Alexander A. P. Pszenny

International Consortium for Atmospheric Research on Transport and Transformation (ICARTT): North America to Europe—Overview of the 2004 summer field study

In the summer of 2004 several separate field programs intensively studied the photochemical, heterogeneous chemical and radiative environment of the troposphere over North America, the North Atlantic Ocean, and western Europe. Previous studies have indicated that the transport of continental emissions, particularly from North America, influences the concentrations of trace species in the troposphere over the North Atlantic and Europe. An international team of scientists, representing over 100 laboratories, collaborated under the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) umbrella to coordinate the separate field programs in order to maximize the resulting advances in our understanding of regional air quality, the transport, chemical transformation and removal of aerosols, ozone, and their precursors during intercontinental transport, and the radiation balance of the troposphere. Participants utilized nine aircraft, one research vessel, several ground-based sites in North America and the Azores, a network of aerosol-ozone lidars in Europe, satellites, balloon borne sondes, and routine commercial aircraft measurements. In this special section, the results from a major fraction of those platforms are presented. This overview is aimed at providing operational and logistical information for those platforms, summarizing the principal findings and conclusions that have been drawn from the results, and directing readers to specific papers for further details.

Aerosol pH in the marine boundary layer: A review and model evaluation

Abstract Impacts of sea-salt-aerosol pH on oxidation processes, sulfur cycling, and surface-ocean fertilization are uncertain; estimates vary from pH 9 and the pH-dependence of some transformations is poorly characterized. We modeled these processes under clean and polluted conditions. At pH 8, S (IV) +O 3 in sea salt is the principal S-oxidation pathway. At pH 5.5, S (IV) oxidation by HOCl dominates. Decreased SO 2 solubility at pH 3 slows S (VI) production. The relative contribution of H 2 SO 4(g) scavenging to S (VI) in sea salt increases with decreasing pH. Significant sea-salt dehalogenation is limited to acidified aerosol. Volatilization rates of BrCl and Br 2 do not vary significantly between pH 5.5 and 3, whereas HCl production via acid displacement increases by a factor of 20. At pH 5.5 and 8, virtually all HNO 3 is scavenged by sea salt. Modeled HNO 3 increases at pH 3 but remains substantially lower than particulate NO - 3 . Discrepancies between measurements and modeled results are assessed based on measurement artifacts, uncertainties in rate and equilibrium constants, organic reactants and surface films, and dynamics.

Chemical and physical characteristics of nascent aerosols produced by bursting bubbles at a model air‐sea interface

inorganic aerosol constituents were similar to those in ambient air. Ca 2+ was significantly enriched relative to seawater (median factor = 1.2). If in the form of CaCO3, these enrichments would have important implications for pH-dependent processes. Other inorganic constituents were present at ratios indistinguishable from those in seawater. Soluble organic carbon (OC) was highly enriched in all size fractions (median factor for all samples = 387). Number size distributions exhibited two lognormal modes. The number production flux of each mode was linearly correlated with bubble rate. At 80% RH, the larger mode exhibited a volume centroid of 5-mm diameter and included 95% of the inorganic sea-salt mass; water comprised 79% to 90% of volume. At 80% RH, the smaller mode exhibited a number centroid of 0.13-mm diameter; water comprised 87% to 90% of volume. The median mass ratio of organic matter to sea salt in the smallest size fraction (geometric mean diameter = 0.13 mm) was 4:1. These results support the hypothesis that bursting bubbles are an important global source of CN and CCN with climatic implications. Primary marine aerosols also influence radiative transfer via multiphase processing of sulfur and other climate-relevant species.

