Simultaneous measurements of particulate and gas-phase water-soluble organic carbon concentrations at remote and urban-influenced locations

The sources, sinks, and overall importance of watersoluble organic carbon (WSOC) in the atmosphere are not well understood. Although the primary historical focus has been on particulate WSOC (WSOCP), here we also present results obtained using a newly developed technique that additionally measures gas-phase water-soluble organic carbon (WSOCG). These first-of-their-kind measurements show that WSOCG can often be more than ten times larger than WSOCP at both urban and remote locations. The average fraction of WSOC residing in the gas phase (fg = WSOCG/(WSOCG + WSOCP)) at five various field sites ranged from 0.64 to 0.93, implying significant differences in WSOC phase partitioning between locations. At Houston, TX, and Summit, Greenland, a repeatable diurnal pattern was observed, with minimum values for fg occurring at night. These trends likely are due, at least in part, to temperature and/or relative humidity related gas-to-particle partitioning. These coincident measurements of WSOC in both the gas and particle phases indicate that a relatively large reservoir of water-soluble organic mass is not taken into account by measurements focused only on WSOCP. In addition, a significant amount of WSOCG is available to form WSOCP or enter cloud droplets depending on the chemical and physical properties of the droplets and/or aerosols present. Citation: Anderson, C., J. E. Dibb, R. J. Griffin, and M. H. Bergin (2008), Simultaneous measurements of particulate and gas-phase water-soluble organic carbon concentrations at remote and urban-influenced locations, Geophys. Res. Lett., 35, L13706, doi:10.1029/2008GL033966

Sulfate and MSA in the air and snow on the Greenland Ice Sheet

Sulfate and methanesulfonic acid (MSA) concentrations in aerosol, surface snow, and snowpit samples have been measured at two sites on the Greenland Ice Sheet. Seasonal variations of the concentrations observed for these chemical species in the atmosphere are reproduced in the surface snow and preserved in the snowpit sequence. The amplitude of the variations over a year are smaller in the snow than in the air, but the ratios of the concentrations are comparable. The seasonal variations for sulfate are different at the altitude of the Ice Sheet compared to those observed at sea level, with low concentrations in winter and short episodes of elevated concentrations in spring. In contrast, the variations in concentrations of MSA are similar to those measured at sea level, with a first sequence of elevated concentrations in spring and another one during summer, and a winter low resulting from low biogenic production. The ratio MSA/sulfate clearly indicates the influence of high-latitude sources for the summer maximum of MSA, but the large impact of anthropogenic sulfate precludes any conclusion for the spring maximum. The seasonal pattern observed for these species in a snowpit sampled according to stratigraphy indicates a deficit in the accumulation of winter snow at the summit of the Greenland Ice Sheet, in agreement with some direct observations. A deeper snowpit covering the years 1985–1992 indicates the consistency of the seasonal pattern for MSA over the years, which may be linked to transport and deposition processes

Enhanced secondary organic aerosol formation due to water uptake by fine particles

This study characterizes the partitioning behavior of a significant fraction of the ambient organic aerosol through simultaneous measurements of gas and particle watersoluble organic carbon (WSOC). During the summer in Atlanta, WSOC gas/particle partitioning showed a strong RH dependence that was attributed to particulate liquid water. At elevated RH levels (\u3e70%) a significant increase in WSOC partitioning to the particle phase was observed and followed the predicted water uptake by fine particles. The enhancement in particle-phase partitioning translated to increased median particle WSOC concentrations ranging from 0.3 –0.9 mgCm3 . The results provide a detailed overview of the WSOC partitioning behavior in the summertime in an urban region dominated by biogenic emissions, and indicate that secondary organic aerosol formation involving partitioning to liquid water may be a significant aerosol formation route that is generally not considered. Citation: Hennigan, C. J., M. H. Bergin, J. E. Dibb, and R. J. Weber (2008), Enhanced secondary organic aerosol formation due to water uptake by fine particles, Geophys. Res. Lett., 35, L18801, doi:10.1029/2008GL035046

