Managers at Glacier Bay National Park must annually determine the allowable number of cruise-ship entries into the park. This decision considers how differences in visitor volume may affect park resources. This study quantified the impacts to air quality and visibility under different ship quotas using simulations with the Weather Research and Forecasting model inline coupled with chemistry. Results of the simulation assuming two entries per day for May 15 to September 15, 2008 (QTA; 248 ship entries representing a 35% increase) were compared to those of the 2008 cruise-ship activity (REF; 184) during that timeframe. A simulation without anthropogenic emissions (CLN) served to assess the overall impacts of cruise-ship emissions on visibility and air quality in Glacier Bay. Compared to REF, the increased entry quotas shifted chemical regimes and aerosol composition, depending upon thermodynamical conditions, and ambient concentrations. On days with notable regime shifts, sulfur-dioxide concentrations deceased while ammonium-sulfate aerosol concentrations increased. The increased quotas also altered the fine-to-coarse aerosol ratios in both directions despite constant ratio of fine-to-coarse aerosol emissions. In Glacier Bay, the days with worst visibility coincided with high relative humidity, although this relationship varied by scenario. On the 20% worst days, mean visibility was slightly better in CLN (mean haze index over Glacier Bay waters = 2.9 dv) than in REF ( = 3.1 dv). While increased emissions in QTA reduced mean visibility by 0.1 dv, the 10th, 50th and 90th percentile of haze indices remained identical to those in REF. Best (worst) visibility occurred on the same days in REF and QTA due to emission impacts, but on different days than in CLN because relative humidity solely governed visibility in CLN. While calm wind played no role for visibility in CLN, wind speed gained similar importance for visibility as relative humidity in REF and QTA. Overall, increasing ship quotas would only marginally affect air quality and visibility as compared to REF, although even small changes in these parameters need careful consideration in the context of conserving the values of Glacier Bay.
The demand for glacier viewing is likely to increase as “last chance” tourism highlights the need to see sensitive places before they disappear [
In 2015, over 95% of all visitors to Glacier Bay arrived aboard cruise ships. Thus, a recurring issue for the park’s superintendent is to decide how many cruise ships should be allowed in the park. Increases in cruise ships equate to more opportunities for enjoying park resources but also mean elevated emissions, which affect both air quality and visibility.
For example, cruise-ships emit sulfur dioxide (SO2), nitrogen oxides (NOx = NO2 + NO, nitric dioxide, nitrogen-dioxide nitric oxide), and non-methane volatile organic compounds (VOC) as well as particulate matter (primary aerosols, e.g. sulfate, nitrate, organic carbon, elemental carbon) [
Cruise-ship emissions are also of concern for visibility, and by extension impacts to visitor experience. Emission-related primary and secondary aerosols can affect the radiation budget in various and counteracting ways [
Haze makes the pollutants from cruise-ship emissions visibile to visitors. In Glacier Bay, pollutants and haze attributed to cruise ships may accumulate under frequently occuring inversions [
Aerosols also affect the irradiance via altered cloud microphysical processes. Enhanced aerosol concentrations due to ship-emissions increase the number of cloud droplets while decreasing their size [
There are also consequences of emissions for aqueous chemistry and deposition. Small droplets are more acidic than large droplets [
Since Glacier Bay is only accessible by water, and off the power grid, establishing and maintaining a monitoring network for air-quality is challenging. In winter, snow cover may make solar panels useless for power production. The typically calm wind conditions [
While few air-quality measurements exist for Glacier Bay, an air-quality modeling study focusing on conditions during the 2008 tourist season when 225 cruise ships entered Glacier Bay revealed that cruise-ship activity did not significantly impact air quality with respect to trace-gas concentrations and/or particulate matter [
Currently, the NPS limits cruise-ship entries in Glacier Bay using both a daily (maximum = 2) and seasonal basis quota. For the seasonal quota, the 153-day season is divided into a 92-day “peak season” (1 June - 31 August) and a 61-day “shoulder” season (May, September). In 2008, the “peak” seasonal quota was set at 153 ships [
The goal of our study is to examine the impacts of increasing the quotas to two entries per day over the tourist season. While the actual tourist season spans 1 May to 30 September, market conditions have resulted in a consistent lack of demand to meet the 122 entries shoulder-season quota. Thus, we defined the tourist season as 15 May to 15 September assuming the “peak season” quota is increased and realized at 184 ships (2 ships per day from 1 June - 31 August) and market conditions during the periods 15 - 31 May, and 1 - 15 September resulted in 2 ships entering the park per day.
