Formation and Degradation of Aerocolloids by Ultraviolet Radiation Alexander Goetzl and Olgierd J. Klejnot Atmospheric Research Group, Altadena, Calif. 91001
Formation of aerocolloids from traces of hydrocarbons in air occurs in a steady airflow exposed for 6-60 sec t o the light of quartz ultraviolet lamps, which generates ozone oxidant. The same oxidant forms aerocolloid in darkness with such reactive hydrocarbons as turpentine and cyclohexene. Enhancing factors are: ultraviolet light, humidity, nucleating matter, and ammonia. The yields are reduced on adding ammonia in the dark and o n shifting the humidity by mixing of airstreams. Degradation of airborne aerocolloid by ozone with short-wavelength uv light is observed, after a peak of formation, a t lower humidities of 5 5 0 z rh. The ultraviolet degradability of impactor deposits depends o n formation conditions and nature of the hydrocarbon.
T
he finding that aerocolloids are formed in flows of atmospheric air exposed t o ultraviolet light for 6-60 sec has led us t o the development of a new instrument, the ultraviolet cascade, useful in studying the phenomenon, tracing natural hydrocarbon patterns, and detecting contaminants in the atmosphere (Goetz and Klejnot, 1967). This paper deals with the effects of flow rate, exposure, and humidity on the yields of oxidant and aerocolloid. Also, the effects of nucleating matter, ammonia, structure of hydrocarbons, and humidity shift are discussed. Procedure Air Control. F o r laboratory studies, air from a compressedair system was subjected t o molecular filtration, after which no condensation nuclei were found with a n Aitken counter. Ultraviolet irradiation of this air gave no indication with a Royco aerosol photometer, but the nuclei count became too high to measure. The air was released a t 5 psia and passed through a charcoal trap (metal can), a set of molecular membranes, a valve with flowmeter, Fl, and the humidity control assembly, to the ultraviolet cascade (Figure 1). A small branch flow in the hydrocarbon dilutor (F2-F5,U, X)was charged with saturated vapor of hydrocarbon. A n excess of the air over the suction of the moving-slide impactor (MSZ) and the Royco photometer (R)was blown off a t Z . For field studies, ambient air was sucked in through t h e humidifier and ultraviolet cascade by the sensor device (MSZ or Royco), and the bypass ( Z ) was closed. The exposures were determined by the branching of the flow. A Hypercon flowmeter (by Goetz) was used t o measure the suction by a compensation method, which avoids flow restrictions. The MSZ and the sampling of natural aerocolloids are described in detail elsewhere (Goetz, 1969).
l
Deceased, January 12, 1970. Present address: 3024 Zane Grey Terrace, Altadena, Calif. 91001.
Humidity Control. The sampled airstream passed through the humidity control assembly, consisting of dry and wet tubes, heating pads, and a mixing valve. Four horizontal tubes were stacked in pairs, the empty dry ones atop the wet tubes. The latter were lined with filter paper that soaked up distilled water standing a t the bottom. The filter paper was impregnated with silver hydroxide t o make it permanently antiseptic without inducing any gas traces into the airstream. Heating pads on the wet tubes could be switched off or operated a t a low or high rate of heating. Thus, a t high flow rates, any required humidities could be maintained by compensating for evaporative cooling. The mixing valve assured a continuous adjustment of humidity by mixing original air and humidified air in any desired ratio. A slide superimposed two pairs of round openings of ‘ 1 2 in. diameter each, opening equal half areas in the center position, shifting either way off-center, and closing the one or the other passage in each extreme position while maintaining a constant total openiw. The adjusted air mixture passed a hygrometer, placed in an airtight box with window and slide valve, which permitted bypassing the hygrometer box during impaction of aerocolloids. Additives Control. Reactants like hydrocarbons, ammonia, and sulfur dioxide were added after humidification, to avoid their ab$orption and subsequent desorption in the humidifier tube. I n the HC dilutor (Figure 1) with the two dilution stages, X and Y , and charging trap U (test tube 300 X 30 mm), a small branch flow was saturated with HC vapors a t room temperature. Part of the mixture was wasted at F;. Gases were introduced directly through flowmeter Fr. The gaseous reagents-methane, acetylene, ethylene, rrans-butene-2-were CP-grade chemicals in lecture bottles (Matheson). Propane was a commercial mixture of propane and butane (liquid gas). All liquid HC samples were of reagent grade (Matheson, Eastman, Baker).
..
