On the volatility and production mechanisms of newly formed nitrate and water soluble organic aerosol in Mexico City

Abstract

Abstract. Measurements of atmospheric gases and fine particle chemistry were made in the Mexico City Metropolitan Area (MCMA) at a site ∼30 km down wind of the city center. Ammonium nitrate (NH 4 NO 3 ) dominated the inorganic aerosol fraction and showed a distinct diurnal signature characterized by rapid morning production and a rapid mid-day concentration decrease. Between the hours of 08:00-12:45, particulate water-soluble organic carbon (WSOC) concentrations increased and decreased in a manner consistent with that of NO − 3 , and the two were highly correlated (R 2 =0.88) during this time. A box model was used to analyze these behaviors and showed that, for both NO − 3 and WSOC, the concentration increase was caused primarily (∼75-85%) by secondary formation, with a smaller contribution (∼15-25%) from the entrainment of air from the free troposphere. For NO − 3 , a majority (∼60%) of the midday concentration decrease was caused by dilution from boundary layer expansion, though a significant fraction (∼40%) of the NO − 3 loss was due to particle evaporation. The WSOC concentration decrease was due largely to dilution (∼75%), but volatilization did have a meaningful impact (∼25%) on the decrease, as well. The results provide an estimate of ambient SOA evaporation losses and suggest that a significant fraction (∼35%) of the fresh MCMA secondary organic aerosol (SOA) measured at the surface volatilized.

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

“…Rapid and intense formation of submicron ammonium nitrate is observed during the morning with important evaporation in the afternoon Hennigan et al, 2008;Aiken et al, 2009), consistent with previous studies . PM 1 nitrate also accounts for a high portion of the NO z budget in Mexico City, exceeding 20% at times ).…”

“…Intense SOA formation in PM 1 was observed during MILA-GRO Herndon et al, 2008;DeCarlo et al, , 2010Hennigan et al, 2008;de Gouw et al, 2009;Aiken et al, 2008Aiken et al, , 2009) at levels much higher than predicted with traditional SOA models de Gouw et al, 2009;Hodzic et al, 2009Hodzic et al, , 2010aFast et al, 2009;Wood et al, 2010). This is consistent with previous observations and modeling from MCMA-2003 Dzepina et al, 2009;Tsimpidi et al, 2010) and the emerging consensus from studies at other polluted locations using a variety of apportionment methods (e.g., Docherty et al, 2008;Hallquist et al, 2009;de Gouw and Jimenez, 2009).…”

“…This is consistent with previous observations and modeling from MCMA-2003 Dzepina et al, 2009;Tsimpidi et al, 2010) and the emerging consensus from studies at other polluted locations using a variety of apportionment methods (e.g., Docherty et al, 2008;Hallquist et al, 2009;de Gouw and Jimenez, 2009). These results were consistent between the WSOC (Hennigan et al, 2008) and AMS techniques (e.g., Kleinman et al, 2008) as has been observed elsewhere Docherty et al, 2008). The substantial contribution of SOA was reported not only in the city, but also at the Altzomoni site located 2 km above basin level ) and from aircraft Kleinman et al, 2008).…”

“…This analysis concludes, using two different techniques, that a substantial fraction (50-80%) of the OA (especially the aged OOA) is essentially nonvolatile and will not evaporate under any atmospheric conditions. However a semivolatile fraction is also present (Cappa and Jimenez, 2010), and Hennigan et al (2008) reported that some of the SOA detected at the T1 site using WSOC measurements evaporated in the afternoons, although to a lesser extent than ammonium nitrate.…”

A critical evaluation of proxy methods used to estimate the acidity of atmospheric particles

“…We evaluate the above methods using ground-based data collected during the Megacity Initiative: Local and Global Research Objectives (MILAGRO) campaign at the T1 site in Mexico City (Molina et al, 2010 (Hennigan et al, 2008). Six-minute integrated measurements were conducted every 15 min.…”