The history of air masses is often investigated using backward trajectories to gain knowledge about processes along the air parcel path as well as possible source regions. Here, we describe a refined approach that incorporates airborne gas, aerosol, and environmental data into back trajectories and show how this technique allows for simultaneous evaluation of air mass history and back trajectory reliability without the need to calculate trajectory errors.We use the HYbrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model and add a simple semi-automated computing routine to facilitate high-frequency coverage of back trajectories initiated along free tropospheric (FT) flight tracks and profiles every 10 s. We integrate our in situ physiochemical data by color-coding each of these trajectories with its corresponding in situ tracer values measured at the back trajectory start points along the flight path. The unique color for each trajectory aids assessment of trajectory reliability through the visual clustering of air mass pathways of similar coloration. Moreover, marked changes in trajectories associated with marked changes evident in measured physiochemical or thermodynamic properties of an air mass add credence to trajectories. This is particularly true when these air mass properties are linked to trajectory features characteristic of recognized sources or processes. This visual clustering of air mass pathways is of particular value for large-scale 3-D flight tracks common to aircraft experiments where air mass features of interest are often spatially distributed and temporally separated.The cluster-visualization tool used here reveals that most FT back trajectories with pollution signatures measured in the central equatorial Pacific reach back to sources on the South American continent over 10 000 km away and 12 days back in time, e. g., the Amazonian basin. We also demonstrate the distinctions in air mass properties between these and trajectories that penetrate deep convection in the Inter-Tropical Convergence Zone. Additionally, for the first time we show consistency of modeled precipitation along back trajectories with scavenging signatures in the aerosol measured for these trajectories.

Introduction Conclusions References

Airborne aerosol measurements in the central equatorial Pacific during PASE (Pacific Atmospheric Sulfur Experiment) revealed that cloud condensation nuclei (CCN) activated in marine boundary layer (MBL) clouds were strongly influenced by entrainment from the free troposphere (FT). About 65% entered at sizes effective as CCN in MBL clouds, while ~25% entered the MBL too small to activate but subsequently grew via gas to particle conversion. The remaining ~10% were inferred to be sea salt aerosol. FT aerosols at low carbon monoxide (CO) mixing ratios (< 63 ppbv) were mostly volatile at 360 °C with a number mode peak of around 30–40 nm dry diameter and tended to be associated with cloud outflow from distant (3000 km or more) deep convection. Higher CO concentrations were commonly associated with trajectories from South America and the Amazon region (ca. ~10 000 km away) and occurred in layers indicative of combustion sources (biomass burning season) partially scavenged by precipitation. These had number modes near 60–80 nm dry diameter with a large fraction of CCN.2 (those activated at 0.2% supersaturation and representative of MBL clouds) prior to entrainment into the MBL. Flight averaged concentrations of CCN.2 were similar for measurements near the surface, below the inversion and in the FT just above the inversion, confirming that subsidence and entrainment of FT aerosol strongly influenced MBL CCN.2. Concurrent flight-to-flight variations of CCN.2 at all altitudes below 3 km also imply MBL CCN.2 concentrations were in quasi-equilibrium with the FT over a 2–3 day timescale. The observed FT transport over thousands of kilometers indicates teleconnections between MBL CCN and cloud-scavenged sources of both natural and/or residual combustion origin. Nonetheless, in spite of its importance, this source of CCN number is not well represented in most current models and is generally not detectable by satellite because of the low aerosol scattering in such layers as a result of cloud scavenging. In addition, our measurements confirm nucleation in the MBL was not evident during PASE and argue against a localized linear relation in the MBL between dimethyl sulfide (DMS) and CCN suggested by the CLAW hypothesis. However, when the FT is not impacted by long-range transport, sulfate aerosol derived from DMS pumped aloft in the ITCZ (Inter-Tropical Convergence Zone) can provide a source of CCN to the boundary layer via FT teleconnections involving more complex non-linear processes.

