Fitz and Motallebi
their replacem ent by volatilizing NH4NO3, this does not
occur during the short period of tim e that the particles are
passing through the denuder.10 The rem aining NH4+ and
NO3– can then be collected on specially treated filters, which
prevent further volatilization.
den uders offer several advan tages over th e den uder
differen ce approach , a sign ifican t capital in vestm en t in
den uder h ardware is n eeded for routin e field m easure-
m en t program s.
An oth er lim itation with con ven tion al den uders is
that it is necessary to rem ove large particles (usually greater
th an 2 µm -aerodyn am ic diam eter) con tain in g th e species
to be m easured, sin ce th ey m ay deposit in th e lon g, n ar-
row ch an n els typically n ecessary to allow gases to diffuse
to th e treated surface. Th is would result in a positive in -
terferen ce. Size-selective in lets, h owever, presen t an oth er
surface to wh ich HNO3 an d NH3 m ay deposit, wh ich could
result in m easurem en ts with a n egative bias. At th e sam e
tim e, NH4NO3 m igh t volatilize from particles collected in
th e in let, resultin g in a positive bias.
Wh ile den uder tech n ology can result in th e accu-
rate m easurem en ts of th ese species, its application to
routin e sam plin g is difficult due to th e cost an d com -
plexity of sam plin g equipm en t. Two approach es h ave
been used in th e past: direct den uder an d den uder dif-
feren ce. In th e den uder differen ce approach ,9,10,14 th e
den uders are n ot extracted after sam plin g; two sam plin g
lin es are used, on e with a den uder an d on e with out.
Th ese den uders are design ed for h igh adsorptive capac-
ity so th ey can be used for m an y sam ple collection pe-
riods before requirin g reactivation or replacem en t. Both
lin es th en collect sam ples on specially coated filters,
wh ich preven t furth er volatilization . Th e filter used on
th e lin e with out th e den uder also retain s th e corre-
spon din g gas-ph ase com pon en t. Both filters are ex-
tracted , an d th e ion of in terest is q u an tified . Th e
con cen tration on th e filter with th e den uder is a m ea-
sure of th e particulate con cen tration , wh ile th at of th e
filter with out th e den uder is th e total con cen tration
for gas an d particulate ph ases. Th e differen ce in con -
cen tration between th e two filters is, th erefore, a m ea-
sure of th e gaseous con cen tration .
We report h erein a n ew approach with th e poten tial
to offer m easurem en t sen sitivity of th e direct den uder
approach with lower capital an d operatin g costs. Th is
den uder is based on diffusion research for devices used to
rem ove very fin e particles. Th ese devices, kn own as diffu-
sion batteries, are used to size-resolve subm icron particles
in situ. Origin ally con structed usin g a sin gle lon g ch an -
n el, th ey were th en m ade m ore com pact, but m ore diffi-
cult to fabricate, by usin g tubin g bun dles. Th ese evolved
to honeycom b structures and finally to wire screens.20 This
developm en t is an alogous to th e developm en t of diffu-
sion den uders for sam plin g sem i-volatile species, wh ich
started with tubin g bun dles,9 progressed to an n ular ge-
om etry,11 an d th en to h on eycom b structures.19
While this approach requires considerably less labor
and capital investm ent than the direct denuder m ethod,
the difference technique is subject to greater m easurem ent
uncertainty for the gas-phase species. In addition, denuder
surfaces m ust be occasionally renewed. This applies to the
denuders for HNO3, which used anodized alum inum de-
nuder surfaces. While they were thought to have infinite
capacity for rem oving HNO3,14 we have recently found that
breakthrough is possible under both laboratory and am bi-
ent air conditions.15
In th e direct den uder approach ,11,13,16,17 th e adsor-
ben t layer of th e den uder is extracted an d an alyzed for
eith er NO3– or NH4+. (Separate den uders are used for HNO3
an d NH3 becau se differen t sorben t coatin gs are re-
q u ired .) Th is m et h o d req u ires t h e d en u d er t o be
recoated after each sam plin g period, usually a labor-
in ten sive laboratory procedure. Norm ally, an n ular de-
n uders are used, wh ich are relatively expen sive precision
sam plin g devices. Th is approach , in wh ich th e gas of
in terest is adsorbed on a den uder coatin g an d quan ti-
fied, is gen erally favored over th e den uder differen ce
m eth od due to its greater sen sitivity.
Our approach uses a fabric as th e den uder substrate,
an alogous to wire screen den uders th at collect very fin e
particles in diffusion batteries. Th e fin er th e m esh , th e
greater th e deposition by eith er particles or gases. In th e
case of particles, th e wire of th e m esh is typically 10 µm
in diam eter with a spacin g of 20–50 µm . Typically, m ore
th an 100 such screen s are n ecessary to rem ove subm i-
cron -sized particles. Sin ce th e diffusion coefficien t for
gases is several orders of m agn itude h igh er th an for par-
ticulate m atter, a sin gle screen , wh ich n eed n ot be as
fin e, could be used. An open -weave fabric, with a typical
th read size of 100 µm spaced on cen ters of 250 µm , leav-
in g an open grid of 150 µm (typical dim en sion s for fab-
ric with a loose weave), would be sufficien t. In addition ,
an adsorben t m aterial would allow th e fabric to be coated
with a variety of ch em icals for selectively rem ovin g tar-
get gases. Th e den uders, th erefore, could be soaked in
solution s of C6H8O7 or H3PO4, NaCl, trieth an olam in e, an d
K2CO3, wh ich would selectively adsorb NH3, HNO3, NO2,
an d SO2, respectively.
Several types of den uders h ave been developed alon g
with specialized size-selective in lets to m in im ize HNO3
adsorption . Th ese design s in clude an n ular,11 tubular,13
coiled tubin g,18 an d h on eycom b19 den uders. Wh ile th ese
Th eor et ica l Ba sis
Th e feasibility of th e con cept was first evaluated by usin g
th e th eory developed for wire diffusion screen s.21 Th e
982 Journal of the Air & Waste Management Association
Volume 50 June 2000