Size-resolved aerosol emission factors and new particle formation/growth activity occurring in Mexico City during the MILAGRO 2006 Campaign

Measurements of the aerosol size distribution from 11 nm to 2.5 microns were made in Mexico City in March 2006, during the MILAGRO (Megacity Initiative: Lo- cal and Global Research Observations) field campaign. Ob- servations at the urban supersite, referred to as T0, could of- ten be characterized by morning conditions with high particle mass concentrations, low mixing heights, and highly corre- lated 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 de- termined using a comparison of peak sizes in particle number and CO2 concentration. Peaks are determined by subtract- ing the measured concentration from a calculated baseline concentration time series. The number emission and vol- ume emission factors for particles from 11 nm to 494 nm are 1.56◊10 15 particles, and 9.48◊10 11 cubic microns per kg of carbon, respectively. The uncertainty of the number emission factor is approximately plus or minus 50 %. 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 pol-