C
D
._
4
M 5I
R
Z
Figure 1. Flow pattern of humidifier, hydrocarbon dilutor, ultraviolet irradiation cascade, and sensors Humidifier chambers (wet, drj); humidifier mixing valve a, 6 ; blpass to impactor c ; dilutor flowmeters Fg, F1, Fa; hydrocarbon charger U ; bleeder cock F a ; mixing stages X, Y ; ultraviolet cascade stages A , B , C, D ; moving-slide impactor M S I ; Rog co aerosol photometer R ; main flowmeter F1;bypass2 Volume 6, Number 2, February 1972 143
Ammonia was added by passing the main airstream over a small cylindrical dish (14 mm id., 8 mm deep, 1.5 cm2surface) with ammonia solution, that was placed in a glass chamber with bypass by a three-way cock. The solutions were dilutions of concentrated NHI (25x) inratios of 1/50,1/100, and 1/500. The ammonia concentrations produced in the air passing the evaporation chamber were estimated by visual comparison of test tubes containing Nessler’s reagent with standard quantities of ammonia, and Nessler’s reagent through which 20 or 60 liters of the air were bubbled at 2 l./min. The following results were obtained:
BZ
Dilution 1/50 1\50 ljl00 l/lOO
Ppm NH, in air 11.0 2.60 1.69 0.66
80 20
80 22
At 0.66 ppm NH8, 20 liters of air carry 0.01 mg of NHI. Apparently, more ammonia evaporates into the humid air than into the dry air. The Ultraviolet Caiscade. The present method of ultraviolet irradiation treats a stf :ady flow of the air sample in a “cascade’ of 1-liter cylinders, f itted with mercury arcs of quartz glasS . _..* (PCQ 90-1, Ultra-Violet rrooucts, mc., >an crauriei, Lain., 34 cm long, 8 mm ad.). The four cylinders mounted vertically in a water tank, an oscillating-type water pump, high voltage transformers, humidifier, hygrometer, and control panel are contained in a portable rack. The four cylinders (chromiumplated and polished inside) are connected in series and fractional flows may be withdrawn after each cylinder (irradiation stage). In the front view of Figure 2, the ultraviolet lamps are on the right in their water tank. On the left are the humidifier with hygrometer and switchboard for the lamp transformers, water pump, and humidifier heating tape. At upper left are the transformer switches for various lamp arrangements. Flows and Exposure Time. The ultraviolet cascade renders identical, optimal flow conditions in each stage. The sampled airflow enters tangentially topside and leaves centrally at the enters the oxi.dant layer formed near reds a mixing of hydrocarbon and 1
- .
I
r
^...
oxidant where the energetic fraction of the light is available (Figure 3). The constant total flow F , was usually adjusted to exceed slightly the suction requirements of the four-slit MSI and Royco photometer (note bubbler 2, Figure 1). It is necessary to keep the conditions constant for several minutes before an impaction, to avoid drifting of the formation of aerocollnid. As the fractional flows are equal, their light exposure in each stage increases in a nearly exponential series. The mean exposure can be varied between a few seconds and 1 min by the flow rate, number of stages, and flow branching. Longer exposures (12S240 sec) are obtained with a constant flowthrough the stages or by adding mnre stages. The exposure time follows from t = V/F in minutes when stage volume V i s 1 liter and F is given in l./min, or in seconds from t = 60/F under the same conditions. Temperature Control. The temperature was held at about 21°C by circulating the water of the water jacket and an external volume of equal size. Aerocolloid Evaluation. The evaluation of the aerocolloid concentration at each stage of the ultraviolet cascade was done with an aerosol photometer, the intake of which was shifted from stage to stage, and by microscope photometry of deposits impacted from selected stages in the moving-slide impactor (Goetz, 1969). Both methods depend on lightscattering by particles of the size 0.1-1.0 bm, called Sn and S (S, obtained with the Royco instrument). The units of S are arbitrary, self-consistent units obtained from readings of a five-decade microamperemeter (divided by loz) and standardized to a certain albedo S of a certain ground glass standard slide by balancing the photocell, microscope, and objective (Ultrapak 6.5 Leitz-Wetzlar) against the lighting system. The low-angle darkfield technique applied measures light scattered at a 90’ angle, whose intensity depends on the adjustment of the light source (Goetz, 1969). The Royco aerosol photometer Model 230 was equipped with an intake gas cock and with a bypass cock to the pump. This avoided the usual rotary port with rubber rings, and gave two settings for the intake suction at 1 or 2 I./min. The moving-slide impactor (MSO employed as the main
Figure 3. Schematic vertical cut through the irradiation stage umt Figure 2. Ultraviolet cascade, front view 144 Environmental Science &Technology
The airflow rotates (arrows) around the central quartz tube and leaves at center of the unit’slower end
To VACUUM
PUMP
&
IMPACTOR NOZZLE. SLITS
SLIDE MOTION I DLI NG
IMPACTION
POS.l A OR 2 A
SLIT CLEAINING
1B
2 8
Figure 4. Flow alternator: Row sheet in front view
Figure 5. Flow alterna