Precipitation composition and its variability in the southern Indian Ocean: Amsterdam Island, 1980–1987

Event precipitation samples have been collected on Amsterdam Island (37° 47′ S, 77° 31′ E) from May 1980 through January 1987 and analyzed for SO4=, NO3−, Cl−, Na+, Mg++, K+, Ca++, H+, HCOOt, and CH3COOt. The objective of this paper is to assess the processes which influence variability in the chemical composition of precipitation at this remote marine site. Back trajectories and gas phase 222Rn measurements were used to identify source regions and their relative contributions to precipitation composition. The technique of cluster analysis was applied to trajectory data as a method for determining to what degree different atmospheric flow patterns influence variability in the observed composition. The dominant source regions for chemical deposition were found to be Madagascar and/or S.E. Africa and the ocean region north and east of Amsterdam Island. A strong seasonal signal in the precipitation composition is illustrated. Processes which influence variability in the chemical composition include the following: seasonality in the source strength of biogenic precursors for non-sea-salt (nss) SO4=, NH4+ and carboxylic acids; seasonality in biomass-burning continental sources for the concentration of NO3− and a portion of nss Cl−; and variations in source region driven by daily changes in meteorology, as well as seasonal and annual differences in transport and removal patterns. In addition, interannual differences in nss SO4= concentration appear to be related to fluctuations in large-scale circulation patterns as defined by the Southern Oscillation.

Variation of marine aerosol acidity with particle size

[1] pHs were measured in minimally diluted extracts of size-segregated aerosols sampled under moderately polluted conditions at Bermuda during spring 1997. Extrapolation to aerosol liquid water contents yielded ambient pHs for most super-μm diameter size fractions in the upper 3s to upper 4s. Aerosols within this size range of each sample approached similar pH. These results are consistent with recent estimates of sea-salt aerosol pHs based on thermodynamic considerations and model calculations. Most pHs for finer aerosol size fractions were in the 1s and 2s. The H + + SO 2- 4 ↔ HSO - 4 equilibrium strongly buffered aerosol pH in all size fractions.

Pollution‐enhanced reactive chlorine chemistry in the eastern tropical Atlantic boundary layer

This study examines atmospheric reactive chlorine chemistry at the Cape Verde Atmospheric Observatory in the eastern tropical Atlantic. During May–June, 2007, Cl2 levels ranged from below detection (∼2 ppt) to 30 ppt. Elevated Cl2 was associated with high HNO3 (40 to 120 ppt) in polluted continental outflow transported in the marine boundary layer (MBL) to the site. Lower Cl2 was observed in recently subsided air masses with multiday free tropospheric oceanic trajectories and in air containing Saharan dust. Model simulations show that the observations of elevated Cl2 in polluted marine air are consistent with initiation of Cl chemistry by OH + HCl and subsequent heterogeneous, autocatalytic Cl cycling involving marine aerosols. Model estimates suggest that Cl atom reactions significantly impact the fates of methane and dimethylsulfide at Cape Verde and are moderately important for ozone cycling.

Chlorine isotopic composition of marine aerosols: Implications for the release of reactive chlorine and HCl cycling rates

We have determined the elemental and the stable chlorine isotopic composition of size-segregated aerosols collected in the marine boundary layer. All size fractions have chlorine deficiencies. Mid-size aerosols are isotopically enriched in δ37Cl and contain significant quantities of non-sea-salt-SO42− and NO3−. This is consistent with the acid displacement of HCl from the aerosols. Coarse aerosols tend to be isotopically depleted and have Cl deficiencies that are not charge balanced by MSA−, SO42− and NO3− with Cl volatilization by a different mechanism. The isotope systematics imply a fractionation factor, α, of 1.00306±0.0005 if the volatilization mechanism is first order with respect to Cl or 1.00280±0.0005 if the volatilization mechanism is zeroth order with respect to Cl. Our data are consistent with the efficient recycling of aerosol Cl through the gas phase.

