Abstract
Abstract. As part of the MILAGRO field campaign, the DOE G-1 aircraft was used to make measurements over and downwind of Mexico City with the objective of determining growth characteristics of aerosols from a megacity urban source. This study focuses on number concentration and size distributions. It is found that a 5-fold increase in aerosol volume is accompanied by about a 5-fold increase in accumulation mode number concentration. There is growth in aerosol volume because there are more accumulation mode particles, not because of an increase in the average size of accumulation particles. Condensation and volume growth laws were examined to see whether either is consistent with observations. Condensation calculations show that the growth of Aitken mode particles into the accumulation mode size range gives the required increase in number concentration. There are minimal changes in the accumulation mode size distribution with age, consistent with observations. Volume-growth in contrast yields a population of large particles, distinctly different from what is observed. Detailed model calculations are required to translate our observations into specific information on the volatility and properties of secondary organic aerosol.
Highlights
The MILAGRO field campaign, conducted in March, 2006 (Molina et al, 2008) was designed so that air masses carrying emissions from Mexico City and other sources could be sampled at varying downwind distances
As in our previous study of aerosol growth in the Mexico City urban plume (K2008), the time evolution of aerosol properties has been determined by 1) defining an urban data set, 2) splitting the urban data set into subsets according to photochemical age, and 3) for each subset performing a linear regression of the aerosol property vs. CO
Regression slopes yield the volume, mass, or number concentration of aerosol per ppm of urban CO above background. When these slopes are plotted as a function of photochemical age they portray the changes in aerosol volume, mass, or number that occur during photochemical aging
Summary
The MILAGRO field campaign, conducted in March, 2006 (Molina et al, 2008) was designed so that air masses carrying. In this study we present results from the DOE G-1 aircraft on the size distribution of aerosol particles and their time evolution over the Mexico City plateau. In that case the formation rate of H2SO4 is proportional to reaction volume, yielding a “volume growth law” dDp/dt∼Dp. In the diffusion controlled case, small particles grow in diameter faster than large ones. Further complicating the derivation of SOA growth laws are major uncertainties as to VOC precursors, kinetics of gas to particle transfer, composition and properties of condensed phase(s), and aerosol-phase reactions (Kroll et al, 2007; Robinson et al, 2007; Kroll and Seinfeld, 2008). As Bowman et al (1997) caution, aerosols are not necessarily in equilibrium and, “the resulting size distribution of the condensed semi-volatile product cannot be uniquely determined without accounting for size-dependent rates of mass transfer between the gas and aerosol phase”. A qualitative explanation is sought by examining simplified versions of condensation and volume growth mechanisms
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