# Presentation: 686932

## The observation of nitric acid-containing particles

**The observation of nitric acid-containing particles**
**in the tropical lower stratosphere**

**Peter J. Popp**
**Aeronomy Laboratory**
**National Oceanic and Atmospheric Administration**
- T.P. Marcy, E.J. Jensen, B. Kärcher, D.W. Fahey, R.S. Gao, T.L. Thompson, K.H. Rosenlof, E.C. Richard, R.L. Herman, E.M Weinstock, J.B. Smith, R.D. May, H. Vömel, J.C. Wilson, A.J. Heymsfield, M.J. Mahoney, A.M. Thompson

**Notes:**
In my time today, I want to talk about some observations we made during the Pre-AVE campaign in Costa Rica in January of 2004, and the discovery of nitric acid-containing particles in the tropical lower stratosphere during two B57 flights south of San Jose.  Time permitting, I’ll also show some very preliminary validation work we’ve done comparing the MLS nitric acid measurements to our in situ measurements from the B57.  As usual, there’s a long list of colleagues I need to thank who have contributed data and/orsome intellectual content to what I’m going to talk about here.

## NOAA CIMS instrument

- Direction of Flight

- Downward-facing Inlet
- (Gas phase only)

- Forward-facing Inlet
- (Particle + gas phase)

**NOAA CIMS instrument**

- SF5- + HNO3    SF4 + HFNO3-

**Notes:**
Many people here are at least somewhat familiar with the CIMS instrument that we fly on the B57.  We measure nitric acid through this ion-molecule reaction scheme here that Tim Marcy did most of the development on.  The point I really need to make here is that it’s our inlet pylon that allows us to make these particle measurements.  We measure nitric acid with two independent channels of detection with two separate inlets.  The downward facing inlet here effectively measures gas-phase HNO3, because any particulate species are inertially stripped from the sampled airstream.  The forward-facing inlet samples both gas- and particle-phase species, and the difference between these two signals lets us determine the amount of HNO3 in the gas phase.

## Nitric acid-containing particle observations

**Nitric acid-containing particle observations**
- Flight date: 29 January 2004

- 5 Hz Data

**Notes:**
As an example of the instrument response we see when we’re observing these particles, this is a plot showing about 35 s of data in the lower stratosphere on January 29 of 2004.  This is 5 Hz data, and the red trace shows the gas-phase signal at about 0.1 ppb, and the signal from the channel with the forward-facing inlet in blue, showing these peaks, which represent the sampling of individual HNO3-containing particles.  We measure HNO3 for 3 seconds out of every 10, and in the remaining time we’re measuring HCl and ClONO2, and doing some housekeeping.  Tim will talk about the HCl measurements later today.  You might notice this particle size scale over here.  Knowing the nitric acid content of the particle and assuming the composition to be NAT, which I’ll talk about in just a minute, we’re able to calculate the size of the particle - the two large particles here are about 3.4 microns. 

## Pre-AVE flight tracks and particle locations

**Notes:**
So like I said, we observed these particles on two flight south of Costa Rica during Pre-AVE, on the 27th and 29th of January, between 3 degrees south and 7 degrees north, over a distance covering about 1100 km.  The individual particle observations are shown here as symbols laid down over the flight tracks.  I should also point out there was a third southbound flight from San Jose during which we didn’t observe these particles.

## Particle observation conditions

- Particle Observations

**Particle observation conditions**
- Flight date: 29 January 2004

- •  Altitude 18 ± 0.1 km
- •  ~1 km above tropopause
- •  Temperature 188 - 192 K
- •  Water vapor ~3.5 ppmv
- •  SNAT ~0.1 - 10
- •  Particle Conc. 6·10-5 cm-3

**Notes:**
So I want to show you the ambient conditions under which we observed these particles.  This plot shows data from the flight of January 29th, and there’s a few things I’ll draw your attention to.  The top panel shows the nitric acid signal, and I’ve highlighted the two regions where we saw these particles.  The series of blue peaks are particle observations, only on a much more compressed scale than the last plot I showed you.  So, a few things, over two flights we observed these particles in a very narrow layer, about 200 m thick right at 18km, and never lower than that.  I should point out that this is as high as the B57 flew during these flights, and we have no idea how deep the particle layer was above the plane on these days.  It could be that we were only sampling the bottom of a much thicker layer.  And you can see from the black trace in this panel here showing the MTP tropopause height that we were about a km above the tropopause when we observed these particles.  Throughout these observations the temperature ranged from 199 to 192 K, and the water vapor was right about 3 and a half ppm.  When you calculate Snat in these air masses you get a range from about 0.1 to 10.  The observation of NAT particles in subsaturated air isn’t unprecedented - we’ve seen this before in the Arctic stratosphere.  These particles can survive for days in slightly subsaturated air - it just means that we’re observing them under non-equilibrium conditions.  And finally, the average particle concentration during this period here, the most intense, was 6 times 10e-5 per cc, which is very dilute but comparable to typical large NAT particle concentrations in the Arctic of 10 to the negative 4 per cc.