Nitric acid phase partitioning and cycling in the New England coastal atmosphere

[1] During summer 2004, HNO 3 and size-resolved aerosols were measured in parallel, and corresponding dry-deposition fluxes were modeled at Appledore Island, Maine, as part of the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) field program. HNO 3 concentrations varied widely on the timescale of hours; however, all days were characterized by a minimum near sunrise. Mixing ratios normally peaked in the early afternoon; maximum and median concentrations of HNO 3 during the campaign were 337 and 22.8 nmol m -3 , respectively. Aerosol NO - 3 exhibited a bimodal size distribution with a primary peak associated with sea salt at ∼4 μm and a secondary sub-μm peak. The median NO - 3 concentrations in sub-and super-μm diameter size fractions were 3.3 and 7.7 nmol m -3 , respectively. Peak HNO 3 and super-μm NO - 3 concentrations were associated with westerly and southwesterly flow regimes respectively. The multiphase cycling of HNO 3 was evaluated as a function of transport sector. Although median total nitrate (HNO 3 + NO - 3 ) concentrations were higher under westerly flow, higher median dry-deposition rates of total nitrate were associated with southwesterly flow. Sea-salt concentrations were ∼3 times greater during southwesterly flow, which shifted the phase partitioning toward particulate NO - 3 . Consequently, under westerly flow, HNO 3 deposited more quickly than aerosol NO - 3 , while for southwesterly flow, the fluxes from the two phases were comparable. On the basis of all data, the median dry-deposition fluxes for HNO 3 and aerosol NO - 3 were 8.2 and 5.6 μmol m -2 d -1 ; super-μm size fractions dominated the NO - 3 flux.

Nanoparticle growth following photochemical α‐ and β‐pinene oxidation at Appledore Island during International Consortium for Research on Transport and Transformation/Chemistry of Halogens at the Isles of Shoals 2004

[1] Nanoparticle events were observed 48 times in particle size distributions at Appledore Island during the International Consortium for Atmospheric Research on Transport and Transformation/Chemistry of Halogens on the Isles of Shoals (ICARTT/CHAiOS) field campaign from 2 July to 12 August of 2004. Eighteen of the nanoparticle events showed particle growth and occurred during mornings when peaks in mixing ratios of a- and b-pinene and ozone made production of condensable products from photochemical oxidation probable. Many pollutants and other potential precursors for aerosol formation were also at elevated mixing ratios during these events, including NO, HNO3 ,N H3, HCl, propane, and several other volatile organic carbon compounds. There were no consistent changes in particle composition, although both submicron and supermicron particles included high maximum concentrations of methane sulfonate, sulfate, iodide, nitrate, and ammonium during these events. Nanoparticle growth continued over several hours with a nearly linear rate of increase of diameter with time. The observed nanoparticle growth rates varied from 3 to 13 nm h � 1 . Apparent nanoparticle aerosol mass fractions (yields) were estimated to range from less than 0.0005 to almost 1 using a- and b-pinene as the presumed particle source. These apparent high aerosol mass fractions (yields) at low changes in aerosol mass are up to two orders of magnitude greater than predictions from extrapolated laboratory parameterizations and may provide a more accurate assessment of secondary organic aerosol formation for estimating the growth of nanoparticles in global models. Citation: Russell, L. M., A. A. Mensah, E. V. Fischer, B. C. Sive, R. K. Varner, W. C. Keene, J. Stutz, and A. A. P. Pszenny (2007), Nanoparticle growth following photochemical a- and b-pinene oxidation at Appledore Island during International Consortium for Research on Transport and Transformation/Chemistry of Halogens at the Isles of Shoals 2004, J. Geophys. Res., 112, D10S21,

Aerosol size distribution and aerosol water content measurements during Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange

Aerosol size distribution data measured during the June 1992 Marine Aerosol and Gas Exchange experiment are analyzed to investigate the characteristics of fine marine aerosol particles measured over the North Atlantic near the Azores Islands. Measured aerosol size distribution data were corrected using the corrected size calibration data based on the optical properties of particles being measured. The corrected size distribution data were then approximated with either one or two lognormal size distributions, depending on air mass conditions. Under clean air mass conditions <3 μm diameter aerosol size distributions typically exhibited two modes, consisting of an accumulation mode and the small end of the sea-salt particle mode. However, under the influence of continental polluted air masses, the aerosol size distribution was dominated by <1 μm diameter particles in a single mode with an increased aerosol concentration. Aerosol water content of accumulation mode marine aerosols was estimated from differences between several series of ambient and dried aerosol size distributions. The average aerosol water fraction was 0.31, which is in good agreement with an empirical aerosol growth model estimate. The average rate of SO4= production in the accumulation mode aerosol water by H2O2 oxidation was estimated to be <7×10−10 mol L−1 s−1, which is an insignificant contributor to the observed non-sea-salt SO4= in the accumulation mode.