Environ. Sci. Technol. 1990, 24, 1021-1026
Interferences from Aldehydes and Peroxyacetyl Nitrate When Sampling Urban Air Organic Acids on Alkaline Traps Danlel Grosjean' and Sucha S. Parmar DGA, Inc., 4526 Telephone Road, Suite 205,Ventura, California 93003
We have investigated the possible conversion of formaldehyde to formate and of acetaldehyde and peroxyacetyl nitrate (PAN) to acetate when sampling organic acids in urban air using alkaline traps. Tests were carried out with cartridges (C18and alumina) and with filters (25and 47 mm, glass or paper) coated with KOH and carbonate, aldehydes concentrations of 8-240 ppb, PAN concentrations of 50-160 ppb, sampling flow rates of 0.4-15 L/min, and sampling times of 1-24 h, in both purified and particle-free ambient air. Disproportionation of aldehydes to acids was negligible, accounting for 1-3% of the aldehyde concentration irrespective of trap type, aldehyde concentration, sampling flow rate, and test duration. Conversion of PAN to acetate on alkaline traps (heterogeneous alkaline hydrolysis) was 11-17% for carbonate and KOH filters, 16-2770 for carbonate cartridges, and 100% for KOH cartridges. Laboratory tests and analysis of ambient data for three recent southern California field studies both indicate negligible contribution of formaldehyde and acetaldehyde to the formic acid and acetic acid, respectively, measured in alkaline filter and cartridge samples. PAN interference as acetate can be neglected for filter samples. For cartridges, PAN contribution to the measured acetate may be 16-loo%, depending upon the assumptions made regarding trap conversion to carbonate by atmospheric C02 during sampling. Other sampling devices were also studied. Nylon filters, which retain nitric acid and oxalic acid, did not collect formic acid and acetic acid. Tests with water microimpingers were successful in the laboratory but not in the field, where the data obtained were consistent with loss of organic acids by volatilization as the sampling solution was progressively acidified during sampling.
Introduction Methods employed to measure gas-phase organic acids in ambient air include long-path Fourier transform infrared spectroscopy, FTIR (1-4) as well as collection in aqueous (5, 6) or alkaline media followed by chromatographic analysis (7-1 I). Alkaline traps employed for the collection of organic acids include hydroxide- and carbonate-impregnated filters (7-9), annular denuders (IO),and C18 cartridges (11). Measurements by long-path FTIR, an in situ method, are not subject to interferences but involve very expensive equipment and are limited to only one acid, formic acid. Conversely, the alkaline trap method makes use of simple, inexpensive equipment and is suitable for many organic acids besides formic acid (9). However, this method may suffer from the following two types of positive bias: (1)alkaline disproportionation of aldehydes to organic acids, (the Cannizzaro reaction), e.g., formaldehyde to formic acid, acetaldehyde to acetic acid, and so on: 2RCHO + OH-- RCH2OH + RCOO(R1) (2) alkaline decomposition of peroxyacyl nitrates to the corresponding carboxylate ions, e.g., peroxyacetyl nitrate (PAN) and peroxypropionyl nitrate (PPN) to acetate and propionate, respectively (12, 13): RC(O)OON02 + 20H- RCOO- + NO, + 02 + H2O (R2) 0013-936X190/0924-1021$02.50lO
These potential interferents are nearly always present in urban air at levels comparable to those of the organic acids. For example, ambient levels of formaldehyde and acetaldehyde in southern California may exceed 30 ppb (14,15),and PAN levels of 30 ppb are not uncommon (16, 17). Limited data indicate that ambient levels of PPN are about 10% of those of PAN (18,19). Thus, if aldehydes and peroxyacyl nitrates are retained on alkaline traps, even only partially, they may contribute a substantial positive bias to the corresponding organic acids measurements. In this article, the issue of positive bias due to aldehydes and peroxyacyl nitrates when sampling organic acids with alkaline traps is addressed on the basis of laboratory studies, field validation experiments, and examination of relevant air quality data with emphasis on the last three major air quality studies carried out in southern California, the 1985 Nitrogen Species Methods Comparison Study, NSMCS (20),the 1986 Carbon Species Methods Comparison Study, CSMCS (21), and the 1987 Southern California Air Quality Study, SCAQS (22). Our study focuses on the two most abundant acids, formic acid and acetic acid, and on the corresponding three most abundant potential interferents, formaldehyde, acetaldehyde, and PAN.
