ALBERT

Description: In spite of impressive advances in recent years, our present understanding of organic aerosol (OA) composition, physical and chemical properties, sources and transformation characteristics is still rather limited, and their environmental effects remain highly uncertain. This paper discusses and prioritizes issues related to organic aerosols and their effects on atmospheric processes and climate, providing a basis for future activities in the field. Four main topical areas are addressed: i) sources of OA; ii) formation transformation and removal of OA; iii) physical, chemical and mixing state of OA; iv) atmospheric modelling of OA. Key questions and research priorities regarding these four areas are synthesized in this paper, and outstanding issues for future research are presented for each topical area. In addition, an effort is made to formulate a basic set of consistent and universally applicable terms and definitions for coherent description of atmospheric OA across different scales and disciplines.

Description: Aerosol loading in the marine environment is investigated using aerosol composition measurements from several research ship campaigns (ICEALOT, MAP, RHaMBLe, VOCALS and OOMPH), observations of total AOD column from satellite (MODIS) and ship-based instruments (Maritime Aerosol Network, MAN), and a global chemical transport model (GEOS-Chem). This work represents the most comprehensive evaluation of oceanic OM emission inventories to date, by employing aerosol composition measurements obtained from campaigns with wide spatial and temporal coverage. The model underestimates AOD over the remote ocean on average by 0.02 (21 %), compared to satellite observations, but provides an unbiased simulation of ground-based Maritime Aerosol Network (MAN) observations. Comparison with cruise data demonstrates that the GEOS-Chem simulation of marine sulfate, with the mean observed values ranging between 0.22 μg m−3 and 1.34 μg m−3, is generally unbiased, however surface organic matter (OM) concentrations, with the mean observed concentrations between 0.07 μg m−3 and 0.77 μg m−3, are underestimated by a factor of 2–5 for the standard model run. Addition of a sub-micron marine OM source of approximately 9 TgC yr−1 brings the model into agreement with the ship-based measurements, however this additional OM source does not explain the model underestimate of marine AOD. The model underestimate of marine AOD is therefore likely the result of a combination of satellite retrieval bias and a missing marine aerosol source (which exhibits a different spatial pattern than existing aerosol in the model).

Description: Ambient particles collected on teflon filters at the Peak of Whistler Mountain, British Columbia (2182 m a.s.l.) during spring and summer 2009 were measured by Fourier transform infrared (FTIR) spectroscopy for organic functional groups (OFG). The project mean and standard deviation of organic aerosol mass concentrations (OM) for all samples was 3.2±3.3 (μg m−3). Measurements of aerosol mass fragments, size, and number concentrations were used to separate fossil-fuel combustion and burning and non-burning forest sources of the measured organic aerosol. The OM was composed of the same anthropogenic and non-burning forest components observed at Whistler mid-valley in the spring of 2008; during the 2009 campaign, biomass burning aerosol was additionally observed from fire episodes occurring between June and September. On average, organic hydroxyl, alkane, carboxylic acid, ketone, and primary amine groups represented 31 %±11 %, 34 %±9 %, 23 %±6 %, 6 %±7 %, and 6 %±3 % of OM, respectively. Ketones in aerosols were associated with burning and non-burning forest origins, and represented up to 27 % of the OM. The organic aerosol fraction resided almost entirely in the submicron fraction without significant diurnal variations. OM/OC mass ratios ranged mostly between 2.0 and 2.2 and O/C atomic ratios between 0.57 and 0.76, indicating that the organic aerosol reaching the site was highly aged and possibly formed through secondary formation processes.

Description: Direct measurements of N2O5 reactivity on ambient aerosol particles were made during September 2009 at the Scripps Institution of Oceanography (SIO) Pier facility located in La Jolla, CA. N2O5 reactivity measurements were made using a custom flow reactor and the particle modulation technique alongside measurements of aerosol particle size distributions and non-refractory composition. The pseudo-first order rate coefficients derived from the particle modulation technique and the particle surface area concentrations were used to determine the population average N2O5 reaction probability, γ(N2O5), approximately every 50 min. Insufficient environmental controls within the instrumentation trailer led us to restrict our analysis primarily to nighttime measurements. Within this subset of data, γ(N2O5) ranged from 〈0.001 to 0.029 and showed significant day-to-day variations. We compare these data to a recent parameterization that utilizes aerosol composition measurements and an aerosol thermodynamics model. The parameterization captures several aspects of the measurements with similar general trends over the time series. However, the parameterization persistently overestimates the measurements by a factor of 1.5–3 and does not illustrate the same extent of variability. Assuming chloride is internally mixed across the particle population leads to the largest overestimates. Removing this assumption only partially reduces the discrepancies, suggesting that other particle characteristics that can suppress γ(N2O5) are important, such as organic coatings or non-aqueous particles. The largest apparent driver of day-to-day variability in the measured γ(N2O5) at this site was the particle nitrate loading, as inferred from both the measured particle composition and the parameterizations. The relative change in measured γ(N2O5) as a function of particle nitrate loading appears to be consistent with expectations based on laboratory data, providing direct support for the atmospheric importance of the so-called "nitrate effect".