## NAT saturation conditions in the tropics

**NAT saturation conditions in the tropics**

- Ozone and temperature profiles: San Cristobal (1° S) and Paramaribo (6° N)
- Water vapor profiles: San Cristobal (1° S) and San Jose (10° N)

- NAT saturated
- region

- Particle obs.
- 80 ± 0.2 hPa

**Notes:**
So thermodynamic considerations point to these particles being composed of NAT, but investigating this possibility in the deep tropics isn’t easy because there is very little nitric acid data available in the tropics.  What we do have, however, is ozone sonde data from the SHADOZ network and  nitric acid to ozone correlation measured in the lower stratosphere during Pre-AVE.  And with this correlation we can develop proxy nitric acid profiles, which is what I’m showing you here.  We have January sonde data from San Cristobal and Paramaribo, as well as water data from San Cristobal, and from San Jose during Pre-AVE compliments of Randy May.  So taking the median values of nitric acid, temperature and water in the UTLS we can calculate Snat around the tropopause, shown by the dashed red line in the third panel, and we find there is a NAT-stable layer about 3 km thick, shown by this light pink ban.  And low and behold, this is where we observed our particles, shown here by the dashed line.

## NAT particle growth times

- At T = 188 K,
- HNO3 = 0.09 pbbv,
- H2O = 3.4 ppmv,
*t* = 8 days (2 μm particle)
*t* = 13 days (3 μm particle)

- Calculated using a diffusional growth model

**Notes:**
So we asked ourselves if we could grow these particles on a reasonable timescale given the low amounts of gas-phase nitric acid we were seeing in the tropics.  So we used a simple diffusional growth model with static conditions typical of what we saw during Pre-AVE, and that’s what I’m showing you here.  And what I’ll draw your attention to are the solid and dashed green lines on this plot, over here on the left at .09 ppb of nitric acid.  At 188 K with 3.4 ppm of water, it takes 8 days to grow a particle to 2 microns and 13 days to grow a particle to 3 microns.  And you can also see that these growth times are strongly dependent upon the available HNO3, and the times drop rapidly when HNO3 increases.  The growth time drops by about half when you double HNO3.

So we looked at how long it would take to grow these particles under observed conditions using a diffusional growth model, and I’ll just point out to you that
HNO3 doesn’t change, only 2 pptv in the gas phase - has implications for no loss of HNO3

## Particle back trajectories

- Particles observed under saturated conditions on 27 January 2004

**Notes:**
Now we wanted to see if these growth times were realistic, so with the help of Eric Jensen we inititiated back trajectories for the particles observed under saturated conditions on January 27.  Well, I say with the help of Eric Jensen but he did all the work.  Nonetheless, we found that we could track most of these particles back to their origin after 6-14 days, and in some cases they had travelled more than 15,000 km during the growth period.

## Conclusions and implications

- Nitric acid trihydrate (NAT) particles were discovered in the tropical lower stratosphere during the Pre-AVE campaign.  Balloon sonde data shows a NAT-stable layer (~3 km thick) in the deep tropics.
- A diffusional growth model indicates these particles require 8-13 days to grow in the tropical lower stratosphere.  Back-trajectory calculations show the particles can travel up to 15,000 km during the growth period.
- It seems reasonable to speculate that tropical and polar NAT particles nucleate via a common process, since low NAT particle concentrations were observed in both regions.
- At the number concentrations and sizes observed, tropical NAT particles will not be observable in satellite extinction measurements.

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**Notes:**
So to conclude, we performed what we think are the first observations of NAT particles in the tropical lower stratosphere during Pre-AVE, and we used sonde data to support these results by showing a NAT-stable layer in the tropics.  A diffusional growth model shows these particles can grow in 8-13 days under observed conditions, and a particled BT model showed they can travel up to 15,000 km during this period.  Since we observed these particles at a low number density similar to that of NAT particles in the Arctic, we’re speculating that they might nucleate via the same slow or selective process as the particles in the polar region.  And one last point to make on this topic is that these particles are too dilute to be observed in satellite extinction measurements.

## NASA Aura and WB-57F flight tracks

- 29 October 2004

**Notes:**
One last thing I’ll show you is some very preliminary validation work that we’ve done, comparing our in situ measurements on the B57 to the MLS nitric acid measurments.  I’ll show you some data from the AVE-October 2004 mission from Houston.  In blue is the MLS ground track on October 29th, with the location of the individual MLS retrieval profiles shown by the blue symbols.  The B57 flight track, originating in Houston, shown by the star, is here in red.  The B57 had to fly east over Louisiana to intercept the MLS ground track, and you can see there were 7 MLS profiles coincident with the 57 flight track.

## HNO3 vertical profiles

**HNO****3**** vertical profiles**

- 29 October 2004

**Notes:**
So I took the in situ CIMS data and plotted it as a vertical profile for this flight, it’s colored according to latitude at the time of measurement, and I just averaged the 7 MLS profiles along the flight track, at four pressure levels, and they are here in black.  And I think by most peoples metric you’d have to say that looks pretty good.  Either that, or we’re both doing something wrong, and in the same fashion.  But given the dramatically different way these measurements are made, you need to be cautious in making this kind of comparison, and I’m going to show more of this work to the MLS validation subgroup on Wednesday, and I’ll talk more about that.

## Many thanks to the air and ground crews

**of the NASA WB-57F aircraft.**

**Notes:**
And finally, I can’t stress this enough, the B57 air and ground crews go above and beyond the call of duty  to help us pull this off every time we fly with them, and without them we wouldn’t get anything done.  And thank you.