Size-resolved aerosol emission factors and new particle formation/growth activity occurring in Mexico City during the MILAGRO 2006 Campaign
1Department of Chemical and Biochemical Engineering, University of Iowa, Iowa City, IA 52242, USA
2Atmospheric Science Division, Brookhaven National Laboratory, Upton, NY 11973, USA
3Department of Civil and Environmental Engineering, University of Iowa, Iowa City, IA 52242, USA
4School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822, USA
5NASA Langley Research Center, Hampton, VA 23681, USA
6Department of Chemistry, University of California-Irvine, Irvine, CA 92697, USA
Abstract. Measurements of the aerosol size distribution from 11 nm to 2.5 microns were made in Mexico City in March, 2006, during the MILAGRO field campaign. Observations at the T0 research site could often be characterized by morning conditions with high particle mass concentrations, low mixing heights, and highly correlated particle number and CO2 concentrations, indicative that particle number is controlled by primary emissions. Average size-resolved and total number- and volume-based emission factors for combustion sources impacting T0 have been determined using a comparison of peak sizes in number and CO2 concentration peaks. The number emission and volume emission factors for particles from 11 nm to 494 nm are 1.23 × 1015 particles, and 7.54 × 1011 cubic microns per kg of carbon, respectively. Uncertainty on the number emission factor is approximately a factor of 1.5. The mode of the number emission factor was between 25 and 32 nm, while the mode of the volume factor was between 0.25 and 0.32 microns. These emission factors are reported as log normal model parameters and are compared with multiple emission factors from the literature. In Mexico City in the afternoon, the CO2 concentration drops during ventilation of the polluted layer, and the coupling between CO2 and particle number breaks down, especially during new particle formation events when particle number is no longer controlled by primary emissions. Using measurements of particle number and CO2 taken aboard the NASA DC-8, this emission factor was applied to the MCMA plume; the primary emission factor predicts less than 50% of the total particle number and the surplus particle count is not correlated with photochemical age. Primary particle volume and number in the size range 0.1–2 μm are similarly too low to predict the observed volume distribution. Contrary to the case for number, the apparent secondary volume increases with photochemical age. The size distribution of the apparent increase, with a mode at ~250 nm, is reported.