evaluation device is a single4.age impactor with four equal and bypasr. The air sample slits arranged in one line. The slide is moved across the line contained 10 cc of an acid -c----L.-. CVIISLLLIII mixed for each test from 4u LL V I ,loR I ~ V L U I I V I I ,itu LL UI of slits at a constant speed, or ..ILLL -a ~"a"..L G P >r;qucn~r;VI speeds (Goetz, 1969). distilled water, and20 cc of 2NH2S06. Neutral stock solution During a single pass, thi2 collecting surface of a metallize:d of KI, which, over a few weeks, acquired the yellow of free . . ..I..IV _"., fnnr n n m l l d I.uuvL~s rihhmr microscope slide recelves SI...miiltnn~rmrlu iodine, was decolorized by a 10-min exposure over coconut of aerocolloid deposits. A synchronous drive allows a variacharcoal and filtration. Iodine liberated by the oxidant was tion of speeds in 10 doubling steps from 2O to 2Q.Thus, it is titrated with a 0.02N standard thiosulfate solution with an possible to vary the deposit density up to 512-fold. Up to 10 end point indication with starch. For a 10-liter air sample, 1 ml such stens can be evaluated in one slide leneth with a micro"~~~ ~~~.. . of 0.02N thiosulfate solution equals 12.25 ppm oxidant. For a . Ftudv of the influence of F., t., and relative humiditv 1Jhotometer or by microscopic counting. A limiting high-de~~. oIn the oxidant level, two connected ultraviolet cascades Ilosit density is usually attained, at which coalescence of the viith eight consecutive stages were used. For constant voltage, I,articles occurs and the coints, as well as the light-scattering, . , . lamps were on, wnue .. one ourier. was connecrea , au eignr IO ;harply fall off. The impactor has a bypass system in order to keep the the bubbler. 1 :.. L L " ..I*..^..:^,^* Le.---A_-:..^ ..., 11"W 111 U L C u L l l a Y I " l ~ L CaJCaUC L V I I > L ' u I I "SIULt., uu,,ng, anu Dark..Reaction Tests. In the dark-reaction tests, a tee at the r ...~" ..^^-I *^ " A A tl." L..,l--"~-l.^.. ":.. na after an impaction (flow alternator, Figure 4). The reported S inlet of ~ a g cc was ucu LU MU L E UJYLUCLLVVLI-~U. YCdata refer to deposits obtained with a pressure drop of Ap = posits were made from both dark stages, C and D.The flows in 90 mm Hg at the slit orifice. AB-CD were 3.0, 3.0, 6.0, 3.5 I./min, the exposures were 20, The flow alternator is an aluminum block (2024 ST4 20, 10, 17 sec, respectively. The HC-air containing 5.0 ppm Alum.) with two bores housing two cocks held in place by a of turpentine, before mixing with the oxidant stream, was diluted to 2.5 ppm in the combined stream (3 l./miu each front plate. Each bore has five outlets to make possible the three functions: impaction, idling (bypass), and slit cleaning stream). For arranging different humidities in the two streams, both the main flow F, and that of the humidifier were branched (reverse flow). One internal duct, one external Tygon conand mixed. with one additional flowmeter and difference nection between impactor head and flow alternator, and two short bores between the cocks provide the necessary reading of flowmeter fi. For rh < l S Z , a silica gel tower channels (Figure 4). The cocks are closely fitted and lubricated was put between pressure regulator and Millipore filter unit. .,,.emecrs were siucnen . 3 aii~u with stopcock grease of a vacuum grade (rubber base, not Ammonia ar auour 3 ppm 1 ~ n cmom dilution. silicone or apiezone which tend to flow). The moving-slide impactor with alternator is shown in Figure 5. The instrument is opened for slide access. Deposits Results and Discussion of the MSI are concentrated in a small area, and their aging, Production and Decay of Oxidant. Oxidant is detected imdesiccation, and degradation by ultraviolet light can be mediately on irradiation of air with quartz mercury arcs. evaluated by measuring repeatedly their light-scattering S. The attained ozone concentrations depend on exposure, Aging of deposits required weeks of storage for completion. humidity, and flow rate besides the flow geometry (Figure 3). Rapid aging was accomplished by heating in a vacuum oven Ozone oxidant was measured by iodometry in acidic solution: for several hours at 100°C. Desiccation was achieved by storing the slides in an evacuated desiccator containing a Oa 2 H+ f 2 I- -+ 0% H,O 1% dish with powdery phosphorus pentoxide. For degradation by ultraviolet irradiation, one of the 34-cm The plot of ozone concentrations vs. exposure suggests thar ultraviolet lamps was mounted horizontally above the slides under all conditions of flow and humidity a concentration laid out side by side, with a 2-in. vertical separation to the of 10 ppm is reached in 1.2 to 2.3 sec. The ozone concentration rises steeply at longer exposures (10-100 sec), and it rises lamp. Exposures used were 30 min, 1,2,8, 16 hr, or sequences inratios 1:3:10 for logarithmic irradiation equivalents. also with the flow rate F and with decreasing relative humidity Oxidant Measurements. For the determination of oxidant, (Figure 6). A decay of the oxidant in the darkened cascade (3 an irradiated airstream of 2 I./min was passed under positive dark stages, BCD) was observed only at high humidity and pressure through a bubbler (intensive washer with deflecting was marked by lower titration values. plate) and a flowmeter. Larger flows were divided with a tee Ultraviolet lamps shielded with Vycor-7910 produced no
."". .-..-. ~~
~~~~
~
~~~
~~~~
~~
~~
~~
~~I ~
..
^...
^^^^^_1^
.
~~
~~
~~~~
~
..
.
1
,
"-..-*"..A
"A"-"
.. . .
+
.
/