Topography-induced steady-state accelerated wind flow in the Alenuihaha Channel between the islands of Hawaii and Maui provides about 100 km of fetch with winds that can nearly double over trade wind speed. Here ship- and aircraft-based observations of meteorological parameters and aerosols in Hawaii's orographic natural "wind tunnel'' are used for the study of sea salt aerosol (SSA) production, evolution, and related optical effects under clean oceanic conditions. There are certain advantages of channel measurements, such as a broad and uniform upstream area usually filled with background aerosol, stationary flow, and known fetch, but also some difficulties, like vigorous entrainment and persistent presence of organized structures (rolls). It is found that marine boundary layer (MBL) rolls are a common occurrence near the Hawaiian Islands even when cloud streets are not visible in satellite imagery. The presence of rolls tends to enhance the variability of ambient aerosol concentration and probably affects production of primary sea salt aerosol and entrainment from above. The possibility of channel measurements of the size-dependent flux of SSA is explored using a concentration buildup method as surface wind speeds range from 7 to 11 m s(-1). Production of SSA particles with dry diameter as small as 0.18 mu m was observed. General agreement with reported SSA fluxes was found.

In the spring of 2008 NASA and NOAA funded the ARCTAS and ARCPAC field campaigns as contributions to POLARCAT, a core IPY activity. During the campaigns the NASA DC-8, P-3B and NOAA WP-3D aircraft conducted over 160 h of in-situ sampling between 0.1 and 12 km throughout the Western Arctic north of 55° N (i.e. Alaska to Greenland). All aircraft were equipped with multiple wavelength measurements of aerosol optics, trace gas and aerosol chemistry measurements, as well as direct measurements of the aerosol size distributions and black carbon mass. Late April of 2008 proved to be exceptional in terms of Asian biomass burning emissions transported to the Western Arctic. Though these smoke plumes account for only 11–14 % of the samples within the Western Arctic domain, they account for 42–47 % of the total burden of black carbon. Dust was also commonly observed but only contributes to 4–12 % and 3–8 % of total light absorption at 470 and 530 nm wavelengths above 6 km. Below 6 km, light absorption by carbonaceous aerosol derived from urban/industrial and biomass burning emissions account for 97–99 % of total light absorption by aerosol. Stratifying the data to reduce the influence of dust allows us to determine mass absorption efficiencies for black carbon of 11.2±0.8, 9.5±0.6 and 7.4±0.7 m2 g−1 at 470, 530 and 660 nm wavelengths. These estimates are consistent with 35–80 % enhancements in 530 nm absorption due to clear or slightly absorbing coatings of pure black carbon particulate. Assuming a 1/λ wavelength dependence for BC absorption, and assuming that refractory aerosol (420 °C, τ = 0.1 s) in low-dust samples is dominated by brown carbon, we derive mass absorption efficiencies for brown carbon of 0.83±0.15 and 0.27±0.08 m2 g−1 at 470 and 530 nm wavelengths. Estimates for the mass absorption efficiencies of Asian dust are 0.034 m2 g−1 and 0.017 m2 g−1. However the absorption efficiency estimates for dust are highly uncertain due to the limitations imposed by PSAP instrument noise. In-situ ARCTAS/ARCPAC measurements during the IPY provide valuable constraints for absorbing aerosol over the Western Arctic, species which are currently poorly simulated over a region that is critically under-sampled.

We describe aerosol optical depth (AOD) measured during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) experiment, focusing on vertical profiles, inter-comparison with correlative observations and fine-mode fraction. Arctic haze observed in <2 km and 2–4 km over Alaska in April 2008 originated mainly from anthropogenic emission and biomass burning, respectively, according to aerosol mass spectrometry and black carbon incandescence measurements. The Ångström exponent for these air masses is 1.4 ± 0.3 and 1.7 ± 0.1, respectively, when derived at 499 nm from a second-order polynomial fit to the AOD spectra measured with the 14-channel Ames Airborne Tracking Sunphotometer (AATS-14) over 354–2139 nm. We examine 55 vertical profiles selected from all phases of the experiment. For two thirds of them, the AOD spectra are within 3% + 0.02 of the vertical integral of local visible-light scattering and absorption. The horizontal structure of smoke plumes from local biomass burning observed in central Canada in June and July 2008 explains most outliers. The differences in mid-visible Ångström exponent are <0.10 for 63% of the profiles with 499-nm AOD > 0.1. The retrieved fine-mode fraction of AOD is mostly between 0.7 and 1.0, and its root mean square difference (in both directions) from column-integral submicron fraction (measured with nephelometers, absorption photometers and an impactor) is 0.12. These AOD measurements from the NASA P-3 aircraft, after compensation for below-aircraft light attenuation by vertical extrapolation, mostly fall within ±0.02 of AERONET ground-based measurements between 340–1640 nm for five overpass events.

