Abstract. An Aerodyne Aerosol Mass Spectrometer (AMS) was deployed at the CENICA Supersite, during the Mexico City Metropolitan Area field study (MCMA-2003) from 31 March-4 May 2003 to investigate particle concentrations, sources, and processes. The AMS provides real time information on mass concentration and composition of the non-refractory species in particulate matter less than 1 µm (NR-PM1) with high time and size-resolution. In order to account for the refractory material in the aerosol, we also present estimates of Black Carbon (BC) using an aethalometer and an estimate of the aerosol soil component obtained from Proton-Induced X-ray Emission Spectrometry (PIXE) analysis of impactor substrates. Comparisons of AMS + BC + soil mass concentration with other collocated particle instruments (a LASAIR Optical Particle Counter, a PM2.5 Tapered Element Oscillating Microbalance (TEOM), and a PM2.5 DustTrak Aerosol Monitor) show that the AMS + BC + soil mass concentration is consistent with the total PM2.5 mass concentration during MCMA-2003 within the combined uncertainties. In Mexico City, the organic fraction of the estimated PM2.5 at CENICA represents, on average, 54.6% (standard deviation σ=10%) of the mass, with the rest consisting of inorganic compounds (mainly ammonium nitrate and sulfate/ammonium salts), BC, and soil. Inorganic compounds represent 27.5% of PM2.5 (σ=10%); BC mass concentration is about 11% (σ=4%); while soil represents about 6.9% (σ=4%). Size distributions are presented for the AMS species; they show an accumulation mode that contains mainly oxygenated organic and secondary inorganic compounds. The organic size distributions also contain a small organic particle mode that is likely indicative of fresh traffic emissions; small particle modes exist for the inorganic species as well. Evidence suggests that the organic and inorganic species are not always internally mixed, especially in the small modes. The aerosol seems to be neutralized most of the time; however, there were some periods when there was not enough ammonium to completely neutralize the nitrate, chloride and sulfate present. The diurnal cycle and size distributions of nitrate suggest local photochemical production. On the other hand, sulfate appears to be produced on a regional scale. There are indications of new particle formation and growth events when concentrations of SO2 were high. Although the sources of chloride are not clear, this species seems to condense as ammonium chloride early in the morning and to evaporate as the temperature increases and RH decreases. The total and speciated mass concentrations and diurnal cycles measured during MCMA-2003 are similar to measurements during a previous field campaign at a nearby location.

[1] Using optical microscopy, we investigated the heterogeneous nucleation of ice in aqueous (NH 4 ) 2 SO 4 -H 2 O particles containing two types of mineral dusts, kaolinite and montmorillonite. The efficacy of montmorillonite and kaolinite to nucleate ice in (NH 4 ) 2 SO 4 -H 2 O particles is similar. The difference in freezing temperatures, compared to the homogeneous freezing temperatures, is found to vary from 8 -20 K and it is larger for particles with concentrations greater than 27 wt %. Our freezing data shows that for temperatures ranging from 239 K to 198 K, ice super-saturations between 1.35 and 1.51 are required for ice to heterogeneously nucleate in NH 4 SO 4 -H 2 O particles containing mineral dust immersions. Based on our results, we conclude mineral dust is an efficient nuclei for ice in NH 4 SO 4 -H 2 O aerosols and as a result, it can initiate the formation of upper tropospheric ice clouds at warmer temperatures and lower super-saturations in comparison to homogeneous freezing.

The global presence of soot has significant effects on regional and global climate, as well as human health. Influence of soot on radiation budget, rain formation and heterogeneous chemistry, and its residence time in the atmosphere are largely dependent on its ability to interact with water. While freshly emitted soot is extremely hydrophobic, oxidation during aging causes soot to become more hydrophilic. Laboratory studies demonstrate that aged soot attracts and retains water, and can be efficiently removed from the troposphere by entrapment in existing liquid cloud droplets or by activation as cloud condensation nuclei.

Heterogeneous freezing of aqueous particles with solid inclusions of crystallized (NH 4 ) 2 SO 4 , ice, and letovicite were studied using optical microscopy and differential scanning calorimetry. For (NH 4 ) 2 SO 4 -H 2 O particles, the heterogeneous freezing temperature was found to be dependent on the morphology of the (NH 4 ) 2 SO 4 solid. If the crystallized solid was in the form of microcrystals, the heterogeneous ice-freezing temperature was close to the eutectic temperature and the critical saturation with respect to ice was close to 1. However, if the solid was in the form of one or two large crystals, the heterogeneous freezing temperature was close to the homogeneous freezing temperature. For particles with one or two large (NH 4 ) 2 SO 4 crystals in equilibrium with (NH 4 ) 2 SO 4 -H 2 O solution, we have estimated an upper limit of 1.5 × 10 -5 s -1 µm -2 for J het (heterogeneous nucleation rate of ice, immersion freezing mode). Our results for NH 4 HSO 4 -H 2 O particles show that when one or two large crystals of either ice or letovicite are present in the solution, the freezing temperature does not deviate significantly from the homogeneous freezing temperature, consistent with the (NH 4 ) 2 SO 4 -H 2 O experiments. Our work shows that the surface area and surface microstructure of crystalline solids present in aqueous aerosols can significantly change the heterogeneous freezing temperature and critical ice saturations and that heterogeneous ice nucleation induced by crystalline salts may be very important in the formation of upper tropospheric clouds.