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

[摘要] The concentration, size, and composition of non-refractory submicron aerosol(NR-PM₁) was measured over Mexico City and central Mexico with aHigh-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 thecampaign the instrument performed very well, and provided 12 s data.The aerosol mass from the AMS correlates strongly with other aerosolmeasurements on board the aircraft. Organic aerosol (OA) species dominatethe NR-PM₁ mass. OA correlates strongly with CO and HCN indicating thatpollution (mostly secondary OA, SOA) and biomass burning (BB) are the mainOA sources. The OA to CO ratio indicates a typical value for aged air ofaround 80 μg m⁻³ (STP) ppm⁻¹. This is within the rangeobserved in outflow from the Northeastern US, which could be due to acompensating effect between higher BB but lower biogenic VOC emissionsduring this study. The O/C atomic ratio for OA is calculated from the HRmass spectra and shows a clear increase with photochemical age, as SOA formsrapidly and quickly overwhelms primary urban OA, consistent with Volkamer etal. (2006) and Kleinman et al. (2008). The stability of the OA/CO while O/Cincreases 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 enhancementat large m/z indicative of larger molecules or more resistance tofragmentation. The main inorganic components show different spatial patternsand size distributions. Sulfate is regional in nature with clear volcanicand petrochemical/power plant sources, while the urban area is not a majorregional source for this species. Nitrate is enhanced significantly in theurban area and immediate outflow, and is strongly correlated with COindicating a strong urban source. The importance of nitrate decreases withdistance from the city likely due to evaporation. BB does not appear to be astrong source of nitrate despite its high emissions of nitrogen oxides,presumably due to low ammonia emissions. NR-chloride often correlates withHCN indicating a fire source, although other sources likely contribute aswell. This is the first aircraft study of the regional evolution of aerosolchemistry from a tropical megacity.

[学科分类] 大气科学