Characterization of a Facility To Simulate In-Cloud Chemical

Chapter 15

Characterization of a Facility To Simulate In-Cloud Chemical Transformations A. W. Gertler, N. F. Robinson, and D.F.Miller

Downloaded by RUTGERS UNIV on January 1, 2018 | http://pubs.acs.org Publication Date: September 3, 1987 | doi: 10.1021/bk-1987-0349.ch015

Energy and Environmental Engineering Center, Desert Research Institute, University of Nevada System, P.O. Box 60220, Reno, NV 89506

Laboratory simulations of aqueous-phase chemical systems are necessary to 1) verify reaction mechanisms and 2) assign a value and an uncertainty to transformation rates. A dynamic cloud chemistry simulation chamber has been characterized to obtain these rates and t h e i r uncertainties. I n i t i a l experimental results exhibited large u n c e r t a i n t i e s , with a 26% variability in cloud liquid water as the major contributor to measurement uncertainty. Uncertainties in transformation rates were as high as factor of ten. Standard operating procedures and computer control of the simulation chamber decreased the v a r i a b i l i t y in the observed l i q u i d water content, experiment duration and final temperature from ± 0.65 to ± 0.10 g m-3, ± 180 to ± 5.3 s and ± 1.73 to ± 0.27°C respectively. The consequences of t h i s improved control over the experimental variables with respect to cloud chemistry were tested for the aqueous transformation of SO2 using a cloud-physics and chemistry model of t h i s system. These results were compared to measurements made p r i o r to the i n s t i t u t i o n of standard operating procedures and computer control to quantify the reduc t i o n in reaction rate uncertainty resulting from those controls.

In-cloud chemical processes transform soluble trace gases into various ionic products. In the case of acid precursors, such as SO2 and NO2, definitions of the significant chemical reactions in aqueous cloud droplets are necessary for the mathematical description of acid deposition. These s i g n i f i c a n t reactions can be inferred from measurements in the real atmosphere (1,2), and they can be i d e n t i f i e d in controlled laboratory experiments (3,4). Since measurements in the real atmosphere may be characterized by large uncertainties (1), laboratory simulation of aqueous phase chemical systems supplement

0097-6156/87/0349-0183$06.00/0 © 1987 American Chemical Society

Johnson et al.; The Chemistry of Acid Rain ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

184

THE CHEMISTRY OF ACID RAIN

these measurements by 1) verifying the end products of reaction mechanisms and 2) assigning a value and an uncertainty to t r a n s f o r mation rates. Quantification of the uncertainty in the measured transformation rates is crucial i f the data are to be used as inputs to models for risk assessment. Without knowledge of the accuracy, precision, and v a l i d i t y of these measured values i t is impossible to develop r e a l i s t i c abatement strategies.


This paper w i l l : •

Describe a cloud chemistry simulation f a c i l i t y to emulate atmospheric aqueous phase interactions among gases, par t i c l e s , and l i q u i d water droplets.

•

Identify the most s i g n i f i c a n t physical variables these interactions.

•

Quantify the uncertainty of gas to solute transformation rates resulting from uncertainty in the control of the cloud-physical parameters.

controlling

System Overview The cloud chemistry simulation chamber (5,6) provides a controlled environment to simulate the ascent of a humid parcel of polluted a i r in the atmosphere. The cloud forms as the pressure and temperature of the moist a i r decreases. By c o n t r o l l i n g the physical conditions influencing cloud growth ( i . e . i n i t i a l temperature, relative humi d i t y , cooling r a t e ) , and the s i z e , composition, and concentration of suspended p a r t i c l e s , chemical transformation rates of gases and par t i c l e s to dissolved ions in the cloud water can be measured. These rates can be compared with those derived from physical/chemical models (7,9) which involve variables such as l i q u i d water content, solute concentration, the gas/liquid i n t e r f a c e , mass transfer, chemi cal equilibrium, temperature, and pressure. A functional representation of the cloud chemistry simulated chamber is shown in Figure 1. The chamber is constructed of a c y l i n d r i c a l aluminum inner shell 1.8 m in diameter, 2.5 m in height with dome-shaped ends that provide a total volume of 6.6 m . The walls are jacketed and a steel outer s k i n , providing temperature u n i formity within 0 . 2 ° C . Access ports serve for evacuation, pressurizat i o n , r e c i r c u l a t i o n , and sampling. A high-capacity water-seal vacuum pump downstream of an automati c a l l y controlled valve provides for chamber evacuation and pressure regulation. The maximum cooling rate is approximately 5°C min"*. The minimum practical pressure is 140 mbar and the minimum tem perature is approximately - 4 0 ° C . Maximum cloud lifetime on the order of 20 min can be r e a l i z e d , limited by nonadiabatic wall conditions and the f i n i t e f a l l speeds of the cloud droplets. Cloud l i q u i d water content (LWC) is measured by a CO2 laser transmissometer ( λ = 10.6 pm) which l i n e a r l y relates observed extinc tion to LWC (10,11). 3


