Project aguila: In situ measurements of Mexico City air pollution by a research aircraft

Atmospheric Environment Vol. 26B, No. 4, pp. 445~451, 1992. Printed in Great Britain.

0957 1272/92 $5.00+0.00 Pergamon Press Ltd

P R O J E C T AGUILA: I N S I T U M E A S U R E M E N T S O F M E X I C O CITY AIR P O L L U T I O N BY A R E S E A R C H A I R C R A F T EVERETT C. NICKERSON,* GUSTAVO SOSA,t HEIDY HOCHSTEIN,f PAULA McCASLIN,++ WINSTON LUKE§ a n d ALLEN SCHANOT§ *Forecast Systems Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO 80303, U.S.A., tProcesos Energ6ticos Alternos y Quimica Ambiental, Instituto Mexicano del Petr61eo, Mexico D.F., :~National Systems and Research, Boulder, CO 80303, U.S.A. and §Research Aviation Facility, National Center for Atmospheric Research, Boulder, CO 80307, U.S.A. (First received 7 February 1992 and in final form 2 April 1992)

Abstract--Measurements of aerosol concentrations, chemical species and meteorological quantities in the air above Mexico City were obtained from an instrumented research aircraft. Concentrations of particles in the size range between 0.12 and 3.12/lm were nearly invariant with height, and typical values were of the order of 5000 cm-3. However, particles smaller than 0.12 #m were confined to the lowest few hundred meters of the atmosphere until the morning temperature inversion dissipated, after which time those particles, together with newly formed particles created by secondary processes, mixed to a greater height above the city. Total particle concentrations near the surface attained values in excess of 60,000 cm- 3. An examination of the corresponding profiles of SO2 suggests that combustion processes are likely sources for the additional small particles. Key word index: Mexico City pollution, pollution profiles, aircraft pollution data, orography.

INTRODUCTION As is the case with many large cities around the world, the air over Mexico City is subject to an infusion of gaseous and particulate contaminants from a variety of sources (Bravo, 1987; Bravo et al., 1988; J~iuregui et al., 1981). However, the quality of the air is degraded by the city's location at an elevation 2.2 km on a high plateau open to the north, but with mountains on the west, south and east that reach altitudes in excess of 5 km. Further compounding the problem is the poor vertical mixing that often occurs during the dry winter months and the local terrain-induced airflow patterns that move contaminants and precursors from one area of the city to another, thereby giving rise to pollutant levels far in excess of what would be expected from just the local emissions. Although previous surface-based measurement programs (Barfoot et al., 1984; Herera and Castro, 1988) have yielded concentrations of particulate matter eight times greater than that measured in Chicago (Barbiaux, 1990), those values are considerably less than the concentrations reported in this paper. High levels of pollution are responsible for more than the severe impairment to visibility that occurs throughout much of the year. Those pollution levels inflict a heavy toll on the general health of the populace (Instituto Nacional de Estadistica, 1990) and also on the buildings and vegetation that are subjected to the concomitant wet and dry pollutant deposition. In February 1991, project AGUILA was carried out in support of a larger surface-based observational program involving the Instituto Mexicano del Pe445

tr61eo (IMP) and the U.S. Department of Energy's Los Alamos National Laboratory. Aguila, Spanish for eagle, is also an acronym for a program of "Acci6n para GUardar al Instante la Limpieza del Aire," which may be roughly translated as a program of action to watch over and protect the quality of the air over Mexico City. An instrumented Beechcraft King Air, operated by the National Center for Atmospheric Research, flew approximately 40 h during the course of 14 flights on 12 different days. The base of operations was Mexico City's Benito Juarez airport, situated at an altitude of 2.2 km, where the atmospheric pressure is 25% less than the typical value at sea level. This reduced pressure has a significant impact on the efficiency of engines, thereby leading to increased emissions of carbon monoxide and hydrocarbons (EPA, 1978). The lower atmospheric density at high altitudes also causes an increase in the coagulation rate for small aerosol particles. Moreover, there is more ultraviolet radiation resulting in increased ozone production from NOx and hydrocarbon precursors. Since atmospheric moisture normally decreases with altitude, particles grow more slowly in the drier atmosphere. Those smaller particles have a smaller settling velocity, resulting in a corresponding increase in particle residence times.

