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Local Meteorology and Air Quality

During certain periods, the air here at Bodega Marine Lab (BML) has been stunningly clean — approaching what might be found in the remote ocean.  We’re finding, somewhat unsurprisingly, that local meteorology heavily affects the composition of particles that we sample at BML.

Here’s a case study from last weekend.  The scene: nothing but Pacific Ocean to our west, with cow pastures, campgrounds, and a popular vacation town to our east and south.  On sunny days under fair weather (high pressure) we experience a diurnal cycle of sea breeze and land breeze.  In aerosol particle terms, we see a very segmented signal: distinct ‘sea breeze’ and ‘land breeze’ aerosol characteristics.  Let’s have a look at the aerosol particle size distributions next to the surface meteorological conditions (from the nearby DRI met station on Bodega Marine Reserve):

SeaBreezeCycle_BBY

Two days of aerosol particle size distributions (top two panels), carbon monoxide and solar radiation (3rd panel from top), ambient temperature and RH, with wind speed and direction on the bottom panel. The correlation of wind direction with the number concentration of small particles (2nd panel from top) is striking.

The sources of particles in air masses that we sample during the sea breeze (wind from 300 degrees) are ideally very limited: particles ejected from the ocean as sea spray, and particles that form through chemical reactions in the atmosphere just above the ocean.  In contrast, the land breeze (wind from about 140 degrees)  carries with it particles from a wide variety of sources: fossil fuel combustion, biomass combustion (fires), food cooking, and agriculture.

If you aren’t familiar with this kind of data, and you have a keen eye, you’ll start to figure out what’s happening just by thinking about what particle sources we’re sampling under the land breeze vs the sea breeze.  Since the sea breeze contains sea spray, you can see that sea spray aerosol particles are typically characterized by larger particle sizes (the colors in the top plot show higher concentrations near the 1 micron diameter mark), compared to the kinds of aerosols observed during the land breeze episodes (smaller proportions of larger particles compared with to the huge number of small particles).  It’s well known to atmospheric chemists that particles that are emitted directly from combustion sources (so-called ‘primary’ combustion aerosol) are very small, and tend to grow as they react with other trace gases in the atmosphere or are incorporated into clouds and fogs.

I can tell you anecdotally, that the composition of the particles is very different between these two episodes.  Sea salt particles (containing mainly sodium chloride) that travel through polluted air quickly react with the nitrogen oxide pollutants to form sodium nitrate particles.  The land breezes also bring with them fresh combustion particles.  You’ll notice some small spikes in carbon monoxide on either end of the sea breeze periods, just before and after the switch — those are actually from fires from local campgrounds directly upwind of us — it was a weekend in a vacation destination town after all!

One really exciting thing for a atmospheric chemist is to see their science in the world around them.  To live in and around the environment; to observe with your own senses what you’re observing with your instruments.  This is why I’m an environmental scientist.

-Doug Collins, Grad Student (UCSD)

Mid-February Wind Review

Greetings from Bodega Head! Today will be a short post. We are still anticipating quite a storm to arrive later in the week, and today should be our last day in the bright sunshine, northerly afternoon winds and relatively high pressure until the beginning of next week.

In anticipation, some of the crew got out for some R&R yesterday afternoon. After so much time in the coastal grassland, its easy to forget that whole communities of giants live just minutes away:

IMG_0807

BBACPAX at the Armstrong Redwoods State Nature Reserve.

Local Conditions:

DRI-BMR-SfcWx_1800UTC0224

Timeseries of local weather conditions at the Bodega Marine Reserve. Credit: DRI-WRCC.

We have had southeasterly winds for most of the morning, leading to elevated accumulation mode particle counts at the trailer (3000 particles per cc) and few course mode particles. Unlike over the weekend, we did not see any NOx, or CO spikes from the campground to our East.  We are nearing the common time for the sea-breeze to come onshore. This has happened between 10 am and 11 am PST every morning during episodes of high pressure, and we are preparing to sample elevated coarse mode (primarily sea salt) aerosols throughout this afternoon. This may be the last day we see a diurnal shift in ambient aerosol conditions for quite some time. The animation I have linked below shows the high-resolution MM5 forecast 10 m winds from this morning through late Tuesday.

