Mexico City aerosol analysis during MILAGRO using high resolution aerosol mass spectrometry at the urban supersite (T0) – Part 1: Fine particle composition and organic source apportionment

Abstract

Abstract. Submicron aerosol was analyzed during the MILAGRO field campaign in March 2006 at the T0 urban supersite in Mexico City with a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) and complementary instrumentation. Mass concentrations, diurnal cycles, and size distributions of inorganic and organic species are similar to results from the CENICA supersite in April 2003 with organic aerosol (OA) comprising about half of the fine PM mass. Positive Matrix Factorization (PMF) analysis of the high resolution OA spectra identified three major components: chemically-reduced urban primary emissions (hydrocarbon-like OA, HOA), oxygenated OA (OOA, mostly secondary OA or SOA), and biomass burning OA (BBOA) that correlates with levoglucosan and acetonitrile. BBOA includes several very large plumes from regional fires and likely also some refuse burning. A fourth OA component is a small local nitrogen-containing reduced OA component (LOA) which accounts for 9% of the OA mass but one third of the organic nitrogen, likely as amines. OOA accounts for almost half of the OA on average, consistent with previous observations. OA apportionment results from PMF-AMS are compared to the PM2.5 chemical mass balance of organic molecular markers (CMB-OMM, from GC/MS analysis of filters). Results from both methods are overall consistent. Both assign the major components of OA to primary urban, biomass burning/woodsmoke, and secondary sources at similar magnitudes. The 2006 Mexico City emissions inventory underestimates the urban primary PM2.5 emissions by a factor of ~4, and it is ~16 times lower than afternoon concentrations when secondary species are included. Additionally, the forest fire contribution is underestimated by at least an order-of-magnitude in the inventory.

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.…”