Chemically-resolved aerosol eddy covariance flux measurements in urban Mexico City during MILAGRO 2006

R Zalakeviciute,J L Jimenez,E Allwine,I M Ulbrich,T M Vanreken,B T Jobson,E Velasco,L T Molina,M L Alexander,S.N Pressley,E Nemitz,B K Lamb

doi:10.5194/acp-12-7809-2012

Abstract

Abstract. As part of the MILAGRO 2006 field campaign, the exchange of atmospheric aerosols with the urban landscape was measured from a tall tower erected in a heavily populated neighborhood of Mexico City. Urban submicron aerosol fluxes were measured using an eddy covariance method with a quadrupole aerosol mass spectrometer during a two week period in March, 2006. Nitrate and ammonium aerosol concentrations were elevated at this location near the city center compared to measurements at other urban sites. Significant downward fluxes of nitrate aerosol, averaging −0.2 μg m−2 s−1, were measured during daytime. The urban surface was not a significant source of sulfate aerosols. The measurements also showed that primary organic aerosol fluxes, approximated by hydrocarbon-like organic aerosols (HOA), displayed diurnal patterns similar to CO2 fluxes and anthropogenic urban activities. Overall, 47% of submicron organic aerosol emissions were HOA, 35% were oxygenated (OOA) and 18% were associated with biomass burning (BBOA). Organic aerosol fluxes were bi-directional, but on average HOA fluxes were 0.1 μg m−2 s−1, OOA fluxes were −0.03 μg m−2 s−1, and BBOA fluxes were −0.03 μg m−2 s−1. After accounting for size differences (PM1 vs PM2.5) and using an estimate of the black carbon component, comparison of the flux measurements with the 2006 gridded emissions inventory of Mexico City, showed that the daily-averaged total PM emission rates were essentially identical for the emission inventory and the flux measurements. However, the emission inventory included dust and metal particulate contributions, which were not included in the flux measurements. As a result, it appears that the inventory underestimates overall PM emissions for this location.

Highlights

Atmospheric aerosols are complex pollutants having considerable direct and indirect effects on environmental quality and climate change (Forster et al, 2007)
Primary organic aerosols (POA) are directly emitted from natural or anthropogenic sources, such as fossil fuel combustion, cooking, and other urban sources. These are usually identified by an Aerosol Mass Spectrometer (AMS) factor referred to as hydrocarbon-like OA (HOA), but cooking aerosols are increasingly identified separately (Allan et al, 2010) or as part of the aerosols associated with biomass burning (BBOA)
Aiken et al (2010) showed that at the T0 site, a Megacity Initiative: Local and Global Research Observations (MILAGRO) urban supersite located about 10 km north of the SIMAT flux tower, nonfire related gas and particle species showed little change between the three periods, while levels of fire-related species were much lower during the 3rd period, compared with the first two periods

Summary

Introduction

Atmospheric aerosols are complex pollutants having considerable direct and indirect effects on environmental quality and climate change (Forster et al, 2007). Primary organic aerosols (POA) are directly emitted from natural or anthropogenic sources, such as fossil fuel combustion, cooking, and other urban sources. These are usually identified by an AMS factor referred to as hydrocarbon-like OA (HOA), but cooking aerosols are increasingly identified separately (Allan et al, 2010) or as part of the aerosols associated with biomass burning (BBOA). Secondary organic aerosols (SOA) are formed by atmospheric oxidation of gasphase species (Jimenez et al, 2009) and are identified as oxygenated organic aerosols (OOA) due to their characteristic mass spectrum with high oxygen content. Semi-volatile OOA (SV-OOA) has a lower O:C ratio, and is associated with less photochemically aged SOA (Jimenez et al, 2009)

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