Interest in stratospheric aerosol and its role in climate have increased over the last decade due to the observed increase in stratospheric aerosol since 2000 and the potential for changes in the ...sulfur cycle induced by climate change. This review provides an overview about the advances in stratospheric aerosol research since the last comprehensive assessment of stratospheric aerosol was published in 2006. A crucial development since 2006 is the substantial improvement in the agreement between in situ and space-based inferences of stratospheric aerosol properties during volcanically quiescent periods. Furthermore, new measurement systems and techniques, both in situ and space based, have been developed for measuring physical aerosol properties with greater accuracy and for characterizing aerosol composition. However, these changes induce challenges to constructing a long-term stratospheric aerosol climatology. Currently, changes in stratospheric aerosol levels less than 20% cannot be confidently quantified. The volcanic signals tend to mask any nonvolcanically driven change, making them difficult to understand. While the role of carbonyl sulfide as a substantial and relatively constant source of stratospheric sulfur has been confirmed by new observations and model simulations, large uncertainties remain with respect to the contribution from anthropogenic sulfur dioxide emissions. New evidence has been provided that stratospheric aerosol can also contain small amounts of nonsulfatematter such as black carbon and organics. Chemistry-climate models have substantially increased in quantity and sophistication. In many models the implementation of stratospheric aerosol processes is coupled to radiation and/or stratospheric chemistry modules to account for relevant feedback processes.
Investigation of the wavelength dependence (725-1025 nm) of the threshold for nanosecond optical breakdown in water revealed steps consistent with breakdown initiation by multiphoton ionization, with ...an initiation energy of about 6.6 eV. This value is considerably smaller than the autoionization threshold of about 9.5 eV, which can be regarded as band gap relevant for avalanche ionization. Breakdown initiation is likely to occur via excitation of a valence band electron into a solvated state, followed by rapid excitation into the conduction band. Theoretical analysis based on these assumptions suggests that the seed electron density required for initiating avalanche ionization amounts to 2.5 x 10 super(15) cm super(-3) at 725 nm and drops to 1.1 x 10 super(12) cm super(-3) at 1025 nm. These results demand changes of future breakdown modeling for water, including the use of a larger band gap than previously employed, the introduction of an intermediate energy level for initiation, and consideration of the wavelength dependence of seed electron density.
From the earliest observations of ozone in the lower atmosphere in the 19th century, both measurement methods and the portion of the globe observed have evolved and changed. These methods have ...different uncertainties and biases, and the data records differ with respect to coverage (space and time), information content, and representativeness. In this study, various ozone measurement methods and ozone datasets are reviewed and selected for inclusion in the historical record of background ozone levels, based on relationship of the measurement technique to the modern UV absorption standard, absence of interfering pollutants, representativeness of the well-mixed boundary layer and expert judgement of their credibility. There are significant uncertainties with the 19th and early 20th-century measurements related to interference of other gases. Spectroscopic methods applied before 1960 have likely underestimated ozone by as much as 11% at the surface and by about 24% in the free troposphere, due to the use of differing ozone absorption coefficients. There is no unambiguous evidence in the measurement record back to 1896 that typical mid-latitude background surface ozone values were below about 20 nmol mol–1, but there is robust evidence for increases in the temperate and polar regions of the northern hemisphere of 30–70%, with large uncertainty, between the period of historic observations, 1896–1975, and the modern period (1990–2014). Independent historical observations from balloons and aircraft indicate similar changes in the free troposphere. Changes in the southern hemisphere are much less. Regional representativeness of the available observations remains a potential source of large errors, which are difficult to quantify. The great majority of validation and intercomparison studies of free tropospheric ozone measurement methods use ECC ozonesondes as reference. Compared to UV-absorption measurements they show a modest (~1–5% ±5%) high bias in the troposphere, but no evidence of a change with time. Umkehr, lidar, and FTIR methods all show modest low biases relative to ECCs, and so, using ECC sondes as a transfer standard, all appear to agree to within one standard deviation with the modern UV-absorption standard. Other sonde types show an increase of 5–20% in sensitivity to tropospheric ozone from 1970–1995. Biases and standard deviations of satellite retrieval comparisons are often 2–3 times larger than those of other free tropospheric measurements. The lack of information on temporal changes of bias for satellite measurements of tropospheric ozone is an area of concern for long-term trend studies.
We describe an episode of a stratospheric intrusion into the free troposphere over Europe followed by a long‐range transport of ozone from the North American boundary layer. Observational data showed ...the presence of a thin tongue of stratospheric air in the free troposphere for at least 36 hours. This filament was found in data from two ozone soundings and was recorded continuously for 26 hours by a high‐resolution ozone lidar. The stratospheric air also intercepted two high Alpine summits, causing elevated ozone and beryllium 7 concentrations. Trajectory, particle dispersion model, and potential vorticity analyses confirmed the stratospheric nature of the tongue. In the lidar data, following the intrusion, pockets of elevated ozone concentrations (80–100 ppb) were found in the free troposphere close to the tropopause. The low potential vorticity values and high water vapor content in these ozone‐rich pockets and trajectory analyses suggest that the ozone was photochemically produced in the boundary layer over eastern North America, followed by rapid uplifting in a warm conveyor belt over the Atlantic Ocean ahead of a frontal system. This was confirmed by ozone and water vapor measurements aboard commercial airliners crossing the warm conveyor belt. The air mass trajectories in both the stratospheric intrusion and the warm conveyor belt were tightly bundled, emphasizing the importance of the coherency of airstreams for long‐range ozone transport.