DISCUSSION Data from the flights of 13 and 19 February are especially interesting because the existence of both

446

E.C. N[CKERSONetal.

morning and afternoon flights provides information on the temporal evolution of the pollution over the urban area. Although there were differences between each flight, some of which were intentional and some of which were imposed by air traffic controllers, the research flight tracks were nearly identical. The standard flight pattern consisted of a climb toward the northeast, eventually reaching an altitude of 4.6 km msl, followed by two or three E-shaped patterns at different altitudes above the metropolitan area. The aircraft then made a final ascent, returning to an altitude of 4.6 km in preparation for a descent sounding back to the airport. However, the descent sounding turned out to be of considerably less value than the ascent sounding owing to the flight restrictions imposed by airport control authorities. Other special flight patterns were flown to coincide with a satellite overpass and to make measurements in the vicinity of a power plant. Figure 1 shows the track of the aircraft for the afternoon flight of 19 February 199l, superimposed on the orography of the valley of Mexico and surrounding mountains. Figure 2 shows only the ascent portion of that flight. Aircraft data are presented for only the initial segments of the flights, thereby ensuring that each sounding corresponds to the same location above the city. The winds on the afternoon of 13 February were from the northwest at speeds less than 5 m s- ~ in the first 1500 m above the city. The winds on 19 February were from the southwest and nearly twice as strong. This difference in the prevailing air flow may be important in a subsequent discussion of the temporal evolution of 0 3 over the city. The two days were generally bright and sunny, with only some scattered upper-level clouds being present. Vertical profiles of total particle concentration, large particle concentration and temperature for the morning flight of 13 February are shown in Fig. 3a. We note that the aircraft recorded total particle concentrations of 60,000 cm 3 in the layer below the low-level temperature inversion of that flight. There was also a pronounced temperature inversion at 3.8 km. Although there was a residual particle concentration from the previous day of 10,000 cm -3 in the layer between the two inversions, the upper-level air was practically devoid of particles. The corresponding profiles of SO 2 and 0 3 shown in Fig. 3b are much smoother than those for the particles, owing to differences in the response times of the instruments. Total particle concentrations were obtained from a TSI Model 3760 CN Counter, the concentrations of large particles from a Particle Measuring Systems Model ASSAP probe, the SO2 from a Thermo Environmental Model 43a and the 0 3 from a Thermo Environmental Model 49. Taking those differences into account, the SO2 profile is in reasonable agreement with the profile of particle concentration. Moreover, the capping of the ozone by the upper temperature inversion and the small values close to the surface are

what one would expect over an urban area in the morning hours. We also note that ozone concentrations in the air above the inversion were less than 30 ppb. The corresponding soundings for the afternoon flight are shown in Fig. 4a, where it is seen that the temperature inversion has disappeared and that the entire aircraft ascent was characterized by an adiabatic lapse rate of temperature. Both the small and large particles were well mixed in the air below the inversion. Light winds and sunny skies provided the setting for a large increase in ozone, with values exceeding 140 ppb (Fig. 4b). The previously noted small values of ozone in the tropospheric air above the city exclude downward mixing as a source of that ozone. The SO 2

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AGUILA: Mexico City air pollution

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produced during the early morning hours was lifted to higher elevations as the day progressed and those concentrations were undoubtedly affected by deposition and gas-particle conversion processes. Figure 5a shows the corresponding particle concentration plots for the morning of 19 February. Total particle concentration values in excess of 45,000 c m - 3 were found in the lowest few hundred meters above the ground, but the air above the temperature inversion had few particles. It should be noted that the values for the large-particle mode shown in Fig. 5a represent conservative estimates of their actual values owing to a rare instrument problem with the ASSAP probe. Particles in the size range 0.12-0.145 #m were subtracted from the count of the large particles measured by the ASSAP probe, and the "large" particle distribution shown in Fig. 5a reflects that reduction. However,

