15 research outputs found
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Ocean-induced Melting of Greenland Ice Shelves
No full textThe Greenland glaciers have been experiencing ongoing acceleration and significant calving events during the last two decades. Ocean-induced melt is a potential trigger for destabilizing the glaciers and ice shelves, and consequently contributing to global sea level rise. However, its mechanism is still uncertain.In this dissertation, we employ observational and numerical methods to improve our under- standings of ocean-induced melt under major Greenland glaciers. Using improved remote sensing data, we calculate melt rates with an improved accuracy. We then employ the Mas- sachusetts Institute of Technology general circulation model (MITgcm) to study ice-ocean interactions beneath an ice shelf in a 2-D configuration at a high resolution. We include ther- mal forcing from the ocean, cavity shape, and for the first time subglacial water discharge at the grounding line. We optimize the heat and salt transfer coefficients to match observed results. The model replicates the general pattern of melting: high near the grounding zone, decreasing rapidly downstream. Melt increases below linear with subglacial discharge and above linear with thermal forcing from the ocean. Next, we investigate the role of the slope of the ice shelf draft in controlling ice shelf melt. The simulations indicate that the melt rate is sensitive to the slope, hence is larger for steeper ice shelves; and the location of the region of high melt migrates toward the grounding line as the slope becomes steeper. In the limit case of a vertical wall, no ice shelf, we know that the locus of ice melt undercuts the glacier.This study provides major new insights on the sensitivity of ice shelf melt to (1) subglacial water discharge: a direct product of ice sheet surface melt (2) thermal forcing from the ocean: a direct product of changes in ocean circulation as a result of wind forcing, and (3) a time-evolving cavity which affects the melt regimes: shallow, nearly flat cavities do not favor high melt; deep, steep cavities favor high melt. These results are important to interpret recent changes on the ice shelves and to inform ice sheet numerical models how to parameterize ice shelf melt in a changing climate
Impact of Subglacial Freshwater Discharge on Pine Island Ice Shelf
No full textSatellite-based estimates of ice shelf melt rates reach similar to 200 m yr-1 close to the grounding line of Pine Island Glacier, in West Antarctica. However, ocean simulations have not yet been able to reproduce such high melt rates, even with high-resolution models. Here, we use a regional model of Pine Island ice shelf cavity and study the impact of subglacial freshwater discharge on simulated ice shelf melt rates and ocean circulation in the cavity. We show that subglacial freshwater discharge substantially enhances ice shelf melting close to the grounding line, successfully simulating high ice shelf melt rates suggested by observations. The buoyant mixture of glacial meltwater plume rises to similar to 27.4 isopycnal surfaces, following topographically constrained current, and spreads into mid-depths at the ice shelf front. The role of freshwater discharge is likely to remain unchanged over the coming decades given the projected evolution of runoff and rainfall over Pine Island basin
Undercutting of marine‐terminating glaciers in West Greenland
No full textMarine-terminating glaciers control most of Greenland's ice discharge into the ocean, but little is known about the geometry of their frontal regions. Here we use side-looking, multibeam echo sounding observations to reveal that their frontal ice cliffs are grounded deeper below sea level than previously measured and their ice faces are neither vertical nor smooth but often undercut by the ocean and rough. Deep glacier grounding enables contact with subsurface, warm, salty Atlantic waters (AW) which melts ice at rates of meters per day. We detect cavities undercutting the base of the calving faces at the sites of subglacial water (SGW) discharge predicted by a hydrological model. The observed pattern of undercutting is consistent with numerical simulations of ice melt in which buoyant plumes of SGW transport warm AW to the ice faces. Glacier undercutting likely enhances iceberg calving, impacting ice front stability and, in turn, the glacier mass balance
Observations and modeling of ocean-induced melt beneath Petermann Glacier Ice Shelf in northwestern Greenland
No full textFormation, ageing and thermal properties of secondary organic aerosol
No full textIn order to properly represent and predict the effects of aerosol in climate systems, an accurate description of their formation and properties is needed. This thesis describes work done to increase the knowledge of processes and properties of atmospherically relevant secondary organic aerosol (SOA) from both biogenic and anthropogenic origin. The common theme for these projects is the use of a Volatility Tandem Differential Mobility Analyser (VTDMA) setup, which in combination with other observations has generated insight into both detailed chemical mechanisms and physical processes that eventually could be suitable for testing in air quality or climate models. During the course of this work, the experimental facility
the Gothenburg Flow Reactor for Oxidation Studies at low Temperatures (G-FROST) and
the VTDMA setup, as well as a corresponding data evaluation methodology, have been
improved and refined.
