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Aerosol charging state at an urban site: new analytical approach and implications for ion-induced nucleation

The charging state of aerosol populations was determined using an Ion-DMPS in Helsinki, Finland between December 2008 and February 2010. We extrapolated the charging state and calculated the ion-induced nucleation fraction to be around 1.3 % ± 0.4 % at 2 nm and 1.3 % ± 0.5 % at 1.5 nm, on average. We present a new method to retrieve the average charging state for a new particle formation event, at a given size and polarity. We improve the uncertainty assessment and fitting technique used previously with an Ion-DMPS. We also use a new theoretical framework that allows for different concentrations of small ions for different polarities (polarity asymmetry). We extrapolate the ion-induced fraction using polarity symmetry and asymmetry. Finally, a method to calculate the growth rates from the behaviour of the charging state as a function of the particle diameter using polarity symmetry and asymmetry is presented and used on a selection of new particle formation events.
AMS and LC/MS analyses of SOA from the photooxidation of benzene and 1,3,5-trimethylbenzene in the presence of NOx: effects of chemical structure on SOA aging

Oxygenated organic aerosol (OOA) observed in remote areas is believed to comprise aged secondary organic aerosol (SOA); however, the reaction processes relevant to SOA chemical aging have hitherto been unclear. We recently measured the mass spectra of SOA formed from the photooxidation of aromatic hydrocarbons using an Aerodyne aerosol mass spectrometer (AMS) and reported that SOA aging is slowed with increasing number of alkyl groups in the precursor molecule. In this study, we selected benzene and 1,3,5-trimethylbenzene (TMB) as precursors to analyze SOA formed from the photooxidation of aromatic hydrocarbons in the presence of NOx using high-resolution time-of-flight AMS (H-ToF-AMS) and liquid chromatography/time-of-flight mass spectrometry (LC/TOF-MS). A van Krevelen diagram was studied using the O/C and H/C ratios obtained by H-ToF-AMS for organics present in SOA. The results showed these organics to be rich in carboxylic acids or hydroxycarbonyls and the O/C ratio of SOA formed by the reaction of 1,3,5-TMB to be lower than that for benzene. Analytical results from LC/TOF-MS showed the particulate products formed by the reaction of 1,3,5-TMB to be richer in ketocarboxylic acids than for benzene. These results indicate that SOA aging proceeds mainly by formation of carboxylic acids and that the rate of SOA aging in laboratory chambers is limited by the oxidation of ketone groups. SOA formed in laboratory chamber experiments is less oxidized than for ambient OOA, not only because the experimental duration is insufficient or the SOA mass loading in the chamber is higher than that of the atmosphere. The laboratory chamber experiments under dry conditions are not able to simulate ketocarboxylic acid photochemical oxidation in the aqueous phase. The fractions of organic peroxides to the total SOA mass were determined by iodometric spectrophotometry to be 12 ± 8% (1,3,5-TMB) and <39% (benzene). Further, it was newly found that, unlike the reaction of benzene, only very small amounts of nitrophenols are produced by the reaction of 1,3,5-TMB.
Using non-negative matrix factorization for the identification of daily patterns of particulate air pollution in Beijing during 2004–2008