Due to the lack of observations in Glacier Bay, no baselines of current or past pollutant concentrations and/or visibility conditions exist for the length of a tourist season or beyond. Therefore, a hierachy of air-quality simulations served to identify potential impacts of increased quotas on pollutant concentrations and visibility. As the quota for cruise ships has not changed since 2008, we used the pollutant concentrations in Glacier Bay in response to the 2008 cruise-ship activity as reference (REF) based on which we assess the impacts on air quality and visibility in response to increased quota (QTA). A simulation without any cruise-ship emissions (CLN) permitted to assess the deviation from an undistrubed atmospheric composition in Southeast Alaska including Glacier Bay.
The Weather Research and Forecasting model [
The chemical mechanism [
In all simulations, biogenic emissions were determined inline based on the meteorological and soil conditions following [
In 2008, 153 cruise ships entered Glacier Bay during the 92-day peak season (1 June - 31 August) and 184 in total between May 15 and September 15. This timeframe is hereafter referred to as the tourist season or season. The reference simulation (REF) utilized this timeframe in 2008 to make inferences on activity-based emissions [
The 2008 activity-based ship emissions were calculated based on each cruise ship’s characteristics (number and type of engines, engine power, maximum speed, fuel-type used, capacity) and the 2008 actual voyage data (ship position, cruise speed, operation mode) [
The emission inventory for increased quotas encompassed the activity-based emissions of 2008 [
For simplicity, we assumed that increases in ship entries reflected the fraction of the current fleet. In the following, we call the cruise-ships of the two major companies, which currently are allowed in Glacier Bay, group 1 and group 2, while we combined the cruise ships from all the other companies into a group referred to as group 3. Again, for simplicity, we applied the 2008 fractional share of entrances of 37%, 50%, and 13% for the additional cruise ships in these groups. Consequently, group 1, group 2, and group 3 had 24, 32, and 8 additional entrances, respectively.
The cruise-ship type with the most entrances in 2008 in each group was chosen as representative for the additional cruise ships of that group. The representative ships of the three groups had total auxilliary engine powers of 17,700, 14,000, 10,100 kW and total main engine powers of 40,000, 39,000, and 40,000 kW with maximum cruise speeds of 12.3, 11.8, and 12.9 m・s−1, respectively. In cruising mode, the auxiliary engine loads amount 50%, 60%, and 30%, respectively. All additional ships used intermediate fuel oil (IFO380) or marine gas oil (MGO) for their main engines, and heavy fuel oil (HFO) or marine diesel oil (MDO) for their auxilliary engines.
The typical paths, which these three cruise ships traveled within Southeast Alaska on their way to, in and from Glacier Bay in 2008 served as routes of the additional cruises. This means additional emissions not only occurred in Glacier Bay, but also in Southeast Alaska along the routes outbound to, and inbound from Glacier Bay. Since ship emissions not only depend on the ship’s characteristics, but also on cruise speed, operation mode, and engine load, activity-based emissions were calculated assuming a typical cruising behavior for the additional ships (
The days with only one or with no cruise-ship entrance in 2008 were assigned to the three groups under consideration of the aforementioned share. To implement the quota of two entrances in Glacier Bay per day on all days, six additional ships were needed as they have to cruise to, in and from Glacier Bay (
The WRF/Chem domain was the same as used by [
Three simulations were performed for 15 May 2008 to 15 September 2008 with identical model setup except for the cruise-ship emissions. One simulation did not consider emissions from cruise-ships. It only considered the natural emissions. This simulation is called CLN as it represents a clean atmosphere. It serves to provide context to understand the overall impacts of cruise-ship emissions on air quality and visibility. Note that the other two simulations considered biogenic emissions as well.
The reference simulation (REF) assumed the activity-based emission of 2008 [
The third simulation (QTA) assumed an increase in cruise-ship volume to two ships per day every day from 15 May - 15 September in Glacier Bay. The goal for this simulation was to compare visibility and air-quality conditions between the existing (REF) conditions and if the park decided to increase the ship-entry quota.
