provides an inexpensive means of introducing students to air-pollution detection and measurement.
where M is an inert body, such as a nitrogen molecule, that receives excess energy. The atmospheric reactions of NO and HN03 play an important role in the determination of the concentration of NOz. The important chemical relationships between these three nitrogen gases in the atmosphere are summarized in Figure 1.
Nitrogen Dioxide Atmospheric Chemistry
Concentrations of N@ in the Atmosphere NO9 concentrations have been measured in a wide variety ofenvironments in the troposphere, ranging from the Arctic to domestic kitchens. Many of these measurements were made using passive samplers. Concentrations have varied h m 0.5 ppb in the Arctic (1) to 100 ppb in enclosed spaces, such as a railway locomotive repair shop (2)and kitchens (3). Wide variations in local NOz levels were observed by Brauer et al. (4). who found concentrations rangmg fmm 27 to 231 ppb in two heated research homes in Chicago and Maryland. NO2 concentrations have also been measured inside motor vehicles (5).With an ambient concentration at the 24-ppb level, the following levels were found in the UK.
NO, is a major pollutant in the atmosphere of modern cities, and concern over its health effects and impad on atmospheric chemistry has lead to much research. Although small amounts of N02are emitted directly into the atmosphere, it arises mostly though the oxidation of nitric oxide (NO). NO is produced naturally by microbial activity in soils, and anthropogenically by fossil fuel combustion. I t is also produced in significant quantities by lightning. Formation and Destruction One of the major oxidation routes of NO in the atmosphere is the following reaction with ozone. NO + 0, + NO, + 0, (1)
A second route for the production of NO, is the reaction of NO with peroxy radicals, which are atmospheric radicals arising from the hydrocarbons in the atmosphere. NO + RO, + NO, + RO (2)
+ hu -t NO + 0
(3)
J NO
(4)
Concentrations measured in outdoor central city areas were found to be
+ o3 + NO2 + o2
NO + R02 + NO2 + RO
With equal rates of formation and destruction of NO,, eqs 1-4 describe a "photostationary" state. Nonreversible Removal
car8
Inputs fmm combustion, tanspon, lightening, etc
@
0+O2'O3
minibuses
-
These two reactions are opposed by the action of sunlight, which dissociates NO,, thus regenerating NO and ozone. NO,
buses
41 P P ~ 51 P P ~ 69 P P ~
2NO + O2
0
+
NO
+
+
h"
2N02
+
NO2
4
OH
+
NO2
+
NO2
+
OH
C
HN03 hv
+
HN03
4
A second and essentially nonreDepo~itionto Wadout by earths surlace Precipitation versible removal process for NO2 during the day is its oxidation by hydroxy radicals to form nitric acid, with subsequent removal of Figure 1. Major sources and sinks and the chemical relations between nitric oxide, nitrogen dioxide, the acid by rainfall. and nitric acid in the atmosphere. (Continued on page A13n Volume 70 Number 5 May 1993 A133
The Modern Student Laboratory: Chemistry and the Environment 30 P P ~ 50 P P ~ 45 P P ~
Laneaster UK (6) London, UK (7) Harlem, NY (8)
Urban values in the vicinity of 15 ppb have been observed (6). Rural concentrations of NO2 reflect population levels and the degree of industrialization. Northern hemisphere concentrations in the range 5-8 ppb have been found (6,9),whereas in the southern bemisphere levels of only 1ppb have been observed (10). Seasonal variations have also been detected worldwide, with winterlsummer ratios varying from 0.8 to 3.6 ppb (10). In contrast to tropospheric concentrations, the amount of NO2 in the stack exhaust gas of a 1200-MW coal-fired power station has been quoted at 2,000 ppb, whereas the concentration of NO. he., [NOzl + [NO11 was stated to be 450,000 ppb (11). N(h Exposure Limits
NOzis a respiratory irritant, and in recent years concern has grown particularly in Europe about NOz levels in the human environment. In response, in 1987 the World Health Organization issued guidelines (12) for human exposure to NOz. They recommended that the l-h average should not exceed 210 ppb, and the 2 4 h average should not exceed 80 ppb. The occupational exposure limit for the working envimnment was also set by the World Health Organization in 1989 at 3,000 ppb (13). It was argued that exposure to such a concentration for five days per week, 8 hlday, is unlikely to have an adverse effect on a healthy person.