Thermal properties could be linked to both formation and ageing processes of SOA. Using
a VTDMA setup, where the thermal characteristics of SOA were measured at a range of
evaporation temperatures, a sigmoidal fit to the data enabled parameterisation of their
volatility properties. The parameters extracted were e.g. the temperature corresponding to
a volume fraction remaining of 0.5 (TVFR0.5) and the slope factor (SVFR), which are measures
of the general volatility and the volatility distribution of the condensed phase products,
respectively. A higher TVFR0.5 indicates lower volatility, while an increase of SVFR states a
broader distribution of vapour pressures. The response of these parameters from changes in
experimental conditions could be linked to processes occurring both in the gaseous and the
condensed phase. In photo-chemical experiments, the change in TVFR0.5 and SVFR could be
described using the OH dose.
The gas phase processes were found to be very important for SOA ageing, driven mainly by
OH radical exposure in the outdoor chamber SAPHIR. However, processes in the condensed
phase, such as plausible non oxidative ageing processes and non-liquid behaviour of SOA
particles, were also observed.
Detailed studies of ozonolysis of the boreal forest monoterpenes β-pinene and limonene were
enabled by precise control of reaction conditions using the G-FROST. The experimental
findings in response to e.g. water and radical conditions emphasized the difference in
ozonolysis reaction paths between endo- and exocyclic compounds. The results support
the recently suggested decomposition of the stabilized Criegee Intermediate via the
hydroperoxide channel in ozonolysis of β-pinene
Tracking PM2.5 and O3 Pollution and the Related Health Burden in China 20132020
No full textBased on the exposure
data sets from the Tracking Air Pollution
in China (TAP, http://tapdata.org.cn/), we
characterized the spatiotemporal variations in PM2.5 and
O3 exposures and quantified the long- and short-term exposure
related premature deaths during 2013–2020 with respect to the
two-stage clean air actions (2013–2017 and 2018–2020).
We find a 48% decrease in national PM2.5 exposure during
2013–2020, although the decrease rate has slowed after 2017.
At the same time, O3 pollution worsened, with the average
April–September O3 exposure increased by 17%. The
improved air quality led to 308 thousand and 16 thousand avoided long-
and short-term exposure related deaths, respectively, in 2020 compared
to the 2013 level, which was majorly attributed to the reduction in
ambient PM2.5 concentration. It is also noticed that with
smaller PM2.5 reduction, the avoided long-term exposure
associated deaths in 2017–2020 (13%) was greater than that
in 2013–2017 (9%), because the exposure–response curve
is nonlinear. As a result of the efforts in reducing PM2.5-polluted days with the daily average PM2.5 higher than
75 μg/m3 and the considerable increase in O3-polluted days with the daily maximum 8 h average O3 higher
than 160 μg/m3, deaths attributable to the short-term
O3 exposure were greater than those due to PM2.5 exposure since 2018. Future air quality improvement strategies for
the coordinated control of PM2.5 and O3 are
urgently needed
Influence of Ozone and Radical Chemistry on Limonene Organic Aerosol Production and Thermal Characteristics
No full textLimonene has a strong tendency to form secondary organic
aerosol
(SOA) in the atmosphere and in indoor environments. Initial oxidation
occurs mainly via ozone or OH radical chemistry. We studied the effect
of O3 concentrations with or without a OH radical scavenger (2-butanol) on the SOA mass and thermal
characteristics
using the Gothenburg Flow Reactor for Oxidation Studies at Low Temperatures
and a volatility tandem differential mobility analyzer. The SOA mass
using 15 ppb limonene was strongly dependent on O3 concentrations
and the presence of a scavenger. The SOA volatility in the presence
of a scavenger decreased with increasing levels of O3,
whereas without a scavenger, there was no significant change. A chemical
kinetic model was developed to simulate the observations using vapor
pressure estimates for compounds that potentially contributed to SOA.
The model showed that the product distribution was affected by changes
in both OH and ozone concentrations, which partly explained the observed
changes in volatility, but was strongly dependent on accurate vapor
pressure estimation methods. The model–experiment comparison
indicated a need to consider organic peroxides as important SOA constituents.
The experimental findings could be explained by secondary condensed-phase
ozone chemistry, which competes with OH radicals for the oxidation
of primary unsaturated products
