ALBERT

Description: 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 (NH4NO3) dominated the inorganic aerosol fraction and showed a distinct diurnal signature characterized by rapid morning production and a rapid mid-day concentration decrease. The concentration increase was due to both secondary formation and entrainment from the free troposphere. A majority (approximately two-thirds) of the midday concentration decrease was caused by dilution from boundary layer expansion, however a significant fraction (approximately one-third) of the nitrate loss was due to particle evaporation. The water-soluble organic carbon fraction of fine particles (WSOC) and nitrate were highly correlated (R2=0.80) for the entire three-day analysis period, however the WSOC-nitrate correlation was highest (R2=0.88) between the hours of 08:00–12:45, indicating similar sources and processing during this period. The results show that WSOC also experienced evaporation losses and that a significant fraction of the MCMA secondary organic aerosol (SOA) measured at the surface was semi-volatile.

Description: A focus of the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) mission was examination of bromine photochemistry in the spring time high latitude troposphere based on aircraft and satellite measurements of bromine oxide (BrO) and related species. The NASA DC-8 aircraft utilized a chemical ionization mass spectrometer (CIMS) to measure BrO and a mist chamber (MC) to measure soluble bromide. We have determined that the MC detection efficiency to molecular bromine (Br2), hypobromous acid (HOBr), bromine oxide (BrO), and hydrogen bromide (HBr) as soluble bromide (Br−) was 0.9±0.1, 1.06+0.30/−0.35, 0.4±0.1, and 0.95±0.1, respectively. These efficiency factors were used to estimate soluble bromide levels along the DC-8 flight track of 17 April 2008 from photochemical calculations constrained to in situ BrO measured by CIMS. During this flight, the highest levels of soluble bromide and BrO were observed and atmospheric conditions were ideal for the space-borne observation of BrO. The good agreement (R2 = 0.76; slope = 0.95; intercept = −3.4 pmol mol−1) between modeled and observed soluble bromide, when BrO was above detection limit (〉2 pmol mol−1) under unpolluted conditions (NO

Description: 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 (NH4NO3) 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 NO3−, and the two were highly correlated (R2=0.88) during this time. A box model was used to analyze these behaviors and showed that, for both NO3− 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 NO3−, a majority (~60%) of the midday concentration decrease was caused by dilution from boundary layer expansion, though a significant fraction (~40%) of the NO3− 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.

Description: Archived Federal Reference Method (FRM) Teflon filters used by state regulatory agencies for measuring PM2.5 mass were acquired from 15 sites throughout the southeastern US and analyzed for water-soluble organic carbon (WSOC), water-soluble ions and carbohydrates to investigate biomass burning contributions to fine aerosol mass. Based on over 900 filters that spanned all of 2007, levoglucosan and K+ were studied in conjunction with MODIS Aqua fire count data to compare their performances as biomass burning tracers. Levoglucosan concentrations exhibited a distinct seasonal variation with large enhancement in winter and spring and a minimum in summer, and were well correlated with fire counts, except in winter when residential wood burning contributions were significant. In contrast, K+ concentrations had no apparent seasonal trend and poor correlation with fire counts. Levoglucosan and K+ only correlated well in winter (r2=0.59) when biomass burning emissions were highest, whereas in other seasons they were not correlated due to the presence of other K+ sources. Levoglucosan also exhibited larger spatial variability than K+. Both species were higher in urban than rural sites (mean 44% higher for levoglucosan and 86% for K+). Positive Matrix Factorization (PMF) was applied to analyze PM2.5 sources and four factors were resolved: biomass burning, refractory material, secondary light absorbing WSOC and secondary sulfate/WSOC. The biomass burning source contributed 13% to PM2.5 mass annually, 27% in winter, and less than 2% in summer, consistent with other souce apportionment studies based on levoglucosan, but lower in summer compared to studies based on K+.