These tubes are often called passive samplers because they rely on the molecular diffusion of a gas alone a tube to an absorber. The method was first deveioped for-use in indusdesimed trial environments (14). with the tubes ori~hally to be attached to the clothing of workers. Atkins used diffusion tube samplers to measure concentrations of NO2in homes that use gas and electricity (151, and later evaluated them for outdoor use in urban euvimnments (16). Since then, d f i s i o n tube samplers have been evaluated and used extensively for remote outdoor measurements of NOz(l, 10,17). Constructionand Theory
A diffusion sampler consists of a tube closed at one end. Two stainless steel meshes coated with tliethanolamine (TEA), an absorber of NOz, are held in place at the closed end of the tube (see Fig. 2). The TEA-coated mesh absorbs NOz, establishing a concentration gradient along the length of the tube. The diffusion of a gas through air is given by Fick's Law.
where F is the flux of gas; D is the diffision coefficient of the gas through air (mol cm4); and z is the length of diffision (cm). The quantity of gas transferred (Qmoles) in t seconds for a cylinder of radius r is given by eqs 7 and 8. Q =F(+
mol
Thus,
Diffusion Tubes
There are a number of instrumental methods for detection of NOz in the atmosphere, but a simple method has recently been developed using diffusion samplers (1, 14). red polyethylene cap
Q=
-D(C - C,)&t
mol
where (C - Co)is the concentration gradient along the cylinder length z. If an efficient absorber is used to remove the gas, then Coeffectively becomes zero. For the collection of NOz with diffusion tube samplers with a typical tube length of 7.1 cm and an internal radius of 0.55 cm, the number of moles of NO2 collected is given by
Hence the total amount that accumulates on the TEA can be written as Q = -72Ct
(10)
where C is the concentration of NOzin the ambient air (mol em3); and a value of 0.154 cm2s-' is used for the diffusion coefficient for NOzin air (14). Therefore, in 1h (3600 s) the diffusiontube absorbs 72C mol of NO2 (eq 10) or absorbs the NO2 from 72 cm3 of airh. The negative sign reflects the fact that the flux is from high to low concentrations. Materials
white polyethylene cap (removed to expose absorbing solution to the atmosphere)
Figure 2. Exploded view of a diffusion tube sampler.
lhbes, Caps, and Stainless Steel Meshes Although the diffision tubes can be purchased readymade, it is more interesting when students make their own. This preparation is a relatively simple and rewarding task. Tubes, caps, and stainless steel meshes (or ready(Continued on next page) Volume 70 Number 5 May 1993 A137
The Modern Student Laboratory: Chemistry and the environment made tubes) can be purchased (Gradko International, 77 Wales St, Winchester, England). The polyethylene caps should be exposed outdoors only once because with repeated use they tend to split or stretch. However, the acrylic tubes and stainless steel meshes can be reused. Chemicals The following chemicals are needed for the preparation of the diffusion tubes and for the spectrophotometric analysis of the nitrite in the exposed tubes. +TEAfor absorption of the NOz (analytical grade) 'Brij-35 (polyoxyethylenelauryl ether)wettingagent for TEA (laboratoryreagent grade) N-l-naphthylethylene-diamine dihydrochloride (NEDA) (analargrade) orthophosphoric acid (analargrade) sulfanilamide (reagent grade) sodium nitrite for preparation of nitrite standards (analar grade)
The Solutions Mesh Coating Solution: 20% vlv aqueous solution of TEA, containing 80 pL of 10% Brij-35 solution/lOmL. Analysis Solution 1: 2% wlv aquwus solution of sulfanilamide in 6% v/v orthophosphoric acid. (Prepare 250 mL of aqueous solution containing 5 g of sulfanilamide and 15 mL of concentrated orthophosphoric / CH2CH2OH NO2 acid.) N-CH2CH20H Analysis Solution 2: 0 . 14 % \ CH2CH20H WIWaqueous solution of NEDA. ( P r e ~ a r e250 mL of aauwus solution containing 0.35 'g NEDA.) Triethanolamine (TEA) Tube Preparation Before assemblv. the individual com~onentsof the samplers should be thoroughly washed in detergent solution and then rinsed and soaked in hieh-our it^ water. Contamination of the tube ~ o m ~ o n e n t s ~ ~ ~ r tthe ic~ acrylic larl~ body, with the relatively high levels of NOzin a laboratory can be avoided by storing the tube components under water until just before sampler preparation. The two stainless steel meshes are placed in a red cap a n d coated with 40 @ of absorber solution using a micropipette. Immediately after coating the meshes, the samplers should be assembled (both caps) to reduce contamination by NOz from the surrounding air. If the tubes can't be used immediately, they should be stored in a plastic bag in a freezer.
