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Aircraft measurements of BrO, IO, glyoxal, NO<sub>2</sub>, H<sub>2</sub>O, O<sub>2</sub>–O<sub>2</sub> and aerosol extinction profiles in the tropics: comparison with aircraft-/ship-based in situ and lidar measurements
Tropospheric chemistry of halogens and organic carbon over tropical oceans
modifies ozone and atmospheric aerosols, yet atmospheric models remain
largely untested for lack of vertically resolved measurements of bromine
monoxide (BrO), iodine monoxide (IO) and small oxygenated hydrocarbons like
glyoxal (CHOCHO) in the tropical troposphere. BrO, IO, glyoxal, nitrogen
dioxide (NO<sub>2</sub>), water vapor (H<sub>2</sub>O) and O<sub>2</sub>–O<sub>2</sub> collision
complexes (O<sub>4</sub>) were measured by the University of Colorado Airborne Multi-AXis Differential
Optical Absorption Spectroscopy (CU AMAX-DOAS) instrument, aerosol
extinction by high spectral resolution lidar (HSRL), in situ aerosol size
distributions by an ultra high sensitivity aerosol spectrometer (UHSAS) and
in situ H<sub>2</sub>O by vertical-cavity surface-emitting laser (VCSEL) hygrometer. Data are presented from two research flights (RF12, RF17) aboard
the National Science Foundation/National Center for Atmospheric
Research Gulfstream V aircraft over the tropical Eastern Pacific Ocean (tEPO) as
part of the "Tropical Ocean tRoposphere Exchange of Reactive halogens and
Oxygenated hydrocarbons" (TORERO) project (January/February 2012). We assess the
accuracy of O<sub>4</sub> slant column density (SCD) measurements in the presence
and absence of aerosols. Our O<sub>4</sub>-inferred aerosol extinction
profiles at 477 nm agree within 6% with HSRL in the boundary layer and
closely resemble the renormalized profile shape of Mie calculations
constrained by UHSAS at low (sub-Rayleigh) aerosol extinction in the free
troposphere. CU AMAX-DOAS provides a flexible choice of geometry, which we
exploit to minimize the SCD in the reference spectrum (SCD<sub>REF</sub>, maximize
signal-to-noise ratio) and to test the robustness of BrO, IO and glyoxal
differential SCDs. The RF12 case study was conducted in pristine marine and
free tropospheric air. The RF17 case study was conducted above the NOAA RV <i>Ka'imimoana</i> (TORERO cruise, KA-12-01) and provides independent validation
data from ship-based in situ cavity-enhanced DOAS and MAX-DOAS. Inside the
marine boundary layer (MBL) no BrO was detected (smaller than 0.5 pptv), and
0.2–0.55 pptv IO and 32–36 pptv glyoxal were observed. The near-surface
concentrations agree within 30% (IO) and 10% (glyoxal) between ship
and aircraft. The BrO concentration strongly increased with altitude to 3.0 pptv at 14.5 km (RF12, 9.1 to 8.6° N; 101.2 to 97.4° W).
At 14.5 km, 5–10 pptv NO<sub>2</sub> agree with model predictions and demonstrate
good control over separating tropospheric from stratospheric absorbers
(NO<sub>2</sub> and BrO). Our profile retrievals have 12–20 degrees of freedom
(DoF) and up to 500 m vertical resolution. The tropospheric BrO vertical column density (VCD) was 1.5 × 10<sup>13</sup> molec cm<sup>−2</sup> (RF12)
and at least 0.5 × 10<sup>13</sup> molec cm<sup>−2</sup> (RF17, 0–10 km, lower limit). Tropospheric IO VCDs correspond to
2.1 × 10<sup>12</sup> molec cm<sup>−2</sup> (RF12) and 2.5 × 10<sup>12</sup> molec cm<sup>−2</sup>
(RF17) and glyoxal VCDs of 2.6 × 10<sup>14</sup> molec cm<sup>−2</sup> (RF12) and 2.7 × 10<sup>14</sup> molec cm<sup>−2</sup> (RF17).
Surprisingly, essentially all BrO as well as
the dominant IO and glyoxal VCD fraction was located above 2 km (IO:
58 ± 5%, 0.1–0.2 pptv; glyoxal: 52 ± 5%, 3–20 pptv). To our
knowledge there are no previous vertically resolved measurements of BrO and
glyoxal from aircraft in the tropical free troposphere. The atmospheric
implications are briefly discussed. Future studies are necessary to better
understand the sources and impacts of free tropospheric halogens and
oxygenated hydrocarbons on tropospheric ozone, aerosols, mercury oxidation
and the oxidation capacity of the atmosphere
Water level changes in Lake Van, Turkey, during the past ca. 600 ka: climatic, volcanic and tectonic controls
Sediments of Lake Van, Turkey, preserve one of the most complete records of continental climate change in the Near East since the Middle Pleistocene. We used seismic reflection profiles to infer past changes in lake level and discuss potential causes related to changes in climate, volcanism, and regional tectonics since the formation of the lake ca. 600 ka ago. Lake Van's water level ranged by as much as 600 m during the past 600 ka. Five major lowstands occurred, at 600, 365-340, 290-230, 150-130 and 30-14 ka. During Stage A, between about 600 and 230 ka, lake level changed dramatically, by hundreds of meters, but phases of low and high stands were separated by long time intervals. Changes in the lake level were more frequent during the past 230 ka, but less dramatic, on the order of a few tens of meters. We identified period B1 as a time of stepwise transgressions between 230 and 150 ka, followed by a short regression between ca. 150 and 130 ka. Lake level rose stepwise during period B2, until 30 ka. During the past 30 ka, a regression and a final transgression occurred, each lasting about 15 ka. The major lowstand periods in Lake Van occurred during glacial periods, suggesting climatic control on water level changes (i.e. greatly reduced precipitation led to lower lake levels). Although climate forcing was the dominant cause for dramatic water level changes in Lake Van, volcanic and tectonic forcing factors may have contributed as well. For instance, the number of distinct tephra layers, some several meters thick, increases dramatically in the uppermost 100 m of the sediment record (i.e. the past 230 ka), an interval that coincides largely with low-magnitude lake level fluctuations. Tectonic activity, highlighted by extensional and/or compressional faults across the basin margins, probably also affected the lake level of Lake Van in the past