GERTLER ET AL.

Facility

To Simulate ln-Cloud

Chemical

Transformations


RECIRCULATION LINE

INTRODUCE AEROSOL AIR/GAS HUMIDITY »

CONTROL WALL TEMP

CHARACTERIZE

- AIR PRES.

AEROSOL CLOUD GAS

SAMPLE PARTICLES ICLOUD WATER-

Figure 1. Cloud chamber schematic.


186

THE CHEMISTRY OF ACID RAIN

The experimentally v e r i f i e d theoretical extinction c o e f f i c i e n t o i s given by

relationship

for

the


e

where ρ is the density of water, c(x) is the wavelength dependent slope of the linearized f i t to the Mie scattering function when plotted against p a r t i c l e s i z e , and LWC is the cloud l i q u i d water con tent. The approximation is valid for droplet diameter less then 28 m (il). A cloud water c o l l e c t o r that separates unactivated ( i n t e r s t i t i a l ) p a r t i c l e s from cloud droplets by jet impaction of these droplets on inert surfaces (12) provides a sample for chemical analyses. The operational procedures of the chamber have recently been upgraded from manual to automatic with the i n s t a l l a t i o n of a dedi cated computer system (Kinetic Systems Inc., CAMAC with DEC LSI 11/23 microprocessor). This allows precise and reproducible control which was impossible under manual operation. Computer assisted operation was achieved through the development of an algorithm which controls the chamber wall temperature and cooling rate while maintaining a specified d i f f e r e n t i a l between a i r and wall temperature (ΔΤ = 0.0 ± 0 . 2 ° C ) . The d i f f e r e n t i a l is maintained by adiabatic cooling of the a i r caused by variation of the rate of evacuation. Standard operating procedures were i n s t i t u t e d along with the com puter control. These consisted of procedure check l i s t s to ensure protocols were followed. Conditions prior to commencement of an experimental run were as follows: I n i t i a l a i r temperature, T-j 24.0 ± 0.5 °C I n i t i a l dew point temperature, T^ 21.0 ± 0.2 °C I n i t i a l pressure (ambient), Ρ 860. ± 15 mb Aerosol concentration, % 7500. ± 200 c n r Run-mean cooling rate, df/dt -1.5 ± 0.05 °C min"* Air-wall temperature difference, ΔΤ 0.0 ± 0.2 °C Reaction i n t e r v a l , A t 10.0 ± 0.1 mi η 3

1

r

A run was not performed until these conditions were met. The system has been characterized with respect to background im p u r i t i e s in the chamber a i r and in the collected cloud water (Table I). Ammonia is a s i g n i f i c a n t impurity. In the absence of inten t i o n a l l y added reagents, i t balances the major anions in the cloud water which are carbonate and formate. The anion/cation balance of background ( i . e . , blank) cloud water is t y p i c a l l y within 10% of unity. Background non-methane hydrocarbon levels are generally less than 20 ppbC. A typical sample (Table I) indicates that the major com ponents are ethane, propane and acetylene. Because only picomolar amounts of these hydrocarbons would exist in the cloud water, the effects of these background levels on aqueous-phase chemistry are expected to be n e g l i g i b l e . The effect of the organic acids is not expected to be s i g n i f i c a n t unless sources of OH exist. Formaldehyde is known to i n h i b i t aqueous SO2 oxidation, but i t s concentration here is i n s i g n i f i c a n t compared to the concentrations of SO2 intentionally


15.

Facility

GERTLER ET AL.


Table I.

To Simulate In-Cloud

Chemical

Concentration

NH

Characterization of a Facility To Simulate In-Cloud Chemical

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