This study extrapolates aerosol data of the CARIBIC project from 1997 until June 2008 in along trajectories to compose large-scale maps and vertical profiles of submicrometre particle concentrations in the upper troposphere and lowermost stratosphere (UT/LMS). The extrapolation was validated by comparing extrapolated values with CARIBIC data measured near the respective trajectory position and by comparing extrapolated CARIBIC data to measurements by other experiments near the respective trajectory positions. Best agreement between extrapolated and measured data is achieved with particle lifetimes longer than the maximum length of used trajectories. The derived maps reveal regions of strong and frequent new particle formation, namely the Tropical Central and Western Africa with the adjacent Atlantic, South America, the Caribbean and Southeast Asia. These regions of particle formation coincide with those of frequent deep convective clouds. Vertical particle concentration profiles for the troposphere and the stratosphere confirm statistically previous results indicating frequent new particle formation in the tropopause region. There was no statistically significant increase in Aitken mode particle concentration between the first period of CARIBIC operation, 1997–2002, and the second period, 2004–2009. However, a significant increase in concentration occurred within the latter period when considering it in isolation.

Particle number concentration in the troposphere is an important parameter controlling the climate and health impacts of atmospheric aerosols. We show that nucleation rates and total particle number concentrations in the troposphere, predicted by different nucleation schemes, differ significantly. Our extensive comparisons of simulated results with land‐, ship‐, and aircraft‐based measurements indicate that, among six widely used nucleation schemes involving sulfuric acid, only the ion‐mediated nucleation (IMN) scheme can reasonably account for both absolute values (within a factor of ∼2) and spatial distributions of particle number concentrations in the whole troposphere. Binary homogeneous nucleation (BHN) schemes significantly underpredict particle number concentration in the lower troposphere (below ∼500 mbar), especially in the boundary layer over major continents (by a factor of up to ∼10). BHN is also insignificant in the upper troposphere based on a recent kinetically self‐consistent nucleation model constrained by multiple independent laboratory data. Previous conclusions about the importance of BHN in the upper troposphere should be revisited. Empirical activation and kinetic nucleation formulas significantly overpredict the particle number concentrations over tropical and subtropical oceans (by a factor of up to ∼10 in the boundary layer), and the overpredictions extend from ocean surface to around ∼400 mbar. This study represents the first comprehensive comparison of global particle number simulations with relevant measurements that have a 3‐D global spatial coverage. Our results suggest that ion‐mediated H2SO4‐H2O nucleation appears to dominate over neutral H2SO4‐H2O nucleation, not only in the lower troposphere but also in the middle and upper troposphere.

: Our long-term goal is to establish an improved understanding of the factors that control the marine aerosol properties and concentrations as they relate to generation processes, mixing processes, their dependence on oceanic and environmental conditions and physicochemical evolution in the marine boundary layer. We expect these efforts to lead to improved modeling and predictability of marine aerosol concentrations and optical properties. One of the components of our work is to describe the chemical composition of submicron, nonrefractory, aerosol over the central Pacific Ocean, a region where little data of this type exists. Our studies involve using an Aerodyne Time-of-Flight Aerosol Mass Spectrometer (ToF-AMS) to chemically characterize the aerosol. We also seek to address the evolving issue of organic aerosol in the marine environment, and deduce potential sources of these organics. More specifically, we wish to continue our investigation of the marine source of organics to the remote marine atmosphere, and add strength to our hypothesis that little to negligible organics can be associated with a natural marine source. Our activities this past year also include analysis of equatorial Pacific aerosol aircraft data (NCAR C-130 PASE experiment) as part of our studying Large Organized Structures (LOS) or rolls and their role in aerosol processes and the vertical mixing of sea-salt and associated optical extinction.