Description: Biomass burning (BB) is a large source of primary and secondary organic aerosols (POA and SOA). This study addresses the physical and chemical evolution of BB organic aerosols. Firstly, the evolution and lifetime of BB POA and SOA signatures observed with the Aerodyne Aerosol Mass Spectrometer are investigated, focusing on measurements at high-latitudes acquired during the 2008 NASA ARCTAS mission, in comparison to data from other field studies and from laboratory aging experiments. The parameter f60, the ratio of the integrated signal at m/z 60 to the total signal in the organic component mass spectrum, is used as a marker to study the rate of oxidation and fate of the BB POA. A background level of f60~0.3% ± 0.06% for SOA-dominated ambient OA is shown to be an appropriate background level for this tracer. Using also f44 as a tracer for SOA and aged POA and a surrogate of organic O:C, a novel graphical method is presented to characterise the aging of BB plumes. Similar trends of decreasing f60 and increasing f44 with aging are observed in most field and lab studies. At least some very aged BB plumes retain a clear f60 signature. A statistically significant difference in f60 between highly-oxygenated OA of BB and non-BB origin is observed using this tracer, consistent with a substantial contribution of BBOA to the springtime Arctic aerosol burden in 2008. Secondly, a summary is presented of results on the net enhancement of OA with aging of BB plumes, which shows large variability. The estimates of net OA gain range from ΔOA/ΔCO(mass) = −0.01 to ~0.05, with a mean ΔOA/POA ~19%. With these ratios and global inventories of BB CO and POA a global net OA source due to aging of BB plumes of ~8 ± 7 Tg OA yr−1 is estimated, of the order of 5 % of recent total OA source estimates. Further field data following BB plume advection should be a focus of future research in order to better constrain this potentially important contribution to the OA burden.

Description: Secondary organic aerosol (SOA) in the southeastern US is investigated by analyzing the spatial-temporal distribution of water-soluble organic carbon (WSOC) and other PM2.5 components from 900 archived 24-h Teflon filters collected at 15 urban or rural EPA Federal Reference Method (FRM) network sites throughout 2007. Online measurements of WSOC at an urban/rural-paired site in Georgia in the summer of 2008 are contrasted to the filter data. Based on FRM filters, excluding biomass-burning events (levoglucosan 〈 50 ng m−3), WSOC and sulfate were highly correlated with PM2.5 mass (r2~0.7). Both components comprised a large mass fraction of PM2.5 (13% and 31%, respectively, or ~25% and 50% for WSOM and ammonium sulfate). Sulfate and WSOC both tracked ambient temperature throughout the year, suggesting the temperature effects were mainly linked to faster photochemistry and/or synoptic meteorology and less due to enhanced biogenic hydrocarbon emissions. FRM WSOC, and to a lesser extent sulfate, were spatially homogeneous throughout the region, yet WSOC was moderately enhanced (27%) in locations of greater predicted isoprene emissions in summer. A Positive Matrix Factorization (PMF) analysis identified two major source types for the summer WSOC; 22% of the WSOC were associated with ammonium sulfate, and 56% of the WSOC were associated with brown carbon and oxalate. A small urban excess of FRM WSOC (10%) was observed in the summer of 2007, however, comparisons of online WSOC measurements at one urban/rural pair (Atlanta/Yorkville) in August 2008 showed substantially greater difference in WSOC (31%) relative to the FRM data, suggesting a low bias for urban filters. The measured Atlanta urban excess, combined with the estimated boundary layer heights, gave an estimated Atlanta daily WSOC production rate in August of 0.55 mgC m−2 h−1 between mid-morning and mid-afternoon. This study characterizes the regional nature of fine particles in the southeastern US, confirming the importance of SOA and the roles of both biogenic and anthropogenic emissions.