Increasing traffic density and a changing car fleet on the one hand as well as various reduction measures on the other hand may influence the composition of the particle population and, hence, the health risks for residents of megacities like Beijing. A suitable tool for identification and quantification of source group-related particle exposure compositions is desirable in order to derive optimal adaptation and reduction strategies and therefore, is presented in this paper. Particle number concentrations have been measured in high time- and space-resolution at an urban background monitoring site in Beijing, China, during 2004–2008. In this study a new pattern recognition procedure based on non-negative matrix factorization (NMF) was introduced to extract characteristic diurnal air pollution patterns of particle number and volume size distributions for the study period. Initialization and weighting strategies for NMF applications were carefully considered and a scaling procedure for ranking of obtained patterns was implemented. In order to account for varying particle sizes in the full diameter range [3 nm; 10 μm] two separate NMF applications (a) for diurnal particle number concentration data (NMF-N) and (b) volume concentration data (NMF-V) have been performed. Five particle number concentration-related NMF-N factors were assigned to patterns mainly describing the development of ultrafine (particle diameter Dp < 100 nm instead of DP) as well as fine particles (Dp < 2.5 μm), since absolute number concentrations are highest in these diameter ranges. The factors are classified into primary and secondary sources. Primary sources mostly involved anthropogenic emission sources such as traffic emissions or emissions of nearby industrial plants, whereas secondary sources involved new particle formation and accumulation (particle growth) processes. For the NMF-V application the five extracted factors mainly described coarse particle (2.5 μm < Dp < 10 μm) variations, generated by processes like dust storm events. Because particle volume depends on particle diameter in a cubic manner, larger particles are emphasized in the latter application. In order to gain insight in the day-by-day varying source-associated composition of the particle burden non-negative linear combinations of individual source-associated pollution patterns were used to approximate the original particle data. Consequently, this NMF-based procedure provides a reasonable numerical-statistical tool for the description of daily patterns of particle pollution, source identification and reconstruction of daily patterns by summarizing weighted factors.
Aerosol chemical composition at Cabauw, The Netherlands as observed in two intensive periods in May 2008 and March 2009

Observations of aerosol chemical composition in Cabauw, the Netherlands, are presented for two intensive measurement periods in May 2008 and March 2009. Sub-micron aerosol chemical composition was measured by an Aerodyne Aerosol Mass Spectrometer (AMS) and is compared to observations from aerosol size distribution measurements as well as composition measurements with a Monitor for AeRosol and GAses (MARGA) based instrument and a Thermal-Desorption Proton-Transfer-Reaction Mass-Spectrometer (TD-PTR-MS). An overview of the data is presented and the data quality is discussed. In May 2008 enhanced pollution was observed with organics contributing 40% to the PM1 mass. In contrast the observed average mass loading was lower in March 2009 and a dominance of ammonium nitrate (42%) was observed. The semi-volatile nature of ammonium nitrate is evident in the diurnal cycles with maximum concentrations observed in the morning hours in May 2008 and little diurnal variation observed in March 2009. Size dependent composition data from AMS measurements are presented and show a dominance of organics in the size range below 200 nm. A higher O:C ratio of the organics is observed for May 2008 than for March 2009. Together with the time series of individual tracer ions this shows the dominance of OOA over HOA in May 2008.
Enhanced solar energy absorption by internally-mixed black carbon in snow grains

Here we explore light absorption by snowpack containing black carbon (BC) particles residing within ice grains. Basic considerations of particle volumes and BC/snow mass concentrations show that there are generally 0.05–109 BC particles for each ice grain. This suggests that internal BC is likely distributed as multiple inclusions within ice grains, and thus the dynamic effective medium approximation (DEMA) (Chýlek and Srivastava, 1983) is a more appropriate optical representation for BC/ice composites than coated-sphere or standard mixing approximations. DEMA calculations show that the 460 nm absorption cross-section of BC/ice composites, normalized to the mass of BC, is typically enhanced by factors of 1.8–2.1 relative to interstitial BC. BC effective radius is the dominant cause of variation in this enhancement, compared with ice grain size and BC volume fraction. We apply two atmospheric aerosol models that simulate interstitial and within-hydrometeor BC lifecycles. Although only ~2% of the atmospheric BC burden is cloud-borne, 71–83% of the BC deposited to global snow and sea-ice surfaces occurs within hydrometeors. Key processes responsible for within-snow BC deposition are development of hydrophilic coatings on BC, activation of liquid droplets, and subsequent snow formation through riming or ice nucleation by other species and aggregation/accretion of ice particles. Applying deposition fields from these aerosol models in offline snow and sea-ice simulations, we calculate that 32–73% of BC in global surface snow resides within ice grains. This fraction is smaller than the within-hydrometeor deposition fraction because meltwater flux preferentially removes internal BC, while sublimation and freezing within snowpack expose internal BC. Incorporating the DEMA into a global climate model, we simulate increases in BC/snow radiative forcing of 43–86%, relative to scenarios that apply external optical properties to all BC. We show that snow metamorphism driven by diffusive vapor transfer likely proceeds too slowly to alter the mass of internal BC while it is radiatively active, but neglected processes like wind pumping and convection may play much larger roles. These results suggest that a large portion of BC in surface snowpack may reside within ice grains and increase BC/snow radiative forcing, although measurements to evaluate this are lacking. Finally, previous studies of BC/snow forcing that neglected this absorption enhancement are not necessarily biased low, because of application of absorption-enhancing sulfate coatings to hydrophilic BC, neglect of coincident absorption by dust in snow, and implicit treatment of cloud-borne BC resulting in longer-range transport.
Geographic and seasonal distributions of CO transport pathways and their roles in determining CO centers in the upper troposphere

