Tracers of stratospheric influence and Asian dust over the Pacific during INTEX B phase II UNH SAGA special thanks to FASTOZ
All of you on the deployment will recall extensive (excessive?) discussion about whether we were seeing strat influence in the mid and upper trop over the Pacific. There is no doubt we were in the lower strat a lot on the flights from Alaska, almost never from Hawaii. The issue is whether STE was an important influence on airmasses we sampled well below the tropopause.
The green branch of this distribution is clearly tropospheric, while the pink is stratospherically impacted (or fully stratospheric). It appears that the stratospheric influence was large. Note that Melody, Brad, and Duncan are also looking at these tracer-tracer relationships and STE, in DC-8 data and RAQMS (see their talks and posters).
However, the distributions of nitric acid and ozone in the upper trop and lower strat over the Pacific during INTEX B are dominated by the observations from the AK flights, which included significant sampling in the lower strat. Out of HI, there is an indication of stratospheric influence, but the signal in nitric acid is weak. The line is “eye-balled” through the pink cloud on previous slide at 2.5 ppt/ppb, clearly the slope in the southern portion of INTEX B region is more like 1 ppt/ppb, or even less. Note that flight 14 is split at 40 N, in middle of BL run after TES spiral.
Beryllium-7 supports major contribution from stratospherically influenced air masses on the ozone distributions in both regions. Note That Scales are Different! Consistent with very few or no encounters with lower stratosphere on flights from HI.
Beryllium-7 also suggests that much of the nitric acid observed on the AK flights was derived from the stratosphere. But, out of HI the trend is inverted, with much less HNO3 than would be expected. What happened to the stratospheric HNO3 that should have been with the Be-7 and O3? Scavenging could be one option, but that would also deplete Be-7. We will come back to this question later, but first a word from one of our sponsors!
Aura Validation Many of the flights out of AK included specific legs designed to underfly MLS, and especially TES limb retrievals. This particular leg was dedicated to MLS validation. There was a lot of structure in HNO3 and O3, both in the LS and UT, as well as a complex tropopause. (This is the kind of situation Melody and Duncan really dig into.)
This leg was focused on here because it was one (out of three total) that had been reprocessed using the new MLS retrieval (version 2.1) by the time of the Aura meeting last fall. Very encouraging, at both levels! The other INTEX B MLS comparisons were near HI, with low in-situ HNO3. Not as good agreement. Also checked several legs from PAVE, none as nice as this one. Lots more MLS data by now, maybe Mark or Nathaniel have updates. Also checked first TES HNO3 from limb against INTEX B Flt 17. Not so encouraging. Check Greg’s poster? Maybe Mark, Susan, Greg or Annemarie have looked at more.
Outflow from Asia was another strong source influencing the composition of the troposphere over the Pacific during INTEX B. In addition to pollution, the Asian outflow also often carried large amounts of dust. This flight is just one example, showing encounters with thick dust cloud(s?) near 5 km. Note the large enhancement in aerosol nitrate with the dust. Also, detectible calcium through much of the troposphere often accompanied by enhanced nitrate. Some nitrate enhancement also with sea-salt in the marine BL.
These plots show calcium and nitrate in all INTEX B phase II samples (blue) and in samples where the Ca/Na ratio suggests that dust was the main source of Ca2+. There was a lot of dust throughout the troposphere. Also, the dust source was often important even in the marine BL (as Cam pointed out in poster at MILAGRO meeting). Note that occasionally seasalt was lofted well into free trop. Circling back to earlier question, it appears that uptake onto dust was a significant sink for nitric acid. Could this be where all the missing stratospheric nitric acid went in the southern region?
Not so obvious that Asian dust was more abundant, or more effective as a sink for nitric acid, in the southern region.
Searching for an inverse relation between dust and nitric acid in the gas phase has also not been broadly fruitful. There are certainly suggestive cases, but also periods when HNO3 and Ca2+ increase together.