Measurement of Mexico City nanoparticle size distributions: Observations of new particle formation and growth

M. Dunn1,2, J.-L. Jiménez2,3, H. Sakurai4, P. H. McMurry4, F. L. Eisele5,6, J. N. Smith5

2. Instrument OverviewThese experiments were carried out using a scanning mobility particle sizer (SMPS), assembled at the University of Minnesota and consisting of a Nanometer Differential Mobility Analyzer (nano-DMA, model 3085; TSI, Inc.) and an Ultrafine Condensation Particle Counter (UCPC, model 3025A; TSI, Inc.). Size distributions were acquired in ca. 3 minute intervals using control and analysis software written in Labview. The SMPS ran in underpressure mode with the UCPC drawing the sample flow through the NDMA at a rate of 1.5 l min-1 and a recirculating blower maintaining a 15 l min-1 sheath flow through the nano-DMA. At the rural site (Santa Ana), outside air was drawn in at 3 l min-1 while the SMPS shared an inlet with other instruments. For the first two weeks of sampling at CENICA, outside air was drawn in at 10.5 l min-1 while the SMPS shared its inlet with an Aerosol Mass Spectrometer (AMS). During the last week of sampling at CENICA, there were no other instruments sharing the inlet, thus flow was drawn in at 1.5 l min-1.

1. IntroductionThe Mexico City metropolitan area, with a population of 16.4 million (estimated in 2000) is the sixth largest urban agglomeration in the world. In spite of this, there have been no reported measurements of the distribution of nanoparticles, with diameters up to 50 nm. Here we report the first such measurements, performed during the MCMA-2003 campaign from April 7 to May 11, 2003. These ground-based measurements were made at a rural, mountain pass site in the SE sector of the Mexico City Federal District and at the CENICA laboratory located near the center of the district. Objectives of this work were to determine the frequency of new particle formation events, determine the atmospheric chemical and meteorological conditions which lead to these events, and observe subsequent aerosol condensational growth of regional nucleation events through the evolution of the size distribution.

1National Center for Atmospheric Research, Advanced Study Program, P.O. Box 3000, Boulder, CO 80307 2University of Colorado-Boulder, Chemistry Dept., 216 UCB, Boulder, CO 80309 3Cooperative Institute for Research in Environmental Sciences, 216 UCB, Boulder, CO 80309 4Mechanical Engineering Dept., University of Minnesota, 111 Church Street SE, Minneapolis, MN 55455 5National Center for Atmospheric Research, Advanced Study Program, P.O. Box 3000, Boulder, CO 80307 6Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332

AcknowledgementsThe authors gratefully acknowledge Mario and Luisa Molina of M.I.T. for overseeing the MCMA-2003 campaign, Ben de Foy of M.I.T., Alejandra Sanchez of CENICA, and Oscar Fentanes of CENICA for providing ghase phase and meteorological data, and Darrel Baumgardner, TelmaCastro, and Mireya Moya of UNAM for invaluable help securing the use of the Santa Ana site. This work was funded by the Advanced Study Program of the National Center for Atmospheric Research.

4. Santa Ana ResultsTwo major nucleation events were recorded in Santa Ana, though close examination of Figure 1 suggests that multiple, less intense events may have also occurred. Figure 1 shows SMPS data in conjunction with gas phase and meteorological measurements during the second major event, on April 19, 2003. Both instances of nucleation occurred around the same time (9:00am) on Saturday mornings with nearly identical changes in the wind pattern and ambient mixing ratios of SO2.

Figure 1. (a) SMPS particle counts differentiated between aerosols with diameters between the lower limit of detection (~3 nm) up to 10 nm and aerosols with diameters greater than 10 nm to the upper limit of detection (~49 nm). (b) Mixing ratios of NOx and SO2 during the sampling period. On April 12 and 19, very large bursts of sub-10 nm particles precede increased counts of Dp > 10 nm aerosols. These events correlate with peaks in ambient SO2 concentration, whereas no consistent relationship exists between NOx or PM10 (not shown) during these events. Events characterized only by an increase in Dp > 10 nm counts (e.g., afternoon of April 11) correlate with NOx, suggesting combustion sources.

Figure 2. (a) Plot of particle size distribution (dN/dlogDp) vs. local time for April 19, 2003. Around 09:00, a plume of up to 106 particles cm-3 spanning the diameter range of 3 to 12 nm was observed. Subsequent evolution of the distributions towards larger particle sizes over next several hours can be interpreted as particle growth. The diameter growth rate over this period is 5 nm hr-1, based on number mean diameter calculated for each size distribution. (b) Gas and meteorological data recorded at this site show that within minutes of this nucleation event’s onset, winds began blowing from the North and the SO2 mixing ratio rose substantially.

