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24 CHAPTER 2. ATMOSPHERE AND REMOTE SENSING
2.1.7 Long-Path-DOAS Measurements of VOCs and HOx precursors in
Mexico City during MCMA-2006
André Merten ( Philip Sheehy 1 , Rainer Volkamer 1,2 )
1 MIT, Cambridge, MA 02139; 2 UC San Diego, La Jolla, CA 92093
Abstract Two Long-Path-DOAS instruments were installed in Mexico City in March 2006 as part of
MCMA2006 field campaign to measure VOC and radical precursors of HOx (Glyoxal, HCHO, HONO)
and other species in a megacity urban environment.
Toluene time series from different light paths
Mexico City March 2006
200
180
160
140
120
100
80
60
40
20
0
04.03 05.03 06.03 07.03 08.03 09.03 10.03 11.03
date [UTC]
toluene light path 1 to reflector at ground
toluene light path 2 to reflector at water tower
Toulene [ppb]
Opitical density
1.0x10 -2
0.5x10 -2
0
-0.5x10 -2
-1.0x10 -2
Naphthalene fit result Mexico City 2006 DOAS#1
08.03.2006 14:32:43 UTC
274 276 278 280 282 284 286
wavelength nm
measurement (RMSE= 6,12E-4 ∆= 4,90E-3)
naphthalene (5,07E-1+/-1,0E-1)ppb
9,58E+9cm -3 CD=1,97E+15 mean OD=3,83E-3
Figure 2.8: left: time series of different light paths make toluene plume visible. right: sample spectrum
of the polyaromatic specie naphtale
Background Two Long-Path-DOAS instruments
were installed in Mexico City in March
2006 during the MCMA-2006 (Mexico City
Metropolean Area) field campaign as part of
the MILAGRO (Megacity Initiative: Local and
Global Research Observations) project which also
involves airborne measurements and field studies
in other areas of Mexcio. The goal of MILA-
GRO is to improve the knowledge of the chemistry
and transport processes in the Mexico City
atmosphere and use this as a model to describe
the conditions in other megacities and their global
impact. The measurements were performed in collaboration
with Molina Center for Energy and the
Enviroment and UCSD.
Methods and results The DOAS1 telescope
primarily measured aromatic volatile organic compounds
(VOCs) as precursors for secondary organic
aerosol (SOA) formation, among other
species like O3 and SO2. The telescope was oriented
alternately to two reflector arrays at an
altitude of 35m and at the ground. Measured
VOCs included benzene derivatives e.g. toluene,
styrene, phenol, cresols and xylenes, as well as
two ring aromatic compounds, e.g., naphthalene
(Figure 2.8 right panel) and methylnaphthalene.
Episodes of remarkably high concentrations of
toluene (e.g.190 ppb on March 8 left panel) and
styrene (14.5 ppb on March 23) were observed.
DOAS2 was dedicated mainly to measure HOx
radical precursors (Glyoxal, HONO, HCHO, O3).
The NO2 measurement shows a mean daily maximum
of 71ppb with maximal values of 133 ppb.
Glyoxal, a product of VOC oxidation, showed
maximum concentrations between 0.5 and 1.4 ppb
. For the first time during a field campaign a new
fibre optics set up was used to couple the light
source in the telescope and to receive the light
from the reflector. This set-up provides a higher
stability of the alignment and improves the spectral
characteristics of the light source. A particular
focus of the combined DOAS setup was to
assess horizontal gradients of species that were
measured by both instruments on different spatial
scales and directions. While similar values
are measured for NO2, SO2, HONO, HCHO and
O3, the concentrations of VOC differ significantly
on the two different light paths probably due to
plumes of solvents moving through the city (left
panel).
Outlook/Future work The extensive data set
combined with other measurements like meteorological
information will allow to evaluate chemical
models describing the air pollution in megacities.