The lifetime of ozone in the troposphere is approximately 3 weeks. Prevailing westerly winds at northern midlatitudes can transport air around the globe in that time. Hence, within these latitudes ...zonal similarity is expected in long‐term changes and seasonal cycles of concentrations of baseline ozone. We quantify the degree of zonal similarity by examining eight in situ baseline ozone data sets near the west coasts of North America and Europe, that is, upwind of those continents and downwind of the Pacific and Atlantic Oceans, where the impacts of local and regional ozone sources have been largely mixed into the troposphere, giving the best‐defined baseline ozone signature. Zonal similarity is found in both long‐term changes and seasonal cycles. The decades‐long increase in Northern Hemisphere, midlatitude baseline mixing ratios (average ~0.60 ppb year−1 from 1980–2000), has ended, with a maximum reached in the mid‐2000s, followed by slow decrease (average = −0.09 ± 0.08 ppb year−1 from 2000 to the present). The year of the ozone maximum exhibits little if any statistically significant difference with location, altitude, or season. The ozone seasonal cycle differs markedly between sea‐level coastal stations representative of the marine boundary layer and the free troposphere sampled at elevated sites and by sondes and aircraft. However, within each of these broad tropospheric layers, the seasonal cycles are similar at all locations. Vertical profiles of the parameters that define the long‐term trends and the seasonal cycle are also similar between North America and Europe.
Plain Language Summary
Ozone in the lower atmosphere (i.e., the troposphere) plays important roles in atmospheric chemistry, air pollution, and climate change. Ozone has a wide variety of sources and sinks that vary temporally and spatially, which complicate our efforts to fully understand ozone's temporal and spatial distribution. Strong sources (primarily man‐made and natural) and sinks of ozone are located in the lower troposphere (i.e., the boundary layer), and input of ozone from the stratosphere is a strong source to the upper troposphere. Here we use long‐term records (~2 decades or more) of measured ozone concentrations at relatively remote locations in the northern midlatitudes (where the majority of man‐made ozone sources are located) to quantify as accurately and precisely as possible ozone's long‐term changes and seasonal cycles. Consistent with the ~3‐week lifetime of ozone in the troposphere, we find substantial similarity in these temporal variations throughout this latitude band. From the start of the measurements in the late 1970s, these concentrations increased until a maximum was reached in the mid‐2000s, followed by slowly decreasing concentrations. Decreasing ozone concentrations in the background troposphere is good news from the perspective of ozone's contributions to urban and regional air pollution and climate change.
Key Points
Long‐term changes and seasonal cycles of lower troposphere baseline ozone are similar throughout northern midlatitudes
At northern midlatitudes baseline ozone increased before 2000, but that increase ended, and average concentrations have since decreased
The year of the baseline ozone maximum (~2006) does not vary significantly with location, altitude, or season
A high-power Raman lidar system has been developed at the high-altitude research station Schneefernerhaus (Garmisch-Partenkirchen, Germany) at 2675 m, at the side of an existing ...differential-absorption lidar. It is based on a 180-W single-line XeCl laser and on two Newtonian telescopes (up to 1.5-m-diameter). In this way a vertical range up to more than 20 km and an accuracy level of the order of 10 % can be achieved for a measurement time of 1 h. Temperature measurements have been demonstrated to altitudes up to 54 km with just 1 % of the full 308-nm backscatter signal. Significantly higher altitudes are expected when using a chopper that cuts off the first 10 km or for 353 nm.
Routine high-quality lidar measurements of ozone, water vapour and aerosol at Garmisch-Partenkirchen since 2007 have made possible more comprehensive atmospheric studies and lead to a growing insight ...concerning the most frequently occurring long-range transport pathways. In this contribution we present as examples results on stratospheric layers travelling in the free troposphere for extended periods of time without eroding. In particular, we present a case of an intrusion layer that subsided over as many as fifteen days and survived the interference by strong Canadian fires. These results impose a challenge on atmospheric modelling that grossly overestimates free-tropospheric mixing.
In August 1998, severe forest fires occurred in many parts of Canada, especially in the Northwest Territories. In the week from August 5 to 11, more than 1000 different fires burned >1 × 106 ha of ...boreal forest, the highest 1‐week sum ever reported throughout the 1990s. In this study we can unambigously show for the first time that these fires caused pronounced large‐scale haze layers above Europe and that they influenced concentrations of carbon monoxide and other trace gases at the surface station Mace Head in Ireland over a period of weeks. Transport took place across several thousands of kilometers. An example of such an event, in which a pronounced aerosol layer was observed at an altitude of 3–6 km over Germany during August 1998, is investigated in detail. Backward trajectories ending at the measured aerosol layer are calculated and shown to have their origin in the forest fire region. Simulations with a particle dispersion model reveal how a substantial amount of forest fire emissions was transported across the Atlantic. The resulting aerosol lamina over Europe is captured well by the model. In addition, the model demonstrates that the forest fire emissions polluted large regions over Europe during the second half of August 1998. Surface measurements at Mace Head are compared to the model results for an anthropogenic and a forest fire carbon monoxide tracer, respectively. While wet deposition removed considerable amounts of aerosol during its transport, forest fire carbon monoxide reached Europe in copious amounts. It is estimated that during August 1998, 32%, 10%, and 58% of the carbon monoxide enhancement over the background level at Mace Head were caused by European and North American anthropogenic emissions and forest fire emissions, respectively.
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