Primary and secondary contributions to aerosol light scattering and absorption in Mexico City during the MILAGRO 2006 campaign

Abstract

Abstract.A photoacoustic spectrometer, a nephelometer, an aethalometer, and an aerosol mass spectrometer were used to measure at ground level real-time aerosol light absorption, scattering, and chemistry at an urban site located in North East Mexico City (Instituto Mexicano del Petroleo, Mexican Petroleum Institute, denoted by IMP), as part of the Megacity Impact on Regional and Global Environments field experiment, MILAGRO, in March 2006. Photoacoustic and reciprocal nephelometer measurements at 532 nm accomplished with a single instrument compare favorably with conventional measurements made with an aethalometer and a TSI nephelometer. The diurnally averaged single scattering albedo at 532 nm was found to vary from 0.60 to 0.85 with the peak value at midday and the minimum value at 07:00 a.m. local time, indicating that the Mexico City plume is likely to have a net warming effect on local climate. The peak value is associated with strong photochemical generation of secondary aerosol. It is estimated that the photochemical production of secondary aerosol (inorganic and organic) is approximately 75% of the aerosol mass concentration and light scattering in association with the peak single scattering albedo. A strong correlation of aerosol scattering at 532 nm and total aerosol mass concentration was found, and an average mass scattering efficiency factor of 3.8 m 2 /g was determined. Comparisons of photoacoustic and aethalometer light absorption with oxygenated organic aerosol concentration (OOA) indicate a very small systematic bias of the filter based measurement associated with OOA and the peak aerosol single scattering albedo.

Photoacoustic optical properties at UV, VIS, and near IR wavelengths for laboratory generated and winter time ambient urban aerosols

“…During temperature inversion periods, the local health officials of Washoe County, Nevada announced air pollution alerts to encourage inhabitants to refrain from extended outdoor activities, and banned residential wood burning (see, http://www.co.washoe.nv.us/ repository/print pr.php?article=8510). The β abs during these conditions was comparable to that of Reno, NV during the California wild fire season in the summer of 2008 and to that in Mexico City (Gyawali et al, 2009;Paredes-Miranda et al, 2009). Figure 3 presents the aerosol time-series of β sca and β abs at 355 nm in conjunction with meteorological parameters from two near surface stations in Reno separated by an altitude difference of 143 m. Figure 3a and b display the temperature gradient normalized by the dry adiabatic lapse rate (−9.8 • C km −1 ) and the wind speed, respectively.…”

The importance of aerosol mixing state and size-resolved composition on CCN concentration and the variation of the importance with atmospheric aging of aerosols

“…The HR-AMS reported size distribution for chemical species vs. vacuum aerodynamic diameter (D va ;DeCarlo et al, 2004), from which volume equivalent diameter (D ve ) was estimated assuming sphericity and using the estimated sizeresolved density (as discussed below). The instrumental intercomparisons and main results of the HR-AMS in this study have been reported in previous publications (Aiken et al, 2008Paredes-Miranda et al, 2009;Huffman et al, 2009;Salcedo et al, 2010;Cappa and Jimenez, 2010) The AMS reported non-refractory inorganic species (sulfate, nitrate, ammonium, and chloride) as well as total organic aerosol (OA). The OA measured by the AMS was classified into different components using Positive Matrix Factorization (Paatero, 1997;Ulbrich et al, 2009) of the highresolution OA spectra, as described by Aiken et al (2009).…”

An overview of the MILAGRO 2006 Campaign: Mexico City emissions and their transport and transformation

“…Additional information on the chemical and physical characteristics of emissions from mobile sources has been obtained from source apportionment analysis of stationary data at T0 Paredes-Miranda et al, 2009), area-averaged emission fluxes at the SIMAT tower Notes: a NO x (expressed as NO 2 ) emissions in the inventory were assumed 90% NO and 10% NO 2 in mass. b Total VOCs emissions from the inventory include evaporative emissions.…”