All simulations were initialized every five days with respect to meteorology using downscaled 1˚ × 1˚, 6 h- resolution National Centers for Environmental Prediction global final analyses data [
All three simulations used identical idealized profiles of clean background concentrations for initialization. On all following days, the chemical fields at the end of the previous day served as initial conditions for the following
day. Identical idealized profiles of clean air background concentrations served as lateral boundary conditions in all three simulations. This design ensured that any differences in atmospheric trace-gas and particulate-matter concentrations, visibility as well as in meteorological fields between CLN and REF, CLN and QTA, as well as REF and QTA were only due to cruise-ship emissions.
The reference simulation was evaluated by means of twice-daily radiosonde data at Yakutat and hourly observations of 2 m temperature, 2 m dewpoint temperature, 10 m wind speed, and wind direction from 31 land and 11 buoy sites [
WRF/Chem captured the temporal evolutions of spatial mean 2 m air and 2 m dewpoint temperatures well, and those of 10 m wind speed and direction acceptably [
The analysis of the impacts of increased quotas on air quality and visibility focused generally on Glacier Bay National Park and Preserve and specifically on Glacier Bay proper (the main fjord area that cruise ships target for itineraries).The total increase in ship emissions into the region and Glacier Bay was determined (
We compared the visibility and air-quality relevant concentrations obtained by REF to those obtained by CLN to assess the impacts of current cruise-ship emissions. Comparison of these quantities gained by REF and QTA served to gain insight into the impacts of increased quotas.
Relative humidity contributes to the reduction of extinction by swelling of soluble sulfates and nitrates [
The haze index (HI) was determined from the WRF/Chem data at ship-height as described by [
In 2008, winds were on average relatively calm in the first half of the season (cf.
Season accumulated emissions for Glacier Bay | ||||||
---|---|---|---|---|---|---|
Simulation | SO2 (mol・km−2) | NOx (mol・km−2) | NH3 (mol・km−2) | VOC (mol・km−2) | PM2.5 (10−3 kg・km−2) | PM10 (10−3 kg・km−2) |
REF | 780.7 | 4 349.9 | 270.6 | 100.7 | 2.0 | 0.22 |
QTA | 956.6 | 5 988.4 | 371.8 | 138.9 | 2.8 | 0.31 |
started out cool followed by a warm episode. After a short cold snap, temperatures increased over summer with absolute day-to-day variations of daily means up to about ±5˚C. September daily means averaged over Glacier Bay ranged between 9.7˚C and 11.9˚C. Except for four short events early in the season, bay-wide daily mean relative humidity always exceeded 75% (cf.
Radiation-cloud-aerosol interactions may affect meteorological quantities [
In Glacier Bay, typically, absolute differences |CLN-REF| and |CLN-QTA| in near-surface meteorological quantities exceeded those of |REF-QTA|. Hourly absolute differences varied in time and space in Glacier Bay. The absolute differences in meteorological quantities remained relatively low, because the concentrations of pollutants relevant in the interactions were relatively low as compared to the concentrations in the studies on aerosols over the continental US [
Daily bay-averaged means of 2 m air temperature, 2 m relative humidity, 10 m wind speed, and sea-level pressure differed among CLN, REF, and QTA on the order of about ±0.1˚C, ±1%, less than ±1 cm and ±1 hPa, respectively for most of the season. This result was partly due to increases and decreases in time and space canceling each other out. However, over the Glacier Bay water, temporally maximum differences ranged between −1.1˚C and 0.8˚C, −22% and 5%, −66 cm・s−1 and 87 cm・s−1, and −1.8 hPa and 0.8 hPa among simulations. Since the impacts of increased entry quotas on radiation-cloud-aerosol interactions were beyond the scope of this study, they are not discussed any further.
In 2008, 16 different cruise ships visited Glacier Bay. These ships varied in terms of passenger volume, main and auxillary engine types, fuel types used, load factors, and maximum cruise speeds. Thus, their emissions differed. Furthermore, their emissions differed in terms of cruise speed, manuvering, total time in the bay and berthing durations. Thus, daily emissions were not just a function of the volume of ships, but also which ship(s) entered Glacier Bay and their operations once in the bay.