Description: The NASA DC-8 aircraft characterized the aerosol properties, chemical composition, and cloud condensation nuclei (CCN) concentrations of the summertime Arctic during the 2008 NASA Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) campaign. Air masses characteristic of fresh and aged biomass burning, boreal forest, Arctic background, and anthropogenic industrial pollution were sampled. Observations were spatially extensive (50–85° N and 40–130° W) and exhibit significant variability in aerosol and CCN concentrations. The chemical composition was dominated by highly oxidized organics (66–94% by volume), with a water-soluble mass fraction of more than 50%. The aerosol hygroscopicity parameter, κ, ranged between κ = 0.08–0.32 for all air mass types. Industrial pollution had the lowest κ of 0.08 ± 0.01, while the Arctic background had the highest and most variable κ of 0.32 ± 0.21, resulting from a lower and more variable organic fraction. Both fresh and aged (long-range transported) biomass burning air masses exhibited remarkably similar κ (0.18 ± 0.13), consistent with observed rapid chemical and physical aging of smoke emissions in the atmosphere, even in the vicinity of fresh fires. The organic hygroscopicity (κorg) was parameterized by the volume fraction of water-soluble organic matter (εWSOM), with a κ = 0.12, such that κorg = 0.12εWSOM. Assuming bulk (size-independent) composition and including the κorg parameterization enabled CCN predictions to within 30% accuracy for nearly all environments sampled. The only exception was for industrial pollution from Canadian oil sands exploration, where an external mixture and size-dependent composition was required. Aerosol mixing state assumptions (internal vs. external) in all other environments did not significantly affect CCN predictions; however, the external mixing assumption provided the best results, even though the available observations could not determine the true degree of external mixing and therefore may not always be representative of the environments sampled. No correlation was observed between κorg and O : C. A novel correction of the CCN instrument supersaturation for water vapor depletion, resulting from high concentrations of CCN, was also employed. This correction was especially important for fresh biomass burning plumes where concentrations exceeded 1.5×104 cm−3 and introduced supersaturation depletions of ≥25%. Not accounting for supersaturation depletion in these high concentration environments would therefore bias CCN closure up to 25% and inferred κ by up to 50%.

Description: Light absorption of fine particle (PM2.5) aqueous extracts between wavelengths of 200 and 800 nm were investigated from two data sets: 24-h Federal Reference Method (FRM) filter extracts from 15 Southeastern US monitoring sites over the year of 2007 (900 filters), and online measurements from a Particle-Into-Liquid Sampler deployed from July to mid-August 2009 in Atlanta, Georgia. Three main sources of soluble chromophores were identified: biomass burning, mobile source emissions, and compounds linked to secondary organic aerosol (SOA) formation. Absorption spectra of aerosol solutions from filter extracts were similar for different sources. Angstrom exponents were ~7±1 for biomass burning and non-biomass burning-impacted 24-h filter samples (delineated by a levoglucosan concentration of 50 ng m−3) at both rural and urban sites. The absorption coefficient from measurements averaged between wavelength 360 and 370 nm (Abs365, in units m−1) was used as a measure of overall brown carbon light absorption. Biomass-burning-impacted samples were highest during winter months and Abs365 was correlated with levoglucosan at all sites. During periods of little biomass burning in summer, light absorbing compounds were still ubiquitous and correlated with fine particle Water-Soluble Organic Carbon (WSOC), but comprised a much smaller fraction of the WSOC, where Abs365/WSOC (i.e., mass absorption efficiency) was typically ~3 times higher in biomass burning-impacted samples. Factor analysis attributed 50% of the yearly average Abs365 to biomass burning sources. Brown carbon from primary urban emissions (mobile sources) was also observed and accounted for ~10% of the regional yearly average Abs365. Summertime diurnal profiles of Abs365 and WSOC showed that morning to midday increases in WSOC from photochemical production were associated with a decrease in Abs365/WSOC. After noon, this ratio substantially increased, indicating that either some fraction of the non-light absorbing fresh SOA was rapidly (within hours) converted to chromophores heterogeneously, or that SOA from gas-particle partitioning later in the day was more light-absorbing. Factor analysis on the 24-h integrated filter data associated ~20 to 30% of Abs365 over 2007 with a secondary source that was highest in summer and also the main source for oxalate, suggesting that aqueous phase reactions may account for the light-absorbing fraction of WSOC observed throughout the Southeastern US in summer.