Temporal and spatial variability of snow accumulation in central Greenland

Snow accumulation records from central Greenland are explored to improve the understanding of the accumulation signal in Greenland ice core records. Results from a “forest” of 100 bamboo poles and automated accumulation monitors in the vicinity of Summit as well as shallow cores collected in the Summit and Crete areas are presented. Based on these accumulation data, a regression has been calculated to quantify the signal-to-noise variance ratio of ice core accumulation signals on a variety of temporal (1 week to 2 years) and spatial (20 m to 200 km) scales. Results are consistent with data obtained from year-round automated accumulation measurements deployed at Summit which suggest that it is impossible to obtain regional snow accumulation data with seasonal resolution using four accumulation monitors positioned over a length scale of ∼30 km. Given this understanding of the temporal and spatial dependence of noise in the ice core accumulation signal, the accumulation records from 17 shallow cores are revisited. Each core spans the time period from 1964 to 1983. By combining the accumulation records, the regional snow accumulation record has been obtained for this period. The results show that 9 of the 20 years can be identified as having an accumulation different from the 20 year mean with 99% confidence. The signal-to-noise variance ratio for the average accumulation signal sampled at annual intervals is 5.8±0.5. The averaged accumulation time series may be useful to climate modelers attempting to validate their models with accurate regional hydrologic data sets

Exploring positive adjustment in people with spinal cord injury.

This study explored adjustment in people with spinal cord injury; data from four focus groups are presented. Thematic analysis revealed four themes, managing goals and expectations, comparison with others, feeling useful and acceptance, showing participants positively engaged in life, positively interpreted social comparison information and set realistic goals and expectations. These positive strategies show support for adjustment theories, such as the Cognitive Adaptation Theory, the Control Process Theory and Response Shift Theory. These results also provide insight into the adjustment process of a person with spinal cord injury and may be useful in tailoring support during rehabilitation

Air-snow exchange of HNO3 and NOy at Summit, Greenland

Ice core records of NO3− deposition to polar glaciers could provide unrivaled information on past photochemical status and N cycling dynamics of the troposphere, if the ice core records could be inverted to yield concentrations of reactive N oxides in the atmosphere at past times. Limited previous investigations at Summit, Greenland, have suggested that this inversion may be difficult, since the levels of HNO3 and aerosol-associated NO3− over the snow are very low in comparison with those of NO3− in the snow. In addition, it appears that some fraction of the NO3− in snow may be reemitted to the atmosphere after deposition. Here we report on extensive measurements of HNO3, including vertical gradients between 1.5 and 7 m above the snow, made during the summers of 1994 and 1995 at Summit. These HNO3 data are compared with NO3− concentrations in surface snow and the first measurements of the concentrations and fluxes of total reactive nitrogen oxides (Ny) on a polar glacier. Our results confirm that HNO3 concentrations are quite low (mean 0.5 nmol m−3) during the summer, while NO3− is the dominant ion in snow. Daytime peaks in HNO3− appear to be due at least partly to emissions from the snow, an assertion supported by gradients indicating a surface source for HNO3− on many days. Observed short-term increases in NO3− inventory in the snow can be too large to be readily attributed to deposition of HNO3− suggesting that deposition of one or more other N oxides must be considered. We found that the apparent fluxes of HNO3 and NOy were in opposite directions during about half the intervals when both were measured, with more cases of HNO3 leaving the snow, against an NOy flux into the snow, than the reverse. The concentrations of NOy are generally about 2 orders of magnitude greater than those of HNO3; hence deposition of only a small, non-HNO3, fraction of this pool could dominate NO3− in snow, if the depositing species converted to NO3−, either in the snowpack or upon melting for analysis

The Carrington event not observed in most ice core nitrate records

The Carrington Event of 1859 is considered to be among the largest space weather events of the last 150 years. We show that only one out of 14 well-resolved ice core records from Greenland and Antarctica has a nitrate spike dated to 1859. No sharp spikes are observed in the Antarctic cores studied here. In Greenland numerous spikes are observed in the 40 years surrounding 1859, but where other chemistry was measured, all large spikes have the unequivocal signal, including co-located spikes in ammonium, formate, black carbon and vanillic acid, of biomass burning plumes. It seems certain that most spikes in an earlier core, including that claimed for 1859, are also due to biomass burning plumes, and not to solar energetic particle (SEP) events. We conclude that an event as large as the Carrington Event did not leave an observable, widespread imprint in nitrate in polar ice. Nitrate spikes cannot be used to derive the statistics of SEPs