The cruise ships of the two major companies (group 1, group 2) and the cruise ships of all the other companies (group 3) visiting Glacier Bay encompassed ships of different sizes. For simplicity, the additional ships in each of the three groups (cf. Section 2.2) were assumed to be of the same type as the cruise ship with the most entrances in that group in 2008.
Compared to an additional cruise ship of group 1, the typical emissions in Glacier Bay per visit of an additional cruise ship of group 2 were 51%, 38%, 418%, 35%, 61%, and 37% higher for PM, VOC, SO2, NOx, CO, and NH3, respectively. The additional cruise ship of group 3 emitted 24%, 27%, ~1%, 24%, 36% and 24% less PM, VOC, SO2, NOx, CO, and NH3 than the ship of group 1. The cruise ship of group 3 emitted 16%, 20%, 19%, 18%, 22% and 17% less PM, VOC, SO2, NO, CO, and NH3 than the cruise ship of group 2 (
These differences resulted from different ratios of fuel types used, and different engine sizes, load factors, cruise speeds, maximum cruise speed, cruise pattern, as well as bethering durations (for a detailed discussion see [
During the “peak season”, six or more cruise ships are a common occurance in some areas of Southeast Alaska [
In QTA, daily emissions increased the highest when six additional ships cruised in Southeast Alaska. Season- accumulated cruise-ship emissions increased not only in Glacier Bay (
Only on seven days of the tourist season, QTA and REF had the same number of cruise ships in the model domain, i.e. identical emissions across the domain in QTA and REF (cf. e.g.
In Glacier Bay, daily-accumulated emissions varied across the season (
their engine powers [
Comparing all daily-accumulated QTA emissions occurring on days with zero emissions in REF to each other indicates the variability of confounding factors in emissions. Days without cruise-ship visits show as days with zero emissions in
In 2008, less cruise-ship visits occurred during the May and September “shoulder season” (averaging one per day or less) than during the June to August “peak season” (up to two per day). Consequently, with respect to time, accumulated emissions increased stronger at the beginning and towards the end of the tourist season than during the “peak season” (
In CLN, particulate matter was mainly PM2.5 from sea-salt. In the fjords, and Icy Strait, season-mean PM2.5 (PM10) concentrations at ship height ranged between 0.7 (0.2) and 0.9 (0.3) mg・m−3 (cf.
In REF and QTA, Glacier Bay daily mean PM10 concentrations remained below 6 mg・m−3 except for five occasions (
In QTA, daily mean PM10 and PM2.5 concentrations typically increased on days with additional cruise-ship
visits with largest increases on days when two additional cruise ships entered Glacier Bay, i.e. when REF had no visits on that day (cf.
There were also some notable, spatially specific differences in PM10 concentrations between REF and QTA. For example, at the mouth of Glacier Bay, in the mid-bay and in front of the glacier as well as along the shores of the Gulf of Alaska, season-mean PM concentrations increased up to about 0.1 mg・m−3 throughout the ABL in response to the increased entry quotas (cf.
In CLN, no emission sources for SO2 existed. Thus, SO2 concentrations in Glacier Bay represented pristine air background values (cf.
The removal processes and physio-chemical fates modify the spatial-temporal distribution of SO2. Consequently, in CLN on season-average, near-surface mean SO2 concentrations were highest over the St. Elias and Coast Mountains (0.8 ppb to 1.6 ppb) due to the low temperatures there (
In Glacier Bay, synoptic and surface conditions governed SO2 concentrations in CLN (cf.
These exceptions of decreased SO2 concentrations despite increased SO2 emissions (cf. e.g.
In contrast to SO2, NOx has two natural sources within the model domain even in absence of anthropogenic emissions (CLN). Soil bacteria emit NOx depending on the soil-temperature and soil-moisture conditions as part of their metabolic activities (e.g. [
Season mean NOx concentrations followed the topographic elevation in CLN with lowest values over the glaciers of Glacier Bay and highest over the Gulf of Alaska and the low elevations inland (e.g. Figures 6(g)-(i)). The opposite was true for PAN (not shown). Furthermore, NOx concentrations increased/decreased with the advection of warm/cold air (cf. e.g.
