Effect of Organic Coatings on Gas-Phase Nitrogen Dioxide
Production from Aqueous Nitrate Photolysis
Dorea I. Reeser,
†
Nana-Owusua A. Kwamena,
†
and D. J. Donaldson*
,†,‡
†
Department of Chemistry, University of Toronto, 80 St. George St., Toronto, Ontario Canada M5S 3H6
‡
Department of Physical and Environmental Sciences, University of Toronto at Scarborough, 1265 Military Trail, Toronto, Ontario
Canada M1C 1A4
ABSTRACT: The influence of stearic acid, octanol, and
octanoic acid monolayer coatings on the release of NO
2
into
the gas phase following aqueous NO
3
-
photolysis was studied
using incoherent broadband cavity-enhanced absorption spec-
troscopy (IBBC-EAS). The different organic compounds,
when present at the aqueous surface, had varying effects on
the gas-phase NO
2
evolved. Stearic acid monolayers lowered
the initial rate of appearance of NO
2(g)
, and its steady-state
concentration was the same as for uncoated solutions after
∼50 min. In the presence of octanol monolayers, both the
steady-state [NO
2(g)
] and its rate of appearance decreased. A simple kinetic phase partitioning model suggests that the rate of
NO
2(g)
evaporation from the aqueous surface is physically inhibited by the long uncompressed stearic acid chains, whereas both
NO
2
evaporation and steady-state NO
2(g)
concentration decrease when octanol is present at the aqueous surface, due to the
enhanced solubility of NO
2
in the less polar octanol environment. Despite its structural similarity to octanol, monolayers of
octanoic acid showed a different effect and slightly increased the steady-state [NO
2(g)
]. We propose that octanoic acid enhances
NO
2(g)
production because of an increase in solution acidity, which increases the quantum yield of NO
2
production from nitrate
photolysis.
■
INTRODUCTION
Nitrogen dioxide is an important tropospheric pollutant since it
contributes to photochemical smog and the formation of acid
rain, and it influences the oxidation capacity of the troposphere
via HO
x
/NO
x
cycles, where NO
x
= NO
2
+ NO and HO
x
=
HO
2
+ OH.
1
Furthermore, it is a respiratory irritant and
reduces plant growth.
2-4
Nitrogen dioxide in the troposphere is
primarily produced via the reaction between nitric oxide and
ozone or peroxy radicals. Combustion is the major source of
NO
x
, and additional sources include lightning, microbial
nitrification/denitrification in soils, oxidation of ammonia, and
aqueous nitrate photolysis.
1
There are several sources of nitrate in natural waters
including the oxidation of ammonia to nitrite with subsequent
oxidation to nitrate, nitrifying bacteria and wash off from
wastewater, sodium nitrate, and ammonium nitrate fertilizers.
In addition to the natural and anthropogenic sources of nitrate
in natural waters, it is also produced as ammonium nitrate and
nitric acid in the troposphere. The formation of nitric acid is a
major sink for NO
x
species in the troposphere, and gas-phase
HNO
3
is easily sequestered by aerosols or other wet surfaces,
forming aqueous nitrate.
1,5
Aqueous HNO
3
is also produced in
the heterogeneous reaction between gas-phase dinitrogen
pentoxide and water surfaces.
1,6-8
The photolysis of aqueous nitrate can lead to the release of
NO
x
, with yields which depend on the wavelength of light
9-12
ν + → +
→ +
→
− −
−
−
h NO NO O (1)
NO O (2)
ONOO (3)
3 2
2
There are two primary pathways of nitrate photolysis due to
two absorption bands of nitrate; a weak n → π* band at ∼305
nm (eqs 1 and 2) and a strong π → π* band at ∼200 nm eqs
1-3. Room-temperature nitrate solutions with a pH between 4
and 11 illuminated with ∼305 nm light show quantum yields of
0.01 and 0.001 for OH and O atom production, respec-
tively,
9-11
with OH from the reaction O
-
+H
2
O ⇌ OH +
OH
-
. Similar solutions illuminated at ∼250 nm give quantum
yields of 0.09 for OH and 0.1 for ONOO
-
.
13,14
Understanding
the chemical and physical processes that influence the evolution
of NO
x
from nitrate photolysis is of interest because its aqueous
deposition as nitric acid is considered to be a major loss process
of tropospheric inorganic nitrogen due to the small quantum
yield from nitrate photolysis under actinic radiation (λ ≥ 290
nm).
Nitrate photolysis may occur in salt and fresh waters, in
aqueous aerosols released from wave breaking of these waters,
Special Issue: Ron Naaman Festschrift
Received: February 12, 2013
Revised: April 4, 2013
Article
pubs.acs.org/JPCC
© XXXX American Chemical Society A dx.doi.org/10.1021/jp401545k | J. Phys. Chem. C XXXX, XXX, XXX-XXX