Concentrations and snow-atmosphere fluxes of reactive nitrogen at Summit, Greenland

Concentrations and fluxes of NOy (total reactive nitrogen), ozone concentrations and fluxes of sensible heat, water vapor, and momentum were measured from May 1 to July 20, 1995 at Summit, Greenland. Median NOy concentrations declined from 947 ppt in May to 444 ppt by July. NOy fluxes were observed into and out of the snow, but the magnitudes were usually below 1 μmol m−2 h−1 because of the low HNO3 concentration and weak turbulence over the snow surface. Some of the highest observed fluxes may be due to temporary storage by equilibrium sorption of peroxyacetylnitrate (PAN) or other organic nitrogen species on ice surfaces in the upper snowpack. Sublimation of snow at the surface or during blowing snow events is associated with efflux of NOy from the snowpack. Because the NOy fluxes during summer at Summit are bidirectional and small in magnitude, the net result of turbulent NOyexchange is insignificant compared to the 2 μmol m−2 d−1 mean input from fresh snow during the summer months. If the arctic NOy reservoir is predominantly PAN (or compounds with similar properties), thermal dissociation of this NOy is sufficient to support the observed flux of nitrate in fresh snow. Very low HNO3 concentrations in the surface layer (1% of total NOy) reflect the poor ventilation of the surface layer over the snowpack combined with the relatively rapid uptake of HNO3 by fog, falling snow, and direct deposition to the snowpack

Comment on “Low time resolution analysis of ice cores cannot detect impulsive nitrate events” by D. F. Smart et al.

Smart et al. (2014) suggested that the detection of nitrate spikes in polar ice cores from solar energetic particle (SEP) events could be achieved if an analytical system with sufficiently high resolution was used. Here we show that the spikes they associate with SEP events are not reliably recorded in cores from the same location, even when the resolution is clearly adequate. We explain the processes that limit the effective resolution of ice cores. Liquid conductivity data suggest that the observed spikes are associated with sodium or another nonacidic cation, making it likely that they result from deposition of sea salt or similar aerosol that has scavenged nitrate, rather than from a primary input of nitrate in the troposphere. We consider that there is no evidence at present to support the identification of any spikes in nitrate as representing SEP events. Although such events undoubtedly create nitrate in the atmosphere, we see no plausible route to using nitrate spikes to document the statistics of such events

Particulate and water-soluble carbon measured in recent snow at Summit, Greenland

Water-soluble organic carbon (WSOC), waterinsoluble particulate organic carbon (WIOC), and particulate elemental carbon (EC) were measured simultaneously for the first time on the Greenland Ice Sheet in surface snow and in a 3-meter snow pit. Snow pit concentrations reveal that, on average, WSOC makes up the majority (89%) of carbonaceous species, followed by WIOC (10%) and EC (1%). The enhancement of OC relative to EC (ratio 99:1) in Greenland snow suggests that, along with atmospheric particulate matter, gaseous organics contribute to snow-phase OC. Comparison of summer surface snow concentrations in 2006 with past summer snow pit layers (2002 – 2005) found a significant depletion in WSOC (20 – 82%) and WIOC (46 – 65%) relative to EC for 3 of the 4 years. The apparent substantial loss of WSOC and WIOC in aged snow suggests that post-depositional processes, such as photochemical reactions, need to be considered in linking ice core records of organics to atmospheric concentrations. Citation: Hagler, G. S. W., M. H. Bergin, E. A. Smith, J. E. Dibb, C. Anderson, and E. J. Steig (2007), Particulate and water-soluble carbon measured in recent snow at Summit, Greenland, Geophys. Res. Lett., 34, L16505, doi:10.1029/2007GL030110