Analysis for Nitrite on the Meshes The nitrite accumulated is analyzed by spectrophotometly. The exposed samplers are placed in a rack, with the red cap and meshes and with the colorless caps removed. Then 0,7 mL of water is added, followed by 0.7 mL of solution 1and 0.1 mL of solution 2. The sampleis are recapped immediately, shaken, and allowed to stand for approximately 15 min for color development. Then the contents of the tubes are transferred to a lowvolume 4-m cell, and the absorbance is measured at 540 nm. If some loss of sensitivity is acceptable, 1-cm2cells can be used. A suitable calibration curve can be prepared using nitrite standards of 0.2,0.4, 1.0, 4.0, and 10 nmol L-'. Chemistry of the Absorption The chemistry of the absorption of NO2 by TEA and its conversion to NOI has recently been clarified. Aoyama and Yashiro (19) have provided convincing evidence t h a t nitrosodiethanolamine (NDELA)is the product of the reaction between NO, and triethanolamine. It was also shown that an identical product was produced by the reaction hetween NO; and DEA (see below).
-
.
a
CH2CH20H / N-CH2CH20H
NO,-
CH,CH20H / VCH2CH,0H
\NO
H
Nitrosodiethanolamine (NDELA)
Diethanolamine PEA)
Nitrosamines are easily hydrolyzed to form nitrite
The initial step in the color-formation process is the diazotization of the sulfanilamide by nitrite.
-
&N
Use of the Diffusion Tube
Tubes should be held vertically with the red cap uppermost and the colorless cap removed. They can be held in place on a stake or building with either reusable adhesive or a terry-clip. To accumulate sufficient nitrite, exposure times should be 3-4 days in urban areas or 2 weeks in a rural area. The exposure period is ended by replacing the colorless cap. Some workers have chosen to expose two or more tubes a t each monitoring site in order to assess their on-site reliability and to minimize data loss by insect invasion or accident (1,17,18). A138
Journal of Chemical Education
The diazonium salt formed then couples with the NEDA to form a purple azo dye (as shown in the reaction a t the top of the next page). Early work done on the spectrophotometric detection of nitrite (20) yielded a n extinction coefficient of 50,000 for this dye. An alternative to the above method of NO, absorption and nitrite analysis, which could be investigated for use in passive samplers, has recently been provided by Kaveeshwar et al. (21).
Volume mixing ratio (ppm) = 3 em3
A v x 1 ~m3 6~xz~.sx ma^ 10
$ ~ ( C H ~ ) ~ - - N H2c1~+
4+&)-g +
+
NHr(CH2),NH3+
2Cl-
SVH2
S%NH2 Calculation of the Atmospheric NCh Concentration The average air concentration of NOz required to yield the nitrite collected by the sampler can be calculated as explained below. The Procedure In 1h a diffusion tube samples 72 cm9 of air. Thus, the
TEA absorbs NOz from the following volume V over an exposure period.
ppb = lo3 ppm Accuracy of the Difision Samplers The accuracy of these samplers has been evaluated by comparison with three other NOzmeasurement techniques: conAzo Dye ventional chemiluminescence, Differential Optical Absorption Spectroscopy, and the European Monitoring and Evaluation Program network standard method TGS-ANSA (22). The diffusion samplers agreed within 15%in this comparison over a concentration range of 1to 12 ppb NOz. We have obsemed (10) that simultaneous exposure of 20 samplers gave a percentage standard deviation of approximately lo%, and reuse of the tubes did not affect performance. Detection Limit The detection limit for the spectrophotometric method suggested here is 10 ppb nitrite, equivalent to an average NO2 concentration of 0.2 ppb during a one-week exposure (i.e,, approldmately 30 ppb hour), Asimilar detection limit was found by Miller (23)using ion chromatography. 10
v=72cm3h"xt
: E
where t is expressed in hours. For the cell suggested above, the number of moles in this spectrophotometer cell will be concentration (mol L-') x volume (0.0015 L)
> a
-E
.
(.I
-. e
E m
-
I
1000
$000
4000
DIst.nD.)m
highway3. (State Figure Nlrogen Highway dioxide1, concentration near Huntly.near Waikato a major Valley, NewNorth Zealand island).(a)The variation over a3-km stretch of the highway. The tubes were ail placed within 5 m ofthe edge of the highway. (b) The &pendence on distance from the highway.
,
10
,