Past studies have identified a variety of pathways by which carbon monoxide (CO) may be transported from the surface to the tropical upper troposphere (UT); however, the relative roles that these transport pathways play in determining the distribution and seasonality of CO in the tropical UT remain unclear. We have developed a method to automate the identification of two pathways ("local convection" and "advection within the lower troposphere (LT) followed by convective vertical transport") involved in CO transport from the surface to the UT. This method is based on the joint application of instantaneous along-track, co-located, A-Train satellite measurements. Using this method, we find that the locations and seasonality of the UT CO maxima in the tropics were strongly correlated with the frequency of local convective transport during 2007. We also find that the "local convection" pathway (convective transport that occurred within a fire region) typically transported significantly more CO to the UT than the "LT advection → convection" pathway (advection of CO within the LT from a fire region to a convective region prior to convective transport). To leading order, the seasonality of CO concentrations in the tropical UT reflected the seasonality of the "local convection" transport pathway during 2007. The UT CO maxima occurred over Central Africa during boreal spring and over South America during austral spring. Occurrence of the "local convection" transport pathway in these two regions also peaked during these seasons. During boreal winter and summer, surface CO emission and convection were located in opposite hemispheres, which limited the effectiveness of transport to the UT. During these seasons, CO transport from the surface to the UT typically occurred via the "LT advection → convection" pathway.
Global modeling of SOA formation from dicarbonyls, epoxides, organic nitrates and peroxides

Recent experimental findings indicate that secondary organic aerosol (SOA) represents an important and, under many circumstances, the major fraction of the organic aerosol burden. Here, we use a global 3-D model (IMPACT) to test the results of different mechanisms for the production of SOA. The basic mechanism includes SOA formation from organic nitrates and peroxides produced from an explicit chemical formulation, using partition coefficients based on thermodynamic principles together with assumptions for the rate of formation of low-volatility oligomers. We also include the formation of low-volatility SOA from the reaction of glyoxal and methylglyoxal on aqueous aerosols and cloud droplets as well as from the reaction of epoxides on aqueous aerosols. A model simulation including these SOA formation mechanisms gives an annual global SOA production of 120.5 Tg. The global production of SOA is decreased substantially to 90.8 Tg yr−1 if the HOx regeneration mechanism proposed by Peeters et al. (2009) is used. Model predictions with and without this HOx (OH and HO2 regeneration scheme are compared with multiple surface observation datasets, namely: the Interagency Monitoring of Protected Visual Environments (IMPROVE) for the United States, the European Monitoring and Evaluation Programme (EMEP), and aerosol mass spectrometry (AMS) data measured in both the Northern Hemisphere and tropical forest regions. All model simulations show reasonable agreement with the organic carbon mass observed in the IMPROVE network and the AMS dataset, however observations in Europe are significantly underestimated, which may be caused by an underestimation of primary organic aerosol emissions (POA) in winter and of emissions and/or SOA production in the summer. The modeled organic aerosol concentrations tend to be higher by roughly a factor of three when compared with measurements at three tropical forest sites. This overestimate suggests that more measurements and model studies are needed to examine the formation of organic aerosols in the tropics. The modeled organic carbon (OC) in the free troposphere is in agreement with measurements in the ITCT-2K4 aircraft campaign over North America and in pollution layers off Asia during the INTEX-B campaign, although the model underestimates OC in the free troposphere in comparison with the ACE-Asia campaign off the coast of Japan.
Microphysical simulations of sulfur burdens from stratospheric sulfur geoengineering

