Fast airborne aerosol size and chemistry measurements above Mexico City and Central Mexico during the MILAGRO campaign

Abstract

Abstract. The concentration, size, and composition of non-refractory submicron aerosol (NR-PM1) was measured over Mexico City and central Mexico with a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) onboard the NSF/NCAR C-130 aircraft as part of the MILAGRO field campaign. This was the first aircraft deployment of the HR-ToF-AMS. During the campaign the instrument performed very well, and provided 12 s data. The aerosol mass from the AMS correlates strongly with other aerosol measurements on board the aircraft. Organic aerosol (OA) species dominate the NR-PM1 mass. OA correlates strongly with CO and HCN indicating that pollution (mostly secondary OA, SOA) and biomass burning (BB) are the main OA sources. The OA to CO ratio indicates a typical value for aged air of around 80 μg m−3 (STP) ppm−1. This is within the range observed in outflow from the Northeastern US, which could be due to a compensating effect between higher BB but lower biogenic VOC emissions during this study. The O/C atomic ratio for OA is calculated from the HR mass spectra and shows a clear increase with photochemical age, as SOA forms rapidly and quickly overwhelms primary urban OA, consistent with Volkamer et al. (2006) and Kleinman et al. (2008). The stability of the OA/CO while O/C increases with photochemical age implies a net loss of carbon from the OA. BB OA is marked by signals at m/z 60 and 73, and also by a signal enhancement at large m/z indicative of larger molecules or more resistance to fragmentation. The main inorganic components show different spatial patterns and size distributions. Sulfate is regional in nature with clear volcanic and petrochemical/power plant sources, while the urban area is not a major regional source for this species. Nitrate is enhanced significantly in the urban area and immediate outflow, and is strongly correlated with CO indicating a strong urban source. The importance of nitrate decreases with distance from the city likely due to evaporation. BB does not appear to be a strong source of nitrate despite its high emissions of nitrogen oxides, presumably due to low ammonia emissions. NR-chloride often correlates with HCN indicating a fire source, although other sources likely contribute as well. This is the first aircraft study of the regional evolution of aerosol chemistry from a tropical megacity.

A simplified description of the evolution of organic aerosol composition in the atmosphere

“…An intermediate slope of −1 is produced by the simultaneous addition of both functional groups, forming a hydroxycarbonyl or carboxylic acid. Organic atmospheric functionalization reactions, including, but not limited to those shown here, generally involve changes not only to O:C but also to H:C. While increasing O:C is often considered an indicator of atmospheric oxidation [e.g., DeCarlo et al, 2008], Figure 1 shows that a non-oxidative process such as hydration reactions can be misinterpreted based on this limited description.…”

“…[6] Figure [Aiken et al, 2008;DeCarlo et al, 2008]. Also shown is the elemental composition of the individual Positive Matrix Factorization factors during MILAGRO.…”

Organic Aerosols in the Earth’s Atmosphere

“…Figure 1A summarizes observed ∆OA/∆CO enhancement ratios from different studies that were influenced by urban emissions. Red lines are estimates of emission ratios (POA) (7,12,13,(22)(23)(24)(25); blue lines were obtained in aged air (POA + SOA) (7,9,(17)(18)(19)22). These results were determined from field studies on three continents and indicate that the ∆OA/∆CO ratios in aged urban air are systematically higher.…”

“…Figure 1B summarizes ∆OA/∆CO emission ratios (36), as well as enhancement ratios observed in aged biomass burning plumes (9,37,38). There is a large overlap between the two groups, which does not permit strong conclusions about net SOA formation in biomass burning plumes to be drawn at present.…”

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].…”