Description: We report properties of marine aerosol and clouds measured in the shipping lanes between Monterey Bay and San Francisco off the coast of Central California. Using a suite of aerosol instrumentation onboard the CIRPAS Twin Otter aircraft, these measurements represent a unique set of data contrasting the properties of clean and ship-impacted marine air masses in dry aerosol and cloud droplet residuals. Below-cloud aerosol exhibited average mass and number concentrations of 2 μg m−3 and 510 cm−3, respectively, which are consistent with previous studies performed off the coast of California. Enhancements in vanadium and cloud droplet number concentrations are observed concurrently with a decrease in cloud water pH, suggesting that periods of high aerosol loading are primarily linked to increased ship influence. Mass spectra from a compact time-of-flight Aerodyne aerosol mass spectrometer reveal an enhancement in the fraction of organic at m/z 42 (f42) and 99 (f99) in ship-impacted clouds. These ions are well correlated to each other (R2〉0.64) both in and out of cloud and constitute 14% (f44) and 3% (f99) of organic mass during periods of enhanced sulfate. High-resolution mass spectral analysis of these masses from ship measurements suggests that the ions responsible for this variation were oxidized, possibly due to cloud processing. We propose that the organic fractions of these ions be used as a metric for determining the extent to which cloud-processed ship emissions impact the marine atmosphere where (f42 〉 0.15; f99 〉 0.04) would imply heavy influence from shipping emissions, (0.05 〈 f42 〈 0.15; 0.01 〈 f99 〈 0.04) would imply moderate, but persistent, influences from ships, and (f42 〈 0.05; f99 〈 0.01) would imply clean, non-ship-influenced air.

Description: The first dataset for summertime boreal forest concentrations of two atmospherically relevant α-dicarbonyl compounds, glyoxal (Gly) and methylglyoxal (Mgly) on PM2.5 aerosol was obtained during the HUMPPA-COPEC-2010 field measurement intensive in Hyytiälä, Finland. Anthropogenic influences over the course of the campaign were identified using trace gas signatures and aerosol particle chemical composition analysis. The data evaluation allowed the identification of different events such as urban pollution plumes, biomass burning and sawmill emissions as sources of high Gly and Mgly concentrations. Mean aerosol concentrations during periods of biogenic influence were 0.81 ng m−3 for Gly and 0.31 ng m−3 for Mgly. Mgly was generally less abundant in PM2.5, probably due to its shorter photolysis lifetime and less effective partitioning into the particle phase due to its smaller effective Henry's Law constant compared to Gly. This is in contrast with previous urban studies which show significantly more Mgly than Gly. Peak concentrations for Gly coincided with nearby sources, e.g. high VOC emissions from nearby sawmills, urban pollution plumes from the city of Tampere located 50 km southwest of the sampling site and biomass burning emissions from wildfires. Calculated ratios of Gly in PM2.5 and total organic matter in PM1 aerosols indicate higher values in less aged aerosols. Irreversible processing of Gly in the particle phase, e.g. via oxidation by OH radicals, organo sulfate or imidazole formation are processes currently discussed in the literature which could likely explain these findings.

Description: Literature reports have differed on the possibilities of discontinuous and continuous (i.e., prompt and nonprompt) deliquescence and efflorescence of aerosol particles in the nanosize regime. Experiments reported herein using a hygroscopic tandem nano-differential mobility analyzer demonstrate prompt deliquescence and efflorescence of ammonium sulfate particles having diameters from 6 to 60 nm. Apparent nonpromptness can be induced both by operation of the experimental apparatus and by interpretation of the measurements, even though the underlying phase transitions of individual particles remain prompt. No nanosize effect on the relative humidity values of deliquescence or efflorescence is observed for the studied size range. Smaller hygroscopic growth factors are, however, observed for the nanoparticles, in agreement with thermodynamic calculations that include the Kelvin effect. A slightly nonspherical shape for dry ammonium sulfate particles is inferred from their hygroscopically induced reconstruction between 5 and 30% relative humidity. Our results provide a further understanding of nanoparticle behavior, especially relevant to the growth rates of atmospheric nanoparticles.

