Characterization of ambient aerosols in Mexico City during the MCMA-2003 campaign with Aerosol Mass Spectrometry: results from the CENICA Supersite

Abstract

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.

PM1 Chemical Characterization during the ACU15 Campaign, South of Mexico City

“…During MILAGRO, a much wider range of instruments at ground sites, on aircrafts, and satellites was used; the urban "supersite" T0 was located north of the city (see Figure 1). During these studies, a large amount of information regarding the PM in the MCMA was collected in real-time with high resolution, using Aerodyne Aerosol Mass Spectrometers (AMSs) [12][13][14][15][16][17][18]. A more recent similar study was published by Guerrero et al [19] describing the PM 1 chemical composition in a site near T0 (Laboratorio de Análisis Ambiental, LAA) during the 2013-2014 winter/spring season, using an Aerodyne Aerosol Chemical Speciation Monitor (ACSM) [20].…”

“…The instrument is not able to detect components such as black carbon (BC), geological or metallic material, and salts with high fusion temperature. The uncertainties for the NR-PM 1 are estimated to be −30 to +10% [13,18].…”

“…A collection efficiency (CE) of 0.5 was assumed to account for detection losses due to bounce of particles off the vaporizer [23]. This value was chosen in accordance to previous studies in Mexico City using other AMSs [13,18].…”

“…During ACU15, NR-PM 1 represented approximately 100% of the PM 2.5 , indicating that the non-refractory components (BC and crustal material, for example) comprised only a small fraction of the fine particles. Table 1 presents the average PM concentrations measured during ACU15, at LAA [19], and during other previous campaigns in Mexico City using an Aerodyne AMS (MCMA-03 [18], and MILAGRO [13]). The concentrations of NR-PM 1 , BC, and PM 2.5 showed some variability among campaigns, with the largest concentrations in 2003 and the lowest in 2015.…”

“…The Popocatepetl Volcano (Popo, see Figure 1) is located 67 km southeast from the center of the MCMA; however, it is usually a minor source, on a regional scale, of SO 2 impacting the MCMA [9,34]. Given that sulfate is formed in a regional scale [13,18], a larger fraction of this component could also indicate that the aerosol was more aged than in other sites during previous campaigns, which is consistent with the patterns of air circulation that have been described in the Mexico City basin by de Foy et al [35] during the month of April 2003 for the MCMA03 campaign. The same patterns were observed during the MILAGRO campaign in 2006 [36], and are expected to have occurred during ACU15 because the three campaigns occurred in the dry season.…”