INVESTIGATIONS OF THE HETEROGENEOUS REACTIONS BETWEEN
AMMONIA AND “NITRIC/OXALIC/NITROUS ACID” AEROSOLS
Thomas Townsend and John R. Sodeau.
Centre for Research into Atmospheric Chemistry, Department of Chemistry,
University College Cork and Environmental Research Institute, Cork, Ireland.
INTRODUCTION
Nitric acid is mainly produced from the
following two reactions, the NOx
precursors formed as a result of lightning
and combustion processes.
CES AL
U
D
RE
ENT
M
N
IO N
IRO
T
V
A
N
E
FIC
I
D
I
AC
NO2 + OH· → HNO3 Day
N2O5 + H2O → 2HNO3 Night
AM
AIM OF THIS WORK
The purpose of the research is to elucidate the
interactions between ammonia with nitric/nitrous acid
aerosols over a range of concentrations and humidities.
Oxalic acid was also introduced and its effects
established by comparing FTIR spectra, [NH4]+
production and size of the particles.
E IN E
L
O
R
AT
F
L
KEY
E /SU IN G
T
A
NITR TI TION
PAR
INSTRUMENTATION
Diffusion dryer
Dilution Unit
P
Aerosol
Aerosol
Generator
Humidifier
Aerosol Flow
Carrier Flow
RELATIVE HUMIDITY
PARTICLE SIZER (SMPS)
Monitors aerosol fraction,
particle size, mass, surface
area, volume.
FTIR Digilab FTS 3000
using an MCT detector
and BaF2 windows:
monitors both gas and
aerosol condensed phase.
NH3/N2
SMPS
TSI 3081
MCT
Aerosol sampling
Reduce and distort visibility
Climate change
Provide surfaces for “new” reactions
0.25%wt HNO3 +
Ammonium
Nitrate or
Nitrate ion
300ccm NH3
HNO3 300ccm NH3
90%RH
0.5%wt HNO3 + C2H2O4
Nitrogen
Dioxide
Catalytic
Converter
HNO3 300ccm NH3
0.75%wt HNO3 +
20%RH
0.25%wt C2H2O4
NO,NO2 levels for nitric and oxalic acid
Size distributions when heating Nitric Acid
3
1.80E+07
0.5%w t nitric acid + 0.75%w t oxalic acid
90%RH
1.60E+07
NH3/NH4+ sampling
Exhaust
0.75%wt C2H2O4
60%RH
Denuder
RH
To
Health implications when inhaled
CHEMILUMINESCENCE
NOx Monitor. Catalytic
oxidation of NH3/NH4+ to NO.
NH
3
Analyser
FTIR
Effects of particles:
RESULTS
AMMONIA
(100ppm standard,
NH3/N2) diluted to ppb
range. Admitted to flow
tube via a 6mm diameter
movable, glass injector.
AEROSOL FLOW-REACTOR
Made of glass, ID: 10cm, maximal
reactive length Z: 80cm.
Operated at room temperature and
atmospheric pressure.
Flow concentration: 2L/mn.
Flow velocity 1cm/s.
Comp. Air
Unit
ES
C
N
A
ENH IC LE
T
PAR
IO N
T
A
M
FOR
Nitrous acid (HONO) is a source of the
hydroxyl radical (OH·) one of the key species in
photochemical cycles for ozone production and
smog. Formation occurs on surfaces in the
presence of water and NOx.
DENUDER Coated with
oxalic acid, removes NH3(g).
EQUATIONS/STRUCTURES
0.25%w t nitric acid + 0.75%w t oxalic acid
1.40E+07
1.20E+07
300ccm NH3
2
1.00E+07
8.00E+06
1.5
6.00E+06
1
4.00E+06
4HNO3(g)(heated) → 4NO2(g) + 2H2O(g) + O2(g)
0.75%w t nitric acid + 0.25%w t oxalic acid
2.5
conc. dN#/cm 3
(RH) tuned between 1% and 95%.