MM5 surface wind loop Credit: DRI-CEFA.

As time passes, the elongated clockwise wind pattern offshore dissipates in favor of strengthening southerlies at the coast, followed by strong southerlies throughout the domain and eventually, it is possible that we get a coastal jet windward of the Sonoma County coastal range. There are also some slight hints that we may see Petaluma Gap Flow – something we will be watching with great interest.

Review of wind conditions for the first half of our study:

We are approaching the halfway point of BBACPAX, and with a prolonged period of stormy weather approaching I thought it prudent to review the primary flow regimes of the past 13 days here at the site.

In this post, I presented an analysis of the typical quiescent day here at BML. The flow we were seeing at our sampling site was dominated by a diurnal cycle related to the large-scale northerlies plus the sea-breeze circulation. For the most part, we saw strong (20 kts or more) Northwesterlies in the late afternoon hours, light south-southeast winds in the early morning, and transition periods in-between.

A wind-rose diagram of the data from the nearby DRI-WRCC station shows these preferred flow regimes very clearly:

BMR_WindRose_DRI_0210-0224

Wind Rose generated from 10-m tower anemometer at the DRI-WRCC Bodega Marine Reserve site. Credit: WRI-WRCC.

This data has been compiled from the period Feb 12 – Feb 24, 2014. This period included one 24-30 hour period of large-scale southerlies associated with a landfalling pacific wintertime cyclone (PWC), but for the most part it was dominated by high pressure and quiescent conditions.

It is interesting to compare the Bodega Marine Reserve wind rose to that from an inland site:

BORR_WindRose_DRI_0210-0224

The wind data in this rose comes from the Blue Oak Ranch Reserve site of the same network. The site is located windward of the Central CA coastal range but inland. Instead of two primary flow regimes here, we see simply one prevailing wind flow direction, WSW.

The diurnal flow reversal we experienced during our first two weeks at Bodega Marine Lab had a profound effect on the aerosol number, size and chemistry we observed. That data is being analyzed now, and I hope to include some relevant blog posts about aerosol and trace gas concentration (with cool figures!) soon.

Next chance for rain:

We are still watching the storm which should arrive Wednesday. Ill post tomorrow about our chances to see unique flow features such as the Coastal Jet and Petaluma Gap Flow.

-Andy

Cross-Pacific Dust

Greetings from Bodega Head!

Overview:

Its a pleasant sunny day with light WNW winds. We are still under the control of the NE Pacific ridge and as a consequence, we are stuck with miserably nice weather.

Yesterdays conditions here at Bodega Marine Lab were considerably less windy than in days past, due to slight offshore flow dominating the local scale. See the figure below for a reference point:

OPC-surface-WestCoast-18

West Coast surface analysis for 18 UTC on Feb 20, 2014. Credit: NOAA-OPC

This flow pattern took some of the bite out of the sea-breeze. The timeseries of our nearby anemometer  shows the calm conditions, but with a shift to offshore flow during the overnight hours (i.e. during the sea-breeze reversal).

BBY_NOAASfcmet_0221_2000UTC

48 hour timeseries of surface weather variables at BBY. Credit: NOAA-HMT.

There isnt much else to report in local weather news. We are still anticipating a storm reaching the Northern California coast next Friday and Saturday. Ill update the forecast of that storm tomorrow.

Long-Range Transported Aerosol:

Yesterday we were excited by the possibility that a small plume of enhanced dust concentration may cross our area. According to global circulation model (GCM) / chemical transport model (CTM) forecasts This plume originated in Asia several days ago, was cut off from it’s parent plume by a mid-Pacific storm, and then would have been steered clockwise around the ridge to our location. We were hoping to draw some of this dust through our sampling manifold, cause it to participate in heterogeneous immersion ice nucleation in our CFDC and then investigate the ice crystal residue chemistry through a combination of a pumped counterflow virtual impactor and an aerosol time-of-flight mass spectrometer.

The impetus for believing the dust would reach us came from a forecast made by the NASA global modeling and assimilation office (GMAO) GEOS-5.