Experimental Methods Alkaline traps subjected to laboratory and field validation studies included small C18and alumina cartridges as well as 47 mm and 25 mm diameter glass and paper filters. The collection efficiency of alkaline filters, measured under field conditions with two traps in series, was 89 f 7% for formic acid and 90 f 6% for acetic acid (8);that of C18 alkaline cartridges was 92 f 10% for formic acid and 90 f 10% for acetic acid (11,23). Also included in some of the tests were microimpingers containing water and nylon filters, which retain strong acids such as nitric acid (24). A list of sampling devices tested is given in Table I along with relevant sampling parameters. The preparation of the alkaline traps has been described in detail elsewhere (8,9,11,23).AU traps employed in the field were mounted downstream of 1.2 pm pore size Teflon filters, which retained ambient particulate matter. The Teflon filters were omitted in the laboratory tests, which were carried out with particle-free air. Polycarbonate filter holders and 1/4-in. Teflon sampling lines were used in all cases. Purified air was obtained by passing ambient air through large beds of activated carbon, silica gel, and Purafil (permanganate on alumina). The air thus purified contained less than 1 ppb hydrocarbons, ozone, oxides of nitrogen, and sulfur dioxide and less than 0.2 ppb formaldehyde, acetaldehyde, formic acid, acetic acid, and PAN. Atmospheric C02 may be retained on alkaline traps, and the conversion of hydroxide to carbonate during sampling may influence the collection efficiency of alkaline traps for both organic acids and the potential interferents. Therefore, laboratory validation testa also included experiments carried out using particle-free ambient air, which contains some 350 ppm COP Ambient air in Ventura, CA, typically contains low levels of formic acid and acetic acid,
0 1990 American Chemical Society
Environ. Sci. Technol., Vol. 24, No. 7, 1990
1021
Table I. Alkaline Traps and Other Sampling Devices Tested for Collection of Organic Acids flow rate, L/min
sampling device 47-mm glass filter, 5 mL of 0.05 N KOH
16-19 11-14
25-mm glass filter, 0.25 mL of 0.1 N KOH
4
47-mm Whatman 41 paper, 2% Na2C03 CIScartridge: 2 mL of 1 N KOH
3.75 12 2
47-mm nylon filters microimpingers, 2 mL of deionized water + 10 p L of chloroform (biocide) annular denuder (26),0.8 g of KOH annular denuder, 0.5 g of NaOH annular denuder, 0.4 g of Na2C03
sampling duration, h 4-6 4, 8
volume of air sampled, m3 3.2-6.0 2.6-6.7
0.9-1.4
0.4
4-6 1 week 4, 8 4, 6, 7, 8 24
12 0.25
4, 8 4, 6, 7, 8
2.9-5.8 0.06-0.12
2 2 2
8
0.96 0.84 0.84
37.8 2.9-5.8 0.5-1.0 0.58
7 7
relevant field study NSMCS (8) CSMCS (9) SCAQS, Palm Springs (11) SCAQS, San Nicolas Island (11) CSMCS (9) SCAQS, Claremont and Long Beach (11) on-going work, 1988-1989, Upland, Perris, Palm Springs CSMCS (9) SCAQS, Claremont and Long Beach (11)
OOctadecyl groups bonded to neutral silica gel, particle size 5 pm, pore size 120 A, carbon load 11%. Table 11. Summary of Aldehyde Interference Tests aldehyde concn: ppb
carbonate filter downstream of aldehyde source
sampling device
aldehyde sourceb
KOH cartridge KOH cartridge KOH cartridge 47-mm filter 0.05 N KOH 47-mm filter 2% Na2C03
PT PT PT SM
+ + + +
KOH cartridge KOH cartridge KOH cartridge KOH cartridge 47-mm filter 0.05 N KOH 47-mm filter 2% Na2C03
PT
-
HCHO 32.5 f 2.5 130 f 10 240 f 20 80 f a 80 f 8
CHSCHO
* *
8.3 0.3 95 f 5 150 10 180 f 12 55 f 4 55 f 4
SM
LD LD LD SM SM
-
+ + + + +
sampling flow rate, L/min
acid concn,’ ppb
matrix air‘
sampling time, h
A A A
6 7.5 13
P
4
0.5 15
2.7 f 0.3 3.0 f 0.6 3.0 i 0.6 2.5 f 0.5
P
4
15
2.0
A A A A
9 13 12.5 6
P
4
0.5 0.5 0.5 0.5 15
P
4
15
0.4 0.7
positive bias, % of aldehyde concn
