Exploratory Field Measurement Campaign in the MCMA
![](../jpgs/field_1_tnail.jpg)
Figure 1. Configuration
of the Mobile
Laboratory for the February 2002
Exploratory MCMA Field Campaign. |
Now in its second phase, the Mexico City Program has moved from assessment
to an active measurement phase. A major activity of the program’s
second phase involves field measurement campaigns to update and improve
the MCMA emissions inventory, and to improve current knowledge of
the chemistry, and dispersion and transport processes of pollutants
emitted to the MCMA atmosphere.
A series of exploratory mobile and fixed-site field measurements,
funded by the Comisión Ambiental Metropolitana (CAM) and
coordinated by Dr. Luisa Molina, were performed during February
2002, and an intensive five-week field measurement campaign is scheduled
to run from late March through early May of 2003. The campaign was
conducted utilizing a mobile laboratory developed at Aerodyne Research,
Inc. (ARI) and equipped with a range of realtime particle and trace
gas diagnostics, as well as various other instruments from Washington
State University (WSU), Montana State University (MSU), and the
Massachusetts Institute of Technology. The Mexican field measurement
team included scientists from the Mexican Petroleum Institute (IMP),
National University of Mexico (UNAM), Metropolitan University (UAM-I,
UAM-A), National Institute of Public Health (INSP), as well as investigators
from Mexican government agencies involved in air quality in the
MCMA—Federal Ministry of the Environment and National Resources
(SEMARNAT), Secretary of the Environment of the Government of the
Federal District (SMA-GDF), and the Secretary of Ecology of the
State of Mexico.
The layout for the ARI Mobile Laboratory, as deployed in Mexico
City last February, is shown in Figure 1. Realtime measurements
of selected trace gases were provided by ARI’s tunable infrared
laser differential absorption spectroscopy (TILDAS) instruments
using lead salt diode lasers. Fast response fine aerosol size distributions
(40–2000 nm) and non-refractory chemical composition were
measured with a novel aerosol mass spectrometer (AMS) designed and
built at ARI. Fixed site rapid response measurements of selected
aromatic and partially oxygenated VOCs were provided by a proton
transfer reaction mass spectrometer (PTR-MS) operated by MSU. A
commercial LICOR non-dispersive infrared (NDIR) unit provides realtime
CO2 measurements and a commercial TSI condensation particle counter
(CPC) provides realtime total number densities for particles with
diameters between 10 and 1000 nm. Commercial air quality monitors
for NO/NO2, CO and O3 supplied by WSU were also included. Ultraviolet
(UV) solar flux was recorded by an Epply total UV radiometer. WSU
collected VOC whole air samples at selected sampling points within
the urban area using portable canister samplers.
During the exploratory field measurement campaign the mobile lab
operated in three modes: First, stationary deployments at selected
RAMA monitoring sites were used to measure diurnal pollution levels
in various sectors of the MCMA and to provide high time resolution
research instrument data for selected pollutants to help validate
and amplify data from the monitoring instruments. Second, mobile
measurements were made through selected city sectors to map ambient
pollutant levels, and sample and characterize mobile and fixed site
emission plumes. Finally, dedicated chase experiments were mounted
to be?er characterize gaseous and particulate emissions from selected
classes of vehicles, including heavyduty diesel trucks, buses, and
colectivos. While detailed analysis of the February 2002 data is
ongoing, a sampling of findings is presented here to indicate the
quality and import of the data.
![](../jpgs/field_2.jpg)
Figure 2. Mobile laboratory
measurements of diurnal gaseous pollutant and fine aerosol
species concentrations at Pedregal, Feb. 15-16, 2002. |
Figure 2 shows a sampling of diurnal, fixed site data taken at
the RAMA station on the grounds of an elementary school in Pedregal.