IA
N
O
M
NO,NO2 ppm
AEROSOL GENERATION
HNO3/HNO2/C2H2O4 aerosol
generated by passing a flow of
air over a heated solution or
via a nebuliser.
Oxalic acid is produced from biomass
burning, vehicle emissions and biogenic
activity. The aerosols formed from
condensation and nucleation play an
important role in cloud formation and
their radiative properties.
2.00E+06
0.5
0.00E+00
Nitrogen Dioxide
Cis
N O
.
O
N
O
-
NH4+
Nitrous acid
O
NH3
O H
------
H3
O+
N
O
H
N
O
H+
Ammonium Nitrate
O
-
O
C
O
O
H3
C
C
H+
O
O-
O
C
NH4
O
O
100
HNO3 20%RH
HNO3 + 100ccm NH3 90%RH
HNO3 + 300ccm NH3
O+
+
Diameter (nm)
0.5%wt HCl +
1000
HNO3 90%RH
HNO3 + 200ccm NH3
0
0
50
Course of experiment (min)
100
150
200
250
Size Distributions for different aerosol concentrations at 50%RH and
300ccm NH3 added
300ccm NH3 90%RH
0.5%wt NaNO2
4.00E+07
0.5%/0.5% wt. nitric acid/oxalic acid
0.75%/0.25% wt. nitric acid/oxalic acid
0.25%/0.75% wt. nitric acid/oxalic acid
3.50E+07
N
2.50E+05
2.00E+05
3.00E+07
Trans
O Organonitrate? O
conc. d#/cm-3
O
10
Ammonium Oxalate
NH4+
HCl(aq) + NaNO2(aq)→ HONO (?) + NaCl(aq)
2.50E+07
1.50E+05
2.00E+07
1.00E+05
1.50E+07
1.00E+07
5.00E+04
5.00E+06
300ccm NH3 10%RH
DISCUSSION
0.00E+00
0
20
40
60
80
100
0.00E+00
120
Diameter (nm)
FTIR: On heating HNO3, it was found that NO2 was present, whereas
at high humidities and using the nebuliser, NO3- absorptions appeared. It
is unclear whether the 1400cm-1 peak is an NH4+ or nitrate absorption. Wavenumber(cm-1) Assigned Component
Nitric and Oxalic acid aerosols gave spectra with characteristic
carboxylic absorptions and nitrate peaks.
1724 -1653
NO2 asymmetric stretch
1443
NH4+ deformation
1678 -1633
NO stretch (HONO)
1378 -1211
NO2 symmetric stretch
CO stretching
1310
CO-/COH stretching
NOH bend (nitrate)
1294 -1265
NOH bend (HONO)
A likely cis/trans structure of HONO has been produced and complex
formation with ammonia or water is possible.
1593
NOx Monitor: NOx data could not be analysed accurately due to
competition between NO/NO2 gas from nitric/nitrous acid and [NH4]+. 1458 -1382
NO2 levels were at first particularly high, however these lowered and
consequentially the [NO] increased as %RH and NH3 were added.
SMPS: Few particles were observed initially for heated HNO3 but more
(106-7,130-160nm) emerged as humidity and ammonia increased.
For nitric/oxalic acid, bimodal size distributions were recorded
suggesting different particles . Smaller amounts of larger particles
(0.75%wt oxalic) implies more incorporation of water and ammonia.
ACKNOWLEDGEMENTS!: I would like to thank the following funding
institutions for supporting this research, as well as the CRAC lab crew!
Wavenumber(cm-1) Assigned Component
CONCLUSION
FUTURE WORK
FTIR spectra produced nitrate, gaseous NO2,
oxalate, ammonium, HONO absorptions with
concentration and humidity being a factor in
appearance of spectra.
Vary the interaction times of ammonia
with the aerosols to calculate kinetic
measurements and uptake coefficients.
Investigate the interactions between
Sizes of the particles correlated
NH3, H2SO4 and other dicarboxylic
well with changing composition e.g. addition acids such as malonic and succinic acid.
of oxalic acid or ammonia.