In contrast to CLN, this general chemical behavior was less obvious in REF and QTA. The cruise-ship emissions disturbed the PAN-NOx equilibrium and it took time for the equilibrium to re-establish. In QTA, throughout the season, NOx concentrations were higher on days with additional entries in Glacier Bay than in REF (cf.
In CLN, biogenic emissions were the only source of VOCs. In CLN, VOC concentrations were high over the Tongass National Forest and low over glaciers and the Gulf of Alaska (not shown). In CLN, REF, and QTA, VOC concentrations increased as summer progressed. While VOC minima matched up well among the three simulations, in REF and QTA, the ship emissions governed the maxima with the latter having higher concen- trations on days with additional entries than the former (not shown).
The cruise-ship emissions of NOx and VOCs also affected the ozone (O3) concentrations and season-mean O3 concentrations were lowest in CLN (
Wet and dry deposition and atmospheric removal of aerosols are of concern for land and marine ecosystems. Acidic deposition may decrease the pH-values of waters. Since WRF/Chem does not have an ocean model, it cannot provide information on how the deposition fluxes would change the pH-values of the waters. However, WRF/Chem provided information on the amount of pollutants deposited into waters, onto glacier, and on land. In nature, in the case of deposition of soot on glaciers, increased quotas may affect the radiation budget. However, the simulations did not consider this impact.
Changes in deposition fluxes contributed to the nonlinear responses of concentrations to the emission increases. On average, deposition of PM was largest over forests, and lowest over water in all simulations. It was lowest in CLN as here PM only had natural sources (e.g. sea-spray, gas-to-particle conversion). In REF, average deposition of PM over forests reached locally up to 4 kg・km−2・h−1, and remained less than 0.4 kg・km−2・h−1 into waters. In QTA, the season-mean PM deposition increased up to about 0.5 kg・km−2・h−1 along the coasts and on the shores of Glacier Bay in QTA as compared to REF due to increased cruise-ship traffic.
Likewise, season-mean SO2 deposition increased up to 0.5 g・km−2・h−1 in Glacier Bay in particular in the fjords of glacier viewing. However, due to advection of pollutants, notable increases also occurred in the areas of Glacier Bay where the NPS prohibits ship traffic. Along the southern shores of Icy Strait and the entrance of Glacier Bay both SO2 and NO2 deposition increased up to 10 g・km−2・h−1, and about 5 g・km−2・h−1, respectively, in QTA compared to REF (not shown).
Relative humidity, PM10 and NO2 concentrations affect visibility [
Statistics | Comparison of mean concentrations in Glacier Bay | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
SO2 (ppb). (-.-) | NOx (ppb), (-.-) | PM10 (μg・m−3), (-.-) | |||||||||||
CLN | REF | QTA | CLN | REF | QTA | CLN | REF | QTA | |||||
Mean | 0.07 | 0.13 | 0.13 | 0.05 | 0.27 | 0.35 | 0.2 | 3.1 | 3.2 | ||||
StDev | 0.03 | 0.05 | 0.06 | 0.01 | 0.2 | 0.2 | 0.03 | 3.3 | 3.4 | ||||
Skewness | 0.571 | 1.931 | 1.843 | 0.929 | 2.640 | 2.567 | 0.574 | 2.846 | 2.768 | ||||
Kurtosis | −0.195 | 9.189 | 8.416 | 1.321 | 19.346 | 17.034 | 0.628 | 9.579 | 9.910 | ||||
Minimum | 0.02 | 0.02 | 0.02 | 0.02 | 0.03 | 0.03 | 0.14 | 0.16 | 0.16 | ||||
Maximum | 0.51 | 0.72 | 0.79 | 0.1 | 3.20 | 3.75 | 0.22 | 1.13 | 1.16 | ||||
scattering of NO2, relative humidity, the concentrations of sulfate, nitrate, organic carbon (OC), elemental carbon (EC), fine soil particles of aerodynamic diameters less than 2.5 μm, and coarse particulate matter (CM) of aerodynamic diameters between 2.5 μm and 10 μm. Thus, any shifts in fine-to-coarse ratio of these aerosols affect visibility.
