Mexico City aerosol analysis during MILAGRO using high resolution aerosol mass spectrometry at the urban supersite (T0) – Part 2: Analysis of the biomass burning contribution and the modern carbon fraction

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 Aerosol Mass Spectrometer (AMS) and complementary instrumentation. Positive Matrix Factorization (PMF) of high resolution AMS spectra identified a biomass burning OA (BBOA) component, which includes several large plumes that appear to be from forest fires within the region. Here, we show that the AMS BBOA concentration at T0 correlates with fire counts in the vicinity of Mexico City and that most of the BBOA variability is captured when the FLEXPART model is used for the dispersion of fire emissions as estimated from satellite fire counts. The resulting FLEXPART fire impact index correlates well with the observed BBOA, CH3CN, levoglucosan, and potassium, indicating that wildfires in the region surrounding Mexico City are the dominant source of BBOA at T0 during MILAGRO. The impact of distant BB sources such as the Yucatan is small during this period. All fire tracers are correlated, with BBOA and levoglucosan showing little background, acetonitrile having a well-known tropospheric background of ~100–150 ppt, and PM2.5 potassium having a background of ~160 ng m−3 (two-thirds of its average concentration), which does not appear to be related to BB sources. We define two high fire periods based on satellite fire counts and predicted fire impacts. We then compare these periods with a low fire period when the impact of regional fires is about a factor of 5 smaller. Fire tracers are very elevated in the high fire periods whereas tracers of urban pollution do not change between these periods. Dust is also elevated during the high BB period but this appears to be coincidental due to the drier conditions and not driven by direct dust emission from the fires. The AMS oxygenated OA factor (OOA, mostly secondary OA or SOA) does not show an increase during the fire periods or a correlation with fire counts, FLEXPART-predicted fire impacts or fire tracers, indicating that it is dominated by urban and/or regional sources and not by the fires near the MCMA. A new 14C aerosol dataset is presented. Both this new and a previously published dataset of 14C analysis suggest a similar BBOA contribution as the AMS and chemical mass balance (CMB), resulting in 15% higher modern carbon during the high vs. low regional fire periods. The new dataset has ~15% more fossil carbon on average than the previously published one, and possible reasons for this discrepancy are discussed. During the low regional fire period, 37% of organic carbon (OC) and 30% total carbon (TC) are from modern sources, suggesting the importance of urban and regional modern carbon sources other than the fires, such as food cooking and regional biogenic SOA. Overall, the fires from the region near the MCMA are estimated to contribute 15–23% of the OA and 7–9% of the fine PM at T0 during MILAGRO, and 2–3% of the fine PM as an annual average. The 2006 MCMA emissions inventory contains a substantially lower impact of the forest fire emissions, although a fraction of these emissions occur just outside of the MCMA inventory area. The ambient BBOA/ΔCH3CN ratio is much higher in the afternoon when the wildfires are most intense than during the rest of the day, which may explain some disagreements between BB impacts from afternoon aircraft flights and those from 24-h ground measurements. Finally, we show that there are large differences in the contributions of the different OA components to the surface concentrations vs. the integrated column amounts.

Elucidating severe urban haze formation in China

“…Hence, our analysis of the aerosol chemical composition indicates dominant secondary aerosol constituents throughout the episodes, i.e., the nucleation mode particles containing mainly secondary organics and PM 2.5 during the polluted period consisting primarily of secondary organics, sulfate, nitrate, and ammonium. Interestingly, while the evolutions in the particle size distribution and mass concentration for the severe PM 2.5 episodes in Beijing are noticeably distinct, the aerosol chemical compositions measured by AMS in our study are similar to those commonly measured in other global urban regions (29,(31)(32)(33). For example, the mass fractions of organic, sulfate, and nitrate measured by AMS are typically 45%, 23%, and 18% in urban regions worldwide (29,(31)(32)(33), comparable to the values of 43%, 17%, and 22% averaged over our entire measurement period in Beijing (Fig.…”

“…Measurements of the chemical composition by an aerosol mass spectrometer (AMS) (29) show continuously increasing mass concentrations of organics, sulfate, and nitrate during the episodes, correlating closely with the evolutions of the PM 2.5 mass concentration and mean particle size (Fig. 2 B and D).…”

“…The size-resolved particle density and hygroscopicity were measured using an aerosol particle mass analyzer and a hygroscopic tandem differential mobility analyzer (43)(44)(45)). An Aerodyne highresolution time-of-flight aerosol mass spectrometer was used to measure the chemical compositions of submicron particles, with a collection efficiency of about 50% (29,46). Black carbon was measured by a single-wavelength (670 nm) Thermo multiangle absorption photometer.…”

Modeling organic aerosols in a megacity: potential contribution of semi-volatile and intermediate volatility primary organic compounds to secondary organic aerosol formation

“…Hodzic et al, 2009;Zavala et al, 2009). A better prediction of the temporal trend of BBOA was achieved when the emissions profile took into account the emissions during the late evening into the shallow nighttime boundary layer (Aiken et al, 2009b). The fair agreement between HOA derived from measurements and predicted POA concentrations reported by ), and Tsimpidi et al (2010 confirms that SVOC are unaccounted for in the POA emissions in Mexico City (i.e., that the inventory is consistent with the POA present after SVOC evaporation).…”

“…In our simulations, diurnal profiles with maximum emissions in the early afternoon (12:00-16:00 LT) are applied. Aiken et al (2009b) achieved a better correlation between the BBOA at T0 and model prediction by using a modified emissions profile which includes smoldering emissions into the late evening and night. It is also likely that some fire sources are missing from the biomass burning emission inventory as already discussed by (Fig.…”