Description: This paper compares measurements of gaseous and particulate emissions from a wide range of biomass-burning plumes intercepted by the NASA DC-8 research aircraft during the three phases of the ARCTAS-2008 experiment: ARCTAS-A, based out of Fairbanks, Alaska, USA (3 April to 19 April 2008); ARCTAS-B based out of Cold Lake, Alberta, Canada (29 June to 13 July 2008); and ARCTAS-CARB, based out of Palmdale, California, USA (18 June to 24 June 2008). Approximately 500 smoke plumes from biomass burning emissions that varied in age from minutes to days were segregated by fire source region and urban emission influences. The normalized excess mixing ratios (NEMR) of gaseous (carbon dioxide, acetonitrile, hydrogen cyanide, toluene, benzene, methane, oxides of nitrogen and ozone) and fine aerosol particulate components (nitrate, sulfate, ammonium, chloride, organic aerosols and water soluble organic carbon) of these plumes were compared. A detailed statistical analysis of the different plume categories for different gaseous and aerosol species is presented in this paper. The comparison of NEMR values showed that CH4 concentrations were higher in air-masses that were influenced by urban emissions. Fresh biomass burning plumes mixed with urban emissions showed a higher degree of oxidative processing in comparison with fresh biomass burning only plumes. This was evident in higher concentrations of inorganic aerosol components such as sulfate, nitrate and ammonium, but not reflected in the organic components. Lower NOx NEMRs combined with high sulfate, nitrate and ammonium NEMRs in aerosols of plumes subject to long-range transport, when comparing all plume categories, provided evidence of advanced processing of these plumes.

Description: Fine particle (PM2.5) light absorption characteristics of aqueous extracts over wavelengths of 250 to 700 nm were investigated based on two data sets; 24-h Federal Reference Method (FRM) filter extracts from 15 southeastern US monitoring sites over the year of 2007 (900 filters), and online measurements from a Particle-Into-Liquid Sampler deployed from July to mid-August 2009 in Atlanta, Georgia. Three main sources of soluble chromophores were identified, biomass burning, mobile source emissions, and compounds linked to Secondary Organic Aerosol (SOA) formation. Absorption spectra of aerosol solutions from different sources were similar. Angstrom exponents were ~7±1 for biomass burning and non-biomass burning-impacted samples (delineated by a levoglucosan concentration of 50 ng m−3) at both rural and urban sites. The absorption coefficient from measurements averaged between wavelength 360 and 370 nm (Abs365, in units m−1) was used as a measure of overall brown carbon absorptivity. Biomass-burning-impacted samples were highest during colder months and Abs365 was correlated with levoglucosan at all sites. During periods of little biomass burning in summer, light absorbing compounds were still ubiquitous and correlated with fine particle Water-Soluble Organic Carbon (WSOC), but comprised a much smaller fraction of the WSOC, where Abs365/WSOC (i.e., mass absorption efficiency) was typically ~3 times higher in biomass burning-impacted samples. Factor analysis attributed 50% of the yearly average Abs365 to biomass burning sources. Brown carbon from primary urban emissions (mobile sources) was also observed and accounted for ~10% of the regional yearly average Abs365. Summertime diurnal profiles of Abs365 and WSOC showed that morning to midday increases in WSOC from photochemical production were associated with a decrease in Abs365/WSOC. After noon this ratio substantially increased, indicating that either some fraction of the non-light absorbing fresh SOA was rapidly (within hours) converted to chromophores heterogeneously, or that SOA from gas-particle partitioning later in the day was more light-absorbing. Factor analysis associated ~20 to 30% of Abs365 over 2007 with a secondary source that was highest in summer and also the main source for oxalic acid, suggesting that aqueous phase reactions may account for the light-absorbing fraction of WSOC observed throughout the southeastern US in summer.