We calculated light extinction from the WRF/Chem results following [
In CLN, REF and QTA, high relative humidity was the main cause of low visibility (
On average over Glacier Bay, CLN haze indices for the 20% best, median (40% - 60%) and 20% worst visibility days were 1.0, 1.9, and 2.9 dv, respectively. The mid points of these visibility ranges are the 10th, 50th and 90th percentile, as which we addressed these ranges hereafter. In REF, the 10th, 50th and 90th percentile of HI were 1.8, 3.0, and 5.2 dv, respectively [
At ship height, on average over Glacier Bay across the tourist season, mean HI were 1.9, 3.2, and 3.3 dv in CLN, REF and QTA, respectively. This visibility is better than the 2018 target set by the US EPA for pristine areas. Whereas, season-mean haze indices varied marginally in space for CLN over Glacier Bay (
It is clear that transport by wind can be an essential removal path for pollutants emitted by cruise ships in Glacier Bay. However, wind can also transport pollutants into Glacier Bay [
In summary, while humidity and natural PM2.5 determined visibility in Glacier Bay absent anthropogenic emissions (CLN), wind speed and the PM10 concentrations were the most influential in determining visibility in Glacier Bay in REF and QTA (
Comparison of the visibility on the seven days with identical emissions in REF and QTA showed bay-wide average increase of 0.2 dv in QTA (
In Glacier Bay at ship height, mean concentrations of PM10 and PM2.5 were 2%, and 3% higher in QTA, while those of NO2 were 5% lower in QTA than in REF on the seven days of domain-wide identical emissions. Concentrations of O3, PAN or VOC differed less than 1% on average over these seven days (NO2 decreased while PAN, VOC, and O3 remained nearly constant). This means that the additional primary aerosols from emissions on previous days together with enhanced secondary aerosol formation degraded the visibility on those seven days with domain-wide identical emissions in REF and QTA.
We compared HI-values of days without cruise-ship visits in Glacier Bay in REF with those of CLN. Pollutants remaining from emissions in Glacier Bay on previous days and/or advection increased the HI by about 46% on these days. On average, concentrations on days without emissions in Glacier Bay in REF were by a factor of 12.2, 2.5, 1.4 2.2, 1.2, and 1.4 higher for PM10, PM2.5, SO2, NO2, PAN, and VOC than in CLN, while O3 concentrations remained about the same. These findings suggest that cruise-ship emissions from previous days, along with formation of secondary PM from precursors (NO, SO2, NH3) caused the on average 1.5 dv higher haze index in REF on cruise-free days as compared to CLN (
Together these findings indicate that while ship emissions contributed to degraded visibility in REF and QTA in Glacier Bay, meteorology primarily governed visibility. Increases in ship-visit quotas as assumed here would have relatively small impacts on visibility and air quality as compared to the situation with the current quota.
The National Park Service has the mandate to manage and preserve natural resources for current and future generations, while also allowing visitation and use. In Glacier Bay, about 97% of the visitors to the park arrive on board of cruise ships because the park does not have roads connecting it to Canada or other parts of Alaska. Since the associated emissions can affect air quality and visitor experience, an important consideration is the extent to which ships degrade these resources in the park. It is also important to understand how potential increases in allowable ship entries, as demand for glacier viewing keeps increasing, may affect air quality.
Since no long-term atmospheric visibility and air-quality monitoring existed in Glacier Bay, we performed a simulation study using WRF/Chem, and used the 2008 cruise-ship activity [
Simulations demonstrated that emissions of cruise-ships had little impact on daily means of bay-averaged meteorological conditions. Differences in bay-wide daily near-surface means ranged around current measurement accuracy (about ±0.1˚C for 2 m air temperature, ±1% for 2 m relative humidity, ±1 cm for 10 m wind speed, ±1 Pa for surface pressure) during most of the season. This finding suggests that effects from cruise-ship visits on weather conditions within the bay would be hard to detect with current instrument techniques.
We also found that ozone concentrations were independent of the cruise-emissions and remained so even when cruise-ship entry quotas increased. Whereas on days when no cruise ships visited Glacier Bay in REF PM10, PM2.5, SO2, NO2, PAN, and VOC concentrations were all higher (by factors ranging from 1.2 - 12.2) compared to CLN, O3 concentrations remained about the same. From these results, we infer that vegetation impacts from changes in O3 concentrations are not a likely result of cruise ship entries into Glacier Bay.
