Abstract. The time evolution of aerosol concentration and chemical composition in a megacity urban plume was determined based on 8 flights of the DOE G-1 aircraft in and downwind of Mexico City during the March 2006 MILAGRO field campaign. A series of selection criteria are imposed to eliminate data points with non-urban emission influences. Biomass burning has urban and non-urban sources that are distinguished on the basis of CH3CN and CO. In order to account for dilution in the urban plume, aerosol concentrations are normalized to CO which is taken as an inert tracer of urban emission, proportional to the emissions of aerosol precursors. Time evolution is determined with respect to photochemical age defined as −Log10 (NOx/NOy). The geographic distribution of photochemical age and CO is examined, confirming the picture that Mexico City is a source region and that pollutants become more dilute and aged as they are advected towards T1 and T2, surface sites that are located at the fringe of the City and 35 km to the NE, respectively. Organic aerosol (OA) per ppm CO is found to increase 7 fold over the range of photochemical ages studied, corresponding to a change in NOx/NOy from nearly 100% to 10%. In the older samples the nitrate/CO ratio has leveled off suggesting that evaporation and formation of aerosol nitrate are in balance. In contrast, OA/CO increases with age in older samples, indicating that OA is still being formed. The amount of carbon equivalent to the deduced change in OA/CO with age is 56 ppbC per ppm CO. At an aerosol yield of 5% and 8% for low and high yield aromatic compounds, it is estimated from surface hydrocarbon observations that only ~9% of the OA formation can be accounted for. A comparison of OA/CO in Mexico City and the eastern U.S. gives no evidence that aerosol yields are higher in a more polluted environment.

Abstract. The time evolution of aerosol concentration and chemical composition in a megacity urban plume was determined based on 8 flights of the DOE G-1 aircraft in and downwind of Mexico City during the March 2006 MILAGRO field campaign. A series of selection criteria are imposed to eliminate data points with non-urban emission influences. Biomass burning has urban and non-urban sources that are distinguished on the basis of CH3CN and CO. In order to account for dilution in the urban plume, aerosol concentrations are normalized to CO which is taken as an inert tracer of urban emission, proportional to the emissions of aerosol precursors. Time evolution is determined with respect to photochemical age defined as –Log10 (NOx/NOy). The geographic distribution of photochemical age and CO is examined, confirming the picture that Mexico City is a source region and that pollutants become more dilute and aged as they are advected towards T1 and T2, surface sites that are located at the fringe of the City and 35 km to the NE, respectively. Organic aerosol (OA) per ppm CO is found to increase 7 fold over the range of photochemical ages studied, corresponding to a change in NOx/NOy from nearly 100% to 10%. In the older samples the nitrate/CO ratio has leveled off suggesting that evaporation and formation of aerosol nitrate are in balance. In contrast, OA/CO increases with age in older samples, indicating that OA is still being formed. The amount of carbon equivalent to the deduced change in OA/CO with age is 56 ppbC per ppm CO. At an aerosol yield of 5% and 8% for low and high yield aromatic compounds, it is estimated from surface hydrocarbon observations that only ~9% of the OA formation can be accounted for. A comparison of OA/CO in Mexico City and the eastern U.S. gives no evidence that aerosol yields are higher in a more polluted environment.

We have measured the photoabsorption spectra of mass selected Ar+n clusters, n=3–40, from 355–1064 nm. The smaller clusters, n<15, display a visible photoabsorption spectrum similar to Ar+3, i.e., a broad, intense band peaking near 520 nm. From n=15–20 this photoabsorption band shifts smoothly to a longer wavelength, peaking near 600 nm for Ar+20. This band does not change appreciably as n increases from 20 to 40. These results clearly demonstrate that the Ar+n clusters have photophysical properties quite different from those of Ar+2. We have also studied the photoabsorption and subsequent photofragmentation of Ar+n cluster ions, n=3–60, at selected visible wavelengths. The ionic photofragment distributions both indicate that photofragmentation proceeds through the loss of individual Ar atoms and place an upper bound of 90 meV on the cluster ion binding energy in the large cluster limit.

Photodissociation of mass-selected (C0 2 )"~ clusters, n ^ 40, at 355 and 308 nm reveals that the same "magic numbers" are present in the fragmentation patterns as in the mass spectrum, demonstrating that these intensity anomalies are due to relative ionic stabilities. The photon energy dependence of the fragmentation pattern shows that photodissociation proceeds by an evaporative mechanism and yields an upper bound, 0.22 ±0.01 eV, for the binding energy of a neutral C0 2 onto the ionic cluster in the large-cluster limit.PACS numbers: 33.80.Eh, 33.80.Gj Atomic and molecular clusters, both neutral and ionic, formed in supersonic expansions, in ionmolecule reactions, and by sputtering from solids and liquids, have been the subject of much research for several decades. 1 A significant fraction of this work has involved the observation and interpretation of mass spectra generated by the ionization of neutral clusters. Such mass spectra often display ion abundance anomalies or "magic numbers/' where clusters of a specific size are much more abundant than neighboring clusters. The explanation for these anomalies has attracted considerable attention 2 " 4 and a consensus is emerging 5,6 that in many cases they reflect the relative stabilities of the positively charged clusters formed by evaporation due to excess energy imparted in the ionization of the neutral beam. Especially compelling evidence for this model was found by Echt et aL 1 in the case of water clusters, where the "magic" cluster H + (H 2 0) 21 is shown to develop over many microseconds as unimolecular decomposition reduces the size of the nascent ions produced by electron-impact ionization. Negative-ion clusters formed by attachment of low-energy electrons to neutral clusters have been observed by several researchers. 8 " 13 The evaporation mechanism has also been invoked to explain magic numbers observed in negatively charged clusters of C0 2 . 12,13 While photoabsorption experiments might clarify this model, the relevant photodetachment 14 " 16 and photodissociation 17,18 studies of the electronic structure of negative ions have primarily been carried out on small, chemically bound systems. Such measurements have only recently been extended 11 ' 19,20 to include negative cluster molecules.We report here a study of the near-uv photodissociation of mass-selected (C0 2 ) rt~, «^40, a situation where the ionic absorber generally sheds more than twelve C0 2 monomers. In this way, we probe the dynamics of magic-number formation occurring exclusively from unimolecular decay of larger ionic clusters, and eliminate any ambiguity resulting from magic numbers conceivably present in the neutral-cluster distribution. We find magic numbers in the photofragment ion distribution occurring at the same cluster sizes that appear as magic numbers in the parent cluster distribution, demonstrating that these effects are due to relative ionic stabilities. The photon energy dependence of the photofragment ion distributions indicates that evaporation is the mechanism for pho...