3. Sampling SitesFrom April 7–20, 2003, measurements were performed at the NE corner of Santa Ana Tlacotenco (hereafter referred to as Santa Ana), a small town resting near the SE edge of the Mexico City Federal District and on the Western rim of a mountain pass that channels out-flowing air from the urban valley. The Eastern range of this mountain pass includes Popocatépetl, an active volcano roughly 20 km ESE from the sampling site. After these two weeks, the instrument was then taken into Mexico City, to the CENICA roof-top laboratory on the Universidad Autónoma Metropolitanacampus in the Iztapalapa delegation, where measurements continued from April 22 to May 11, 2003. A mix of commercial and residential areas exist in immediate proximity to the CENICA site; large traffic ways are within several city blocks.

The above picture is the western view of Mexico City from atop a flux tower constructed on the roof of the CENICA building (image from the MCMA-2003 website).

The above picture is a northeastern view of the Valley of Mexico from the Santa Ana sampling site (courtesy of Peter DeCarlo).

CENICA in the city, Santa Ana in the pass: These maps detail the locations of the sampling sites in relation to the Federal District. The right image gives a topographical view of the site locations in the Valley of Mexico.

CENICA

Santa Ana

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6. DiscussionNucleation events have been observed in Mexico City and in the surrounding metropolitan area. Events seen in Santa Ana strongly suggest that sulfuric acid (H2SO4) acted as the nucleating agent because they occurred when the mixing ratio of SO2 was elevated in the daylight hours only. The fact that particles in the 3-10 nm Dp range were detected at the onset of an event also indicate that nucleation occurred in close proximity to the sampling site. No consistent relationship between NOx, CO, or PM10 and Santa Ana nucleation events exists. NOx does correlate well with CO, however, and higher values of either of these species coincide with increased 10-40 nm Dp particle concentrations only, suggesting combustion sources.

Similar conclusions relating CO, NOx SO2 and the measured aerosol size distribution can be made for data collected inside the city at CENICA, yet there were a few particle events that occurred simultaneous to elevated SO2 concentrations that did not increase the concentration of the lowest detectable Dp. These events may still be an indication that nucleation is occurring inside the city, but not in close proximity to the sampling site. Suspected nucleation events also correlate with minima in PM10 and PM2.5 mass concentrations; this fact may support the argument that aerosol coagulation and condensation on background aerosol suppress nucleation in the presence of higherconcentrations of condensable gases.

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AMS Main Species (Dva = 25-150 nm ) SO4 NH4 NO3 Organics / 4

AMS Organic Markers (Dva = 25-150 nm ) m/z 57 (Mostly Combustion Marker) m/z 44 Oxidized / SOA Marker

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5. CENICA ResultsBecause improper flows were set for the system between April 22 – May 1, only the sampling period between May 2–11, 2003 will be considered. Many nanoparticle plumes were observed during this period. Most of these plume events are growth of Dp > 10 nm particles that coincide with elevated mixing ratios of NOx, NOx and SO2 or SO2 alone, as can be seen in Figure 3.

Figure 3. (a) Particle counts through the SMPS system differentiated between aerosols with diameters from the lower limit of detection (~3 nm) up to 10 nm and aerosols with diameters greater than 10 nm to the upper limit of detection (~49 nm). (b) Mixing ratios of NOx and SO2 over the same time period. In the morning hours, many peaks in the mixing ratio of SO2 coincide with elevated NOx levels.

Each morning, highly concentrated plumes of Dp > 10 nm particles were measured and coincided with elevated concentrations of NOx, CO, and/or organics. Suspected nucleation events are characterized by high counts of Dpabove and below 10 nm (see, for example, May 3 in Figure 4a). The higher SO2mass concentrations during these events also extend into sizes below those normally seen in background SO2 aerosol (Figure 4c). The Dp > 10 nm plume event in the afternoon of May 2 correlates with peak concentrations of SO2, suggesting that nucleation may have occurred at some distance from the site and particles grew to these larger sizes before detection.

Furthermore, suspected regional nucleation events at CENICA occurred in the afternoon and a few hours after the wind direction had changed direction but while it was still blowing from the North. Almost every afternoon, PM10 and PM2.5 mass concentrations reached their lowest levels (Figure 4e); in Santa Ana such a trend was not seen in PM10 concentrations. These PM minima correlate very well with suspected nucleation events, which suggests that the reduction in background surface area may have allowed for condensational growth of nucleated particles to dominate over coagulation.

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Figure 4. Coincident measurements particle size, composition, and gas and meteorological data at CENICA for May 2-4, 2003. (a) SMPS particle counts for 3 nm < Dp ≤ 10 nm and 10 nm < Dp < 49 nm. (b) Mass concentration profiles of aerosol sulfate, nitrate, ammonium, and organics measured with an Aerosol Mass Spectrometer. (c) Aerosol sulfate and organic mass concentration vs. aerosol vacuum diameter and local time. (d) Plot of SMPS particle size distribution (dN/dlogDp) vs. local time. (e) Gas and meteorological data recorded at this site.

d dN/dlogDp