Overall, however, there was a general nonlinear relationship between increased emissions from increased cruise-ship traffic and concentration changes. The altered emissions meant different chemical regimes and shifts in the reaction paths thereby again modifying the concentrations as well as deposition fluxes.
Differences in chemical paths due to cruise-ship emissions explain why on some days in REF higher SO2 concentrations occurred compared to QTA despite more cruise-ship entries in QTA. This non-intuitive finding resulted from the competition of SO2 and NOx for NH3 in aerosol formation, and NOx being involved in many other chemical reaction paths. Based on these findings, we infer that the chemical regimes of wet, dry and occult deposition in Glacier Bay depend on the combination, and amount of emissions as well as on ambient temperature and relative humidity conditions.
We also conclude that due to the nonlinear relationship between emissions, concentrations and impacts, and because gas-to-particle growth in the accumulation mode shifts PM2.5 to PM10 and shrinking of particles shifts PM10 to PM2.5 that emissions can both degrade and increase visibility, depending upon ambient meteorological and chemical conditions. While we found that in most cases, degraded air-quality results of emissions, due to the otherwise pristine atmosphere, it is important to consider carefully the complexity of atmospheric chemistry and anthropogenic emissions in Glacier Bay.
CLN simulations demonstrated that relative humidity was the primary driver causing low visibility in Glacier Bay (R = 0.68). In contrast to CLN, haze indices in REF (and QTA) were highly correlated (R > 0.7) with the particulate matter concentrations. This means that particularly in this humid area of frequent inversions [
Although our findings indicate the meteorology largely dictates visibility, we also conclude that cruise-ship emissions contributed to degrade visibility and may affect which days have best/worst visibility. In contrast to CLN, in REF and QTA, haze indices correlated highly with the particulate matter concentrations (R > 0.7) and wind speed (R = 0.83). The correlation between HI and relative humidity in REF or QTA decreased as compared to CLN. Together these findings mean that in this region of frequent inversions [
The study showed that in a fjord like Glacier Bay, primary and secondary pollutants form cruise-ship emissions still affect the chemical composition and regime of the ABL a day later. The additional primary aerosols from previous days together with enhanced secondary aerosol formation led to about 0.2 dv degraded visibility in QTA on days with identical emissions in REF and QTA.
Days without cruise-ship visits in Glacier Bay in REF had about 46% higher haze indices than the same days in CLN. These findings also suggest that the cruise-ship emissions in Glacier Bay dominated the degradation in visibility in the bay, but the emissions from previous days and in the upwind (generally from the Pacific Ocean) contributed to the degradation.
Combining results from this and related studies, we infer that increases in cruise-ship quota will only marginally affect air quality and visibility in Glacier Bay as compared to the current quota. Most poor visibility days namely were due to ambient conditions (relative humidity, calm wind) although with increases in ship entries there will be higher likelihood that visitor experience may be affected when haze forms from ship emissions during inversions and on humid days. While calm wind hardly affected visibility in CLN, wind speed gained similar importance as relative humidity when cruise-ship emissions (REF, QTA) occurred in Glacier Bay.
Finally, we highlight that total emissions varied dramatically among cruise-ships due to differences in size, passenger volume, engine type and power, etc. Thus, a future consideration for cruise-ship entries may be to consider the ship-specific attributes for mitigating emissions per visitor. Such an initiative would be consistent with similar on-going efforts in the West Norwegian Fjords World Heritage Site (termed the Green Fjord 2020 initiative) that seeks elevated sustainability of individual operations of cruise ships to minimize impacts to air and water quality while still maintaining levels of visitation.
We thank Gerhard Kramm, and the anonymous reviewers for fruitful discussions, the Geophysical Institute’s Research Computing Support for access to HPC and providing CPU time and storage, and the National Park Service (Cooperative Ecosystem Studies Unit; contract P11AT30883/P11AC90465) for financial support.
NicoleMölders,ScottGende, (2015) Impacts of Cruise-Ship Entry Quotas on Visibility and Air Quality in Glacier Bay. Journal of Environmental Protection,06,1236-1256. doi: 10.4236/jep.2015.611109