Recent microphysical studies suggest that geoengineering by continuous stratospheric injection of SO2 gas may be limited by the growth of the aerosols. We study the efficacy of SO2, H2SO4 and aerosol injections on aerosol mass and optical depth using a three-dimensional general circulation model with sulfur chemistry and sectional aerosol microphysics (WACCM/CARMA). We find increasing injection rates of SO2 in a narrow band around the equator to have limited efficacy while broadening the injecting zone as well as injecting particles instead of SO2 gas increases the sulfate burden for a given injection rate, in agreement with previous work. We find that injecting H2SO4 gas instead of SO2 does not discernibly alter sulfate size or mass, in contrast with a previous study using a plume model with a microphysical model. However, the physics and chemistry in aircraft plumes, which are smaller than climate model grid cells, need to be more carefully considered. We also find significant perturbations to tropospheric aerosol for all injections studied, particularly in the upper troposphere and near the poles, where sulfate burden increases by up to 100 times. This enhanced burden could have implications for tropospheric radiative forcing and chemistry. These results highlight the need to mitigate greenhouse gas emissions rather than attempt to cool the planet through geoengineering, and to further study geoengineering before it can be seriously considered as a climate intervention option.
The equilibrium response to idealized thermal forcings in a comprehensive GCM: implications for recent tropical expansion

Several recent studies have shown the width of the tropical belt has increased over the last several decades. The mechanisms driving tropical expansion are not well known and the recent expansion is underpredicted by state-of-the art GCMs. We use the CAM3 GCM to investigate how tropical width responds to idealized atmospheric heat sources, focusing on zonal displacement of the tropospheric jets. The heat sources include global and zonally restricted lower-tropospheric warmings and stratospheric coolings, which coarsely represent possible impacts of ozone or aerosol changes. Similar to prior studies with simplified GCMs, we find that stratospheric cooling – particularly at high-latitudes – shifts jets poleward and excites Northern and Southern Annular Mode (NAM/SAM)-type responses. We also find, however, that modest heating of the midlatitude boundary layer drives a similar response; heating at high latitudes provokes a weaker, equatorward shift and tropical heating produces no shift. Over 70 % of the variance in annual mean jet displacements across 27 experiments is accounted for by a newly proposed "Expansion Index", which compares mid-latitude tropospheric warming to that at other latitudes. We find that previously proposed factors, including tropopause height and tropospheric stability, do not fully explain the results. Results suggest recently observed tropical expansion could have been driven not only by stratospheric cooling, but also by mid-latitude heating sources due for example to ozone or aerosol changes.
Differences in the reactivity of ammonium salts with methylamine

The heterogeneous uptake of methylamine (MA) onto (NH4)2SO4, NH4HSO4, NH4NO3 and NH4Cl was investigated using a Knudsen cell reactor coupled to a quadrupole mass spectrometer, in situ Raman spectrometer and theoretical calculations. Exchange reactions were observed between MA and NH4NO3, (NH4)2SO4, and NH4Cl were observed at 298 K. Simple acid-base reaction for MA was found taking place on NH4HSO4. CH3NH3NO3 and CH3NH3Cl are not stable at low pressure and have higher dissociation vapor pressure than methylammonium sulfate. The observed uptake coefficients of MA on (NH4)2SO4, NH4HSO4, NH4NO3 and NH4Cl at 298 K were measured to be 6.30±1.03×10−3, 1.78±0.36×10−2, 8.79±1.99×10−3 and 2.29±0.28×10−3, respectively. A linear free energy relationship was found for the heterogeneous reactions between MA and NH4Cl, (NH4)2SO4 and NH4NO3. Namely, the natural logarithm of uptake coefficients of MA on these ammonium salts is linearly related to the electrostatic potential of the H atom in the NH4+ group.