since the profile is bounded on the left by the zero line and on the right by the independent measurement of total particle concentration, the error does not appear to be especially noteworthy. None of the large-particle data presented in any of the other figures appear to exhibit this problem. As was the case for the morning of 13 February, Fig. 5b shows that the large values of SO 2 were confined to the boundary layer. Ozone was nearly constant with height, having a value of 40 ppb. The afternoon soundings of particles and temperature shown in Fig. 6a again reveal a well-mixed adiabatic layer. The corresponding profiles of SO2 and 03 shown in Fig. 6b indicate a much smaller rate of ozone production than for 13 February. One possible explanation for that difference is the stronger synoptic flow from the southwest on 19 February, resulting in increased

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CONCLUSION

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mixing and horizontal ventilation of the ozone precursors northward, away from the metropolitan area. The previously discussed values of total concentration include particles that range in size from 0.01 to 3.0 #m, but the TSI particle counter does not provide any information on the size distribution of the particles. However, an independent measurement of the larger particles in the size range from 0.12 to 3.12 #m was made with the ASSAP probe which provides a particle size histogram. In Fig. 7 we show the size spectra for both the morning and afternoon flights on 19 February (the spectra for 13 February are similar). The width of the bins varies from 0.025 pm for the first bin to 0.375 #m for the 15th bin, and we note that there was a continuous decrease in number concentration with increasing particle size. The spectral width of

Aircraft data obtained during project AGUILA provide the most comprehensive picture to date of the temporal and spatial variability of the state of the atmosphere above Mexico City. Unfortunately, however, it was not possible to determine the elemental composition of the airborne particles. The deleterious health effects that result from exposure to those particles are related to both their size and composition. Medical evidence indicates it is the total number of particles rather than just the total mass or weight of the particles that is important. Furthermore, the retention of airborne particles within the body depends very much on the hygroscopicity, as well as the elemental, composition of those particles (Schlesinger, 1988). It is therefore appropriate to emphasize the number concentration rather than the volume or mass distributions in order to provide a basis for assessing the seriousness of the pollutant levels. The data presented here represent but a small portion of a very large database that may be useful in the initialization and verification of complex chemical models. Numerical models (Nickerson et al., 1986; Chaumerliac et al., 1991), including three-dimensional depictions of the meteorology and chemistry in the air above Mexico city, are expected to play an important role in assessing the impact of proposed control strategies. Those models require input data at all

AGUILA: Mexico City air pollution computational grid points, not just at the surface. The vertical profiles presented in the previous section emphasize the need for caution in assuming a vertical distribution of either gases or particles based only on a knowledge of their values near the ground. Surface monitoring networks can give an indication of the horizontal variability of the pollutant distribution, but they cannot provide information on the vertical structure and hence the residual values of pollutant concentrations that may have been left over from the previous day. The first two images were rendered on a Stardent 3000 computer using AVS-III graphics. This versatile graphical analysis tool will greatly enhance our ability to examine the complete aircraft data set obtained from project A G U I L A , in that any of the quantities measured by the aircraft can be color coded and displayed in the same manner. But most importantly, AVS permits the viewing of the data in a threedimensional framework superimposed on the terrain and viewed from an arbitrary location. The data presented in this paper provide information on concentrations of particulate matter and chemical species in the air above the city where previously none existed. However, we strongly urge that future research flights collect in situ samples of airborne particles in order to determine their elemental composition. That knowledge would aid in the identification of particle precursors, thereby providing additional information that could be useful in the promulgation of air quality control strategies. Acknowledgements We thank F. Guzman and M. Ruiz of the IMP for providing the funds for the aircraft program, and P. Middleton, N. Chaumerliac, G. Sisniega and R. Schlesinger for informative discussions. Special thanks go to M. Leiva, V. H. P~iramo and E. J~iuregui for their support and encouragement. The authors are particularly grateful to a reviewer for pointing out the likelihood of instrument error in one of the figures.

451 REFERENCES

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