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Atmospheric Pollutants B. R. Appel Air and Industrial Hygiene Laboratory, California Department of Health Services, 2151 Berkeley Way, Berkeley, C A 94704-9980
Nitric acid, particulate nitrate, and particulate organic carbon may be termed "labile" atmospheric pollutants; this name reflects the ease with which they undergo physical or chemical changes while an integrated sample is being collected. Manual sampling methods of varying accuracy are described for these species, together with sources of error. For nitric acid and particulate nitrate, the filter pack method, usually with Teflon and nylon filters in tandem, is the least accurate but simplest sampling procedure. More accurate techniques use diffusion denuders to separate gaseous HN0 from fine particulate nitrate. Most methods remain subject to interference from nitrous acid. Sampling techniques for particulate organic C remain relatively primitive. Tandem filter sampling permits correction for the error due to sorption of gaseous C on the filter medium. Denuder-based techniques, such as those described here for polyaromatic hydrocarbons, may hold the key to future development of improved samplers for particulate organic C as well. 3
THE
O B J E C T I V E O F I N T E G R A T I V E A T M O S P H E R I C S A M P L I N G is to
measure
accurately the concentrations of species of interest averaged o v e r the s a m p l i n g t i m e . H o w e v e r , a substantial n u m b e r of species are difficult to sample because of c h e m i c a l or p h y s i c a l changes o c c u r r i n g d u r i n g or after c o l l e c t i o n . S u c h pollutants are r e f e r r e d to as " l a b i l e " . T h i s chapter focuses o n integrative s a m p l i n g t e c h n i q u e s for selected l a b i l e species, i n c l u d i n g particulate nitrate a n d its c o r r e s p o n d i n g gas-phase species, n i t r i c a c i d ; particulate c a r b o n ; a n d particle-phase, p o l y c y c l i c aromatic h y d r o c a r b o n s . T h e t e r m s " v a p o r - p h a s e " , "gaseous", a n d "gas-phase" are u s e d i n t e r c h a n g e a b l y , as are t h e t e r m s " p a r ticulate" and "particle-phase". 0065-2393/93/0232-0001$11.00/0 © 1993 American Chemical Society
Newman; Measurement Challenges in Atmospheric Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1993.
2
MEASUREMENT CHALLENGES IN ATMOSPHERIC CHEMISTRY
T h e n e e d to d e t e r m i n e accurately the phase-specific concentrations o f these pollutants reflects several concerns: C o m p a r e d to gaseous materials, particle-phase materials m a y penetrate m o r e d e e p l y i n t o the h u m a n r e s p i ratory tract; particle-phase pollutants scatter l i g h t m u c h m o r e effectively than gaseous materials, a n d t h e y thus have a greater c o n t r i b u t i o n to v i s i b i l i t y r e d u c t i o n ; gaseous n i t r i c a c i d has a m u c h h i g h e r d e p o s i t i o n v e l o c i t y t h a n particulate nitrates a n d can b e a substantial c o n t r i b u t o r to the acidification of lakes, streams, forests, a n d vegetation. I n some instances s p e c i a l i z e d t e c h n i q u e s w e r e d e v e l o p e d to d e t e r m i n e concentrations of specific l a b i l e pollutants t h r o u g h the use of t e c h n i q u e s that a v o i d (or n e a r l y avoid) sample c o l l e c t i o n ; these t e c h n i q u e s are often r e f e r r e d to as " r e a l - t i m e " methods. T h e s e m e t h o d s , as w e l l as analytical m e t h o d s to b e a p p l i e d to c o l l e c t e d samples, are not discussed. T h e l i t e r a t u r e c i t e d is i n t e n d e d to i l l u m i n a t e the topics c o v e r e d b u t i n no sense represents an exhaustive r e v i e w . F u r t h e r m o r e , n u m e r o u s a l t e r native s a m p l i n g techniques are o m i t t e d i n favor of m o r e c o m p r e h e n s i v e treatments of w h a t I j u d g e to be the most generally useful strategies. T h e most i m p o r t a n t o m i s s i o n is p r o b a b l y i m p a c t o r c o l l e c t i o n , a t e c h n i q u e that m a y r e d u c e s a m p l i n g errors w i t h some l a b i l e species (J). It is useful to consider the general c o m p o s i t i o n of a t m o s p h e r i c aerosols. S u c h c o m p o s i t i o n varies m a r k e d l y d e p e n d i n g o n s a m p l e r location (e.g., u r b a n or rural), p r o x i m i t y of significant sources of aerosols a n d t h e i r gaseous precursors, a n d meteorology (e.g., w i n d speed, i n v e r s i o n h e i g h t , a n d s u n light intensity). A n t h r o p o g e n i c particulate matter is c o n c e n t r a t e d i n fine particles (i.e., less than 2.5 μπι i n diameter), whereas n a t u r a l aerosols (e.g., w i n d - b l o w n soil, sea salt, p o l l e n , a n d spores) are c o n c e n t r a t e d i n larger particles. T a b l e I , taken f r o m reference 2 a n d references c i t e d t h e r e i n , details average concentrations i n the fine a n d coarse ( 2 . 5 - 1 5 μπι) particle fractions o b t a i n e d f r o m a r u r a l a n d an u r b a n location. T h e materials that r e a d i l y u n d e r g o phase a n d c h e m i c a l changes are c o n c e n t r a t e d i n the fine fraction. T h e major constituents s h o w n for the fine fraction i n c l u d e carbonaceous materials, sulfate, a m m o n i u m , a n d nitrate. A l t h o u g h T a b l e I is generally self-explanatory, the carbonaceous m a t e r i a l measurements r e q u i r e c o m m e n t . Because of its c h e m i c a l c o m p l e x i t y , car bonaceous m a t e r i a l is f r e q u e n t l y c h a r a c t e r i z e d o n l y o n the basis of c a r b o n measurements. T h e s e measurements attempt to d i v i d e the carbonaceous m a t e r i a l i n t o "organic C " a n d " e l e m e n t a l C " . C a r b o n present i n carbonate salts, f r e q u e n t l y a m i n o r c o n t r i b u t o r to the total particulate c a r b o n , can be d e t e r m i n e d i n d e p e n d e n t l y . E l e m e n t a l c a r b o n is a m o n g the most i m p o r t a n t pollutants i n v i s i b i l i t y r e d u c t i o n . P o l y c y c l i c aromatic h y d r o c a r b o n s ( P A H s ) are r e l a t i v e l y m i n o r constituents of the particulate c a r b o n b u t are of great interest i n h e a l t h effects studies. P A H s can also serve as m o d e l c o m p o u n d s i n d e v e l o p i n g i m p r o v e d s a m p l i n g t e c h n i q u e s for semivolatile carbonaceous materials.
Newman; Measurement Challenges in Atmospheric Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1993.
1.
APPEL
Sampling of Selected Labile Atmospheric
3
Pollutants
Table I. Average Aerosol Composition for Fine and Coarse Particles at a Rural, Forested Location (Great Smoky Mountains, Tennessee) and an Urban Location (Houston, Texas) Smoky Mountains Composition Total mass SO4 NO32
NH H C (organic) C (elemental) C (total) Al Si S Cl Κ Ca Ti V Μη Fe Ni Cu Zn As Se Br Pb 4
+
+
Fine
24,000 ± 12,000 ± 300 ± 2280 ± 114 2220 ± 1100 ± 3300 ± 20 ± 38 ± 3744 ±