Situated in the southwest portion of the Federal District, Pedregal
receives a pollutant flow when the normal wind pattern blows southwest
from the metropolitan and industrial areas. Even though the weather
was not unusually hot or sunny, tremendous photochemical activity
can be observed, with ozone levels exceeding 250 ppbv on the afternoon
of Feb. 15, and 200 ppbv on the afternoon of Feb. 16 (gray trace
in second panel from top). One key to this photochemical activity
are the high levels of formaldehyde (green trace in center panel,
measured with the TILDAS instrument); HCHO levels of 20 to 40 ppbv
build up in the morning, preceding the photochemical oxidant peaks
by several hours. Data from the core urban and industrial areas
to the northwest show even higher morning HCHO buildups, even on
completely cloudy mornings with no appreciable photochemical activity
(in general the weather during the Feb. 2002 campaign was unusually
cloudy, rainy and cool), demonstrating large emission sources for
this key photochemical trigger species.
![](../jpgs/field_3_tnail.jpg)
Figure 3. Comparison of
transient mobile source exhaust plume: HCHO/CO2 correlations
from traffic in the MCMA (02/02) and metropolitan Boston (03/02). |
Finally, the top panel of AMS data shows a photochemical “bloom”
of fine (mode peak at 400-500 nm) secondary aerosols composed primarily
of ammonium nitrate, ammonium sulfate, and partially oxidized VOCs.
The simultaneous early afternoon production of large concentrations
of photochemical oxidants (ozone, nitrogen dioxide, organic nitrates)
and secondary fine aerosol particles demonstrates how closely connected
the photochemical oxidant and fine aerosol problems are in the MCMA,
and suggests they will have to be dealt with in concert, not as
separate issues.
Data taken during transverse MCMA “mapping” activities
with the mobile laboratory confirms that a significant emission
source of HCHO is present and persistent. Using the fast response
(1s) TILDAS and NDIR measurements, the amount of HCHO present when
the mobile laboratory samples a transient CO2 plume, predominantly
from mobile vehicle sources, can be evaluated. Figure 3 shows the
correlation (in blue) between the concentration of CO2 and HCHO
in MCMA transient plumes during one transect, while the same correlation
for metropolitan Boston traffic in mid-March 2002 is shown in red.
The average HCHO/CO2 ratio in the MCMA plumes is 0.0002. The exploratory
MCMA field campaign also showed the utility of using the mobile
laboratory in “chase” mode, where classes of motor vehicles
of interest can be specifically sampled to better quantify their
emissions characteristics as a function of driving cycles. In this
mode a target vehicle of the desired type is followed as the mobile
laboratory’s sampling probe moves in and out of its exhaust
plume.
MCMA-2003 Field Campaign
The MIT/CAM MCMA experimental team has committed to an intensive,
five-week field campaign in the spring of 2003. The campaign’s
field measurement phase is planned to extend from late March to
early May and is designed to cover the height of the annual photochemical
season just prior to the onset of the rainy season. This period
will include measurements taken before, during, and after Holy Week
(April 14-20, 2003), when vehicular traffic is historically reduced
as city residents leave for the holiday. By taking measurements
before, during and after this period, it is likely that a better
assessment of the vehicular emissions contribution to the air quality
reduction in this megacity can be assessed with regard to oxidant
and aerosol levels.
Unlike the February 2002 exploratory field campaign that focused
primarily on mobile laboratory measurements, the 2003 campaign will
also involve a highly instrumented “supersite” located
at the field campaign headquarters at the National Center for Environmental
Research and Training (Centro Nacional de Investigación y
Capacitación Ambiental or CENICA). The fixed supersite will
be enhanced by the deployment of one or two research grade differential
optical analysis spectroscopy (DOAS) instruments and a research
LIDAR. It will also host collaborative measurement campaigns by
several US research groups, including Drs. Jeffrey Gaffney and Nancy
Marley of the US Department of Energy’s (DOE) Argonne National
Laboratory, and Drs. Janet Arey and Roger Atkinson of the University
of California at Riverside. In addition, similar to February 2002
exploratory campaign, extensive meteorological data as well as a
wide range of fixed site chemical data will be available from collaborating
Mexican research groups.
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