Description: The concentration of gases and aerosol particles have been measured at the mountain site of Altzomoni, 4010 m in altitude, located 60 km southeast of Mexico City, 50 km east of Puebla and 70 km northeast of Cuernavaca. The objective of this study was to evaluate the properties of gases and particles in the Regional Mixed Layer (RML) of Mexico's Megapolis. Altzomoni is generally above the RML from late evening until late morning at which time the arrival of the RML is marked by increasing concentrations of CO and aerosol particles that reach their maxima in mid-afternoon. The average diurnal cycles for fourteen days in March, 2006 were evaluated during which time the synoptic scale circulation had three principal patterns: from the east (E), southwest (SW) and west northwest (WNW). The original hypothesis was that air arriving from the direction of Mexico City would have much higher concentrations of anthropogenic gases and particles than air from Puebla or Cuernavaca, due to the relatively large differences in populations. In fact, not only were the average, maximum concentrations of CO and O3 (0.3 and 0.1 ppmv) approximately the same for air originating from the WNW and E, but the average maximum concentrations of Peroxyacyl nitrates (PAN,PPN) and particle organic matter (POM) in air from the E exceeded those in air from the WNW. Comparisons of measurements from the mountain site with those made by aircraft during the same period, using the same type of aerosol mass spectrometer, show that the total masses of POM, NO3−, SO42− and NH4+ were approximately the same from aircraft measurements made over Mexico City and when winds were from the east at the mountain site. In contrast 75% of the total aerosol mass at the mountain site was POM whereas over Mexico City the fraction of POM was less than 60%. The measurements suggest the occasional influence of emissions from the nearby volcano, Popocatepetl, as well as possible incursions of biomass combustion; however, the large concentrations of O3, PAN and POM suggest that secondary processes are the major source for these gases and particles. The similar concentrations in gases and particles when air is coming from the E and NWN raises the possibility of recirculation of air from Mexico City and the importance of this mechanism for impacting the regional air quality.

Description: Dimethylsulfide (DMS) emitted from the ocean is a biogenic precursor gas for sulfur dioxide (SO2) and non-sea-salt sulfate aerosols (SO42−). During the VAMOS-Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) in 2008, multiple instrumented platforms were deployed in the Southeastern Pacific (SEP) off the coast of Chile and Peru to study the linkage between aerosols and stratocumulus clouds. We present here observations from the NOAA Ship Ronald H. Brown and the NSF/NCAR C-130 aircraft along ~20° S from the coast (70° W) to a remote marine atmosphere (85° W). While SO42− and SO2 concentrations were distinctly elevated above background levels in the coastal marine boundary layer (MBL) due to anthropogenic influence (~800 and 80 pptv, respectively), their concentrations rapidly decreased west of 78° W (~100 and 25 pptv). In the remote region, entrainment from the free troposphere (FT) increased MBL SO2 burden at a rate of 0.05 ± 0.02 μmoles m−2 day−1 and diluted MBL SO42 burden at a rate of 0.5 ± 0.3 μmoles m−2 day−1, while the sea-to-air DMS flux (3.8 ± 0.4 μmoles m−2 day−1) remained the predominant source of sulfur mass to the MBL. In-cloud oxidation was found to be the most important mechanism for SO2 removal and in situ SO42− production. Surface SO42− concentration in the remote MBL displayed pronounced diel variability, increasing rapidly in the first few hours after sunset and decaying for the rest of the day. We theorize that the increase in SO42− was due to nighttime recoupling of the MBL that mixed down cloud-processed air, while decoupling and sporadic precipitation scavenging were responsible for the daytime decline in SO42−.

Description: Aerosol produced during the burning of olive tree branches was characterized with both direct source sampling (using a mobile smog chamber) and with ambient measurements during the burning season. The fresh particles were composed of 80% organic matter, 8–10% black carbon (BC), 5% potassium, 3–4% sulfate, 2–3% nitrate and 0.8% chloride. Almost half of the fresh olive tree branches burning organic aerosol (otBB-OA) consisted of alkane groups. Their mode diameter was close to 70 nm. The oxygen to carbon (O : C) ratio of the fresh otBB-OA was 0.29 ± 0.04. The mass fraction of levoglucosan in PM1 was 0.034–0.043, relatively low in comparison with most fuel types. This may lead to an underestimation of the otBB-OA contribution if levoglucosan is being used as a wood burning tracer. Chemical aging was observed during smog chamber experiments, as f44 and O : C ratio increased, due to reactions with OH radicals and O3. The otBB-OA AMS mass spectrum differs from the other published biomass burning spectra, with a main difference at m/z 60, used as levoglucosan tracer. In addition to particles, volatile organic compounds (VOCs) such as methanol, acetonitrile, acrolein, benzene, toluene and xylenes are also emitted. Positive matrix factorization (PMF) was applied to the ambient organic aerosol data and 3 factors could be identified: OOA (oxygenated organic aerosol, 55%), HOA (hydrocarbon-like organic aerosol, 11.3%) and otBB-OA 33.7%. The fresh chamber otBB-OA AMS spectrum is close to the PMF otBB-OA spectrum and resembles the ambient mass spectrum during olive tree branches burning periods. We estimated an otBB-OA emission factor of 3.5 ± 0.9 g kg−1. Assuming that half of the olive tree branches pruned is burned in Greece, 2300 ± 600 tons of otBB-OA are emitted every year. This activity is one of the most important fine aerosol sources during the winter months in Mediterranean countries.