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Air pollutant concentrations in urban areas exhibit strong spatial and temporal trends because of variations in land use and source distributions. While traffic is an important urban source of pollutants such as NO2 and particulate matter (PM), other sources can also contribute to observed spatiotemporal patterns. PM exposures are further complicated because urban-scale variations in fine particulate matter mass (PM2.5) concentrations can be decoupled from variations in PM physicochemical properties such as composition and the concentration of ultrafine particles (UFPs). Our goal was to quantify spatial gradients in both pollutant concentrations and PM characteristics over spatial scales ranging from hundreds of meters to several kilometers, along with sources influencing the observed spatial patterns. Continuous air pollutant concentrations were measured with a distributed network that consisted of a low-cost sensor package to measure pollutant gases (CO, NO2, O3, CO2), optical measurements of particulate black carbon (BC) and PM2.5, and UFP using condensation particle counters. The sampling sites were classified according to nearby land use for traffic, population, and restaurant density. Mean concentrations of BC and UFP vary by factors of approximately 4 and 3, respectively, across five typical urban sites in a 16 km2 sampling domain. Differences in traffic volume and composition (e.g., diesel bus activity) can describe some, but not all of the observed spatial and temporal differences between sites. Specifically, two sites separated by ~ 500 m in a downtown central business district have significant differences in BC (factor of 2) and UFP (~ 30%), though similar PM2.5 (< 10%), due to varying influences of diesel bus traffic and restaurant cooking emissions. The large spatial gradients have implications for data collection to inform spatial models such as land use regression (LUR). From the standpoint of site selection, the two downtown sites are nominally identical (e.g., both are high traffic, high-source activity sites) but have significant differences in measured pollutant concentrations and source impacts. [ABSTRACT FROM AUTHOR]

We present a 2700-year annually resolved chronology and snow accumulation history for the Roosevelt Island Climate Evolution (RICE) ice core, Ross Ice Shelf, West Antarctica. The core adds information on past accumulation changes in an otherwise poorly constrained sector of Antarctica. The timescale was constructed by identifying annual cycles in high-resolution impurity records, and it constitutes the top part of the Roosevelt Island Ice Core Chronology 2017 (RICE17). Validation by volcanic and methane matching to the WD2014 chronology from the WAIS Divide ice core shows that the two timescales are in excellent agreement. In a companion paper, gas matching to WAIS Divide is used to extend the timescale for the deeper part of the core in which annual layers cannot be identified. Based on the annually resolved timescale, we produced a record of past snow accumulation at Roosevelt Island. The accumulation history shows that Roosevelt Island experienced slightly increasing accumulation rates between 700 BCE and 1300 CE, with an average accumulation of 0.25±0.02 m water equivalent (w.e.) per year. Since 1300 CE, trends in the accumulation rate have been consistently negative, with an acceleration in the rate of decline after the mid-17th century. The current accumulation rate at Roosevelt Island is 0.210±0.002 m w.e. yr -1 (average since 1965 CE, ±2σ), and it is rapidly declining with a trend corresponding to 0.8 mm yr -2. The decline observed since the mid-1960s is 8 times faster than the long-term decreasing trend taking place over the previous centuries, with decadal mean accumulation rates consistently being below average. Previous research has shown a strong link between Roosevelt Island accumulation rates and the location and intensity of the Amundsen Sea Low, which has a significant impact on regional sea-ice extent. The decrease in accumulation rates at Roosevelt Island may therefore be explained in terms of a recent strengthening of the ASL and the expansion of sea ice in the eastern Ross Sea. The start of the rapid decrease in RICE accumulation rates observed in 1965 CE may thus mark the onset of significant increases in regional sea-ice extent. [ABSTRACT FROM AUTHOR]

We present a 2700-year annually resolved timescale for the Roosevelt Island Climate Evolution (RICE) ice core, and reconstruct a past snow accumulation history for the coastal sector of the Ross Ice Shelf in West Antarctica. The timescale was constructed by identifying annual layers in multiple ice-core impurity records, employing both manual and automated counting approaches, and constitutes the top part of the Roosevelt Island Ice Core Chronology 2017 (RICE17). The maritime setting of Roosevelt Island results in high sulfate influx from sea salts and marine biogenic emissions, which prohibits a routine detection of volcanic eruptions in the ice-core records. This led to the use of non-traditional chronological techniques for validating the timescale: RICE was synchronized to the WAIS Divide ice core, on the WD2014 timescale, using volcanic attribution based on direct measurements of ice-core acidity, as well as records of globally-synchronous, centennial-scale variability in atmospheric methane concentrations. The RICE accumulation history suggests stable values of 0.25m water equivalent (w.e.) per year until around 1260CE. Uncertainties in the correction for ice flow thinning of annual layers with depth do not allow a firm conclusion about long-term trends in accumulation rates during this early period but from 1260CE to the present, accumulation rate trends have been consistently negative. The decrease in accumulation rates has been increasingly rapid over the last centuries, with the decrease since 1950 CE being more than 7 times greater than the average over the last 300 years. The current accumulation rate of 0.22±0.06mw.e.yr-1 (average since 1950CE, ±1σ) is 1.49 standard deviations (86th percentile) below the mean of 50-year average accumulation rates observed over the last 2700 years. [ABSTRACT FROM AUTHOR]

Using helium buffer gas cooling, we have prepared dense samples of ground-state molecular calcium monohydride (CaH X²) at cryogenic temperatures. We have used optical pumping to polarise the spin state of the CaH molecules and we have measured the inelastic collisions of molecular CaH with atomic helium at temperatures from 2 to 7 K. The measured CaH electronic spin depolarisation rate coefficient increases rapidly with increasing temperature, increasing from 2 x 10-13cm³s-1 toover lO-11 cm³ s-1. The strong dependence of rate coefficient on temperature is attributed to the CaH population in the first excited rotational state. [ABSTRACT FROM AUTHOR]