Understanding the effect of anthropogenic combustion upon aerosol optical depth (AOD), clouds, and their radiative forcing requires regionally representative aerosol profiles. In this work, we examine more than 1000 vertical profiles from 11 major airborne campaigns in the Pacific hemisphere and confirm that regional enhancements in aerosol light scattering, mass, and number are associated with carbon monoxide from combustion and can exceed values in unperturbed regions by more than one order of magnitude. Related regional increases in a proxy for cloud condensation nuclei (CCN) and AOD imply that direct and indirect aerosol radiative effects are coupled issues linked globally to aged combustion. These profiles constrain the influence of combustion on regional AOD and CCN suitable for challenging climate model performance and informing satellite retrievals.

Remote sensing of cloud condensation nuclei (CCN) would help evaluate the indirect effects of tropospheric aerosols on clouds and climate. To assess its feasibility, we examined relationships of submicron aerosol composition to CCN activity and optical properties observed during the MILAGRO/INTEX-B aircraft campaigns. An indicator of CCN activity, κ, was calculated from hygroscopicity measured under saturation. κ for dry 100 nm particles decreased with increasing organic fraction of non-refractory mass of submicron particles (OMF) as 0.34–0.20×OMF over Central Mexico and 0.47–0.43×OMF over the US West Coast. These fits represent the critical dry diameter, centered near 100 nm for 0.2% supersaturation but varied as κ(−1/3), within measurement uncertainty (~20%). The decreasing trends of CCN activity with the organic content, evident also in our direct CCN counts, were consistent with previous ground and laboratory observations of highly organic particles. The wider range of OMF, 0–0.8, for our research areas means that aerosol composition will be more critical for estimation of CCN concentration than at the fixed sites previously studied. Furthermore, the wavelength dependence of extinction was anti-correlated with OMF as −0.70×OMF+2.0 for Central Mexico's urban and industrial pollution air masses, for unclear reasons. The Angstrom exponent of absorption increased with OMF, more rapidly under higher single scattering albedo, as expected for the interplay between soot and colored weak absorbers (some organic species and dust). Because remote sensing products currently use the wavelength dependence of extinction albeit in the column integral form and may potentially include that of absorption, these regional spectral dependencies are expected to facilitate retrievals of aerosol bulk chemical composition and CCN activity over Central Mexico.

Supplement Table 1.Slope, intercept and R2 of the regression line for the scattering comparison between the OPC and nephelometer at 450, 550 and 700 nm.

Research flights with the National Center for Atmospheric Research (NCAR) C-130 airborne laboratory were conducted over the equatorial ocean during the Pacific Atmospheric Sulfur Experiment (PASE). The focused, repetitive flight plans provided a unique opportunity to explore the principal pathways of sulfur processing in remote marine environments in close detail. Fast airborne measurements of SO 2 using the Drexel University APIMS (Atmospheric Pressure Ionization Mass Spectrometer) instrument further provided unprecedented insight into the complete budget of this important sulfur gas. In general, turbulent mixing in the marine boundary layer (MBL) continuously depletes SO 2 due to the shallow convection of the tropical trade wind regime by venting the gas into the buffer layer (BuL) above. However, on nearly one-third of the flights a net import of SO 2 into the MBL from the BuL was observed. Concurrent measurements of the DMS budget allowed for a heterogeneous S(IV) oxidation rate to be inferred from the SO 2 budget residual. The average heterogeneous loss rate was found to be 0.05 h −1 , which taken in conjunction with the observed aerosol surface area distributions and O 3 levels indicates that the supermicron aerosols maintain a near neutral pH. The average dry deposition velocity of SO 2 was found to be 0.4 cm s −1 , about 30% lower than predicted by standard parameterizations. The yield of SO 2 from DMS oxidation was found to be near unity. The mission averages indicate that approximately 57% of the SO 2 in the MBL is lost to aerosols, 27% is subject to dry deposition, 7% is mixed into the BuL, and 10% is oxidized by OH.