Abstract
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 (Megacity Initiative: Local and Global Research Observations) field campaign. Observations at the urban supersite, referred to as T0, 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 particle number and CO2 concentration. Peaks are determined by subtracting the measured concentration from a calculated baseline concentration time series. The number emission and volume emission factors for particles from 11 nm to 494 nm are 1.56 × 1015 particles, and 9.48 × 1011 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 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, the determined primary emission factor was applied to the Mexico City Metropolitan Area (MCMA) plume to quantify the degree of secondary particle formation in the plume; the primary emission factor accounts for 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 explain 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.
Highlights
Numerous studies have shown the adverse effects of particulate matter (PM) on human health, with increased interest placed on ultrafine particles which become more toxic per unit mass with decreasing size (Mills et al, 2009; Oberdorster et al, 2005; Osornio-Vargas et al, 2003; Delfino et al, 2005)
Observations were made by the University of Iowa and by the Department of Energy Brookhaven National Laboratory in Mexico City during the MILAGRO field campaign in March 2006, in order to characterize the T0 urban size distribution, to study the primary emissions around T0, and to characterize new particle formation and ultrafine particle growth
The average number emission factor as a function of time of day, and day of the month were examined in order to determine the impact that meteorological changes, biomass burning, or new particle formation may have on the overall emission factor
Summary
Numerous studies have shown the adverse effects of particulate matter (PM) on human health, with increased interest placed on ultrafine particles which become more toxic per unit mass with decreasing size (Mills et al, 2009; Oberdorster et al, 2005; Osornio-Vargas et al, 2003; Delfino et al, 2005). Primary and secondary particles vie for growth through condensation of low volatility gas phase species, which can homogeneously nucleate to form new particles as previously mentioned (Wang and Penner, 2009). Any uncertainty in the size distributions or rates of primary emissions or of secondary particle formation may lead to large uncertainty in the predicted CCN concentration (Adams and Seinfeld, 2002). Accurate knowledge of the size distribution and number concentration of atmospheric particles has been determined to be critical for prediction of CCN concentration, global models representing aerosol number concentration assume the number and size of particles from mass emissions (Chang et al, 2009; Pierce and Adams, 2007; Yu et al, 2010), due to the fact that inventories of anthropogenic emissions are based on mass rather than number concentration. Both the emission factor itself, and the method of its recovery ( if refined using fast number size distribution measurements), may be valuable for continued refinement of size resolved emission factors
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