GEOS-5_dustAOT550NA_23UTC0220

Tropospheric dust AOT forecast valid at 3 pm PST on Feb 20, 2014. Credit: NASA GMAO.

The GEOS-5 is a global circulation model which contains some simplified aspects of a chemical transport model. Dust mass mixing ratio is forecast based on emission from a saltation-imapction type emission model and the GEOS-5 3 dimensional winds. The model shortwave emission model is used to calculate the aerosol optical thickness (AOT) which would result from the dust mass and vertical distribution in each atmospheric column.

Unfortunately, we did not detect elevated dust levels at the surface yesterday afternoon in BBY, meaning that the point forecast for our area of the GEOS-5 domain may have busted. The caveat here is that we are only privy to the total column dust AOT through the web portal. It could be that dust in this plume simply never reached the surface.

Let’s examine further cross-pacific dust transport and how we might verify global model (such as GEOS-5) forecasts.

Recall that in a previous post I posted an image from MODIS-Aqua which indicated a region of high total aerosol optical thickness leaving the Asian continent in a large plume. That image was a composite of all “swaths” (the footprint of a satellite instrument) of MODIS level 2 aerosol optical thickness (AOT) at 550 nm from February 17. Here is the corresponding image from 02-18:

MODIS-Aqua_AsiaPlume_0218

North Pacific swaths of MODIS level 2 AOT from Feb 18, 2014. Credit NASA-GSFC.

There is still a large area (the size of the entire western US) of elevated AOT east of Japan. The satellite carrying MODIS, Aqua, crossed this aerosol plume near 16:30 UTC. Lets see what the GEOS-5 dust forecast looked like for this time and date:

GEOS-5_dustAOT550Glob_17UTC0218

Tropospheric dust AOT forecast valid at 17 UTC on Feb 18, 2014. Credit: NASA GMAO.

The model does predict a plume of moderately optically thick dust extending east from Asia over Japan at the correct date and time. For now, we will not worry about whether the model gets the magnitude of AOT correct, but rather simply worry about a “qualitative plume” arriving at the correct location in space and time. We should note that the meridional extent of the plume in GEOS-5 is smaller than it appears in MODIS AOT.

The next question we should ask is “Is the dust confined to a discrete vertical layer?” followed by “If so, how elevated is the dust layer?” To partially answer this question we can turn to CALIOP. CALIOP is an active, polarized, visible light emitting/receiving interferometer. It flies on CALIPSO in the same orbit and slightly behind Aqua. When the atmosphere is optically thin (no thick clouds) CALIOP can estimate the vertical profile of aerosol optical extinction (related to AOT) at nadir. Lets look at what CALIOP saw along this path:

CLPSO_plumeJapan_0218_swathmap

One orbital path of the CALIPSO satellite on Feb 18, 2014. Credit: NASA-LARC.

The pink section of the above swath passed over the approximate area of our plume. The measurement along the pink section looked like:

CLPSO_plumeJapan_0218_532nfeatmask

532 nm attenuated backscatter (top) and target classification (bottom) from CALIOP along a path which crossed the northeast Pacific on Feb 18, 2014.

The top plot is the time-altitude cross section of 532 nm attenuated backscatter. The plume we are interested can be seen between 134.5 and 139 E longitude as a shallow near-surface layer of yellow shading. The bottom plot is generated based on backscatter and the 532 nm depolarization ratio. After data is collected, regions of significant backscatter signals are classified based on their likely size and shape relative to Mie spheres. According to the classification algorithm, this near-surface layer is full of aerosol.

So far, so good. We identified a layer of aerosol in MODIS data, its timing and position was forecast by GEOS-5, we saw an aerosol layer with CALIOP, and are now confident that this aerosol layer is confined to the lowest 3 km.

What about the aerosol plume that was supposed to cross the California coast yesterday? Unfortunately, the data from the CALIPSO swath which passed over this area is not yet available to view online. However we can check MODIS:

MODIS-Aqua_AsiaPlume_0220

North Pacific swaths of MODIS level 2 AOT from Feb 20, 2014. Credit NASA-GSFC.

MODIS identified very low AOT in the cloud-free pixels near the north-central California coast during its overpass on Feb 20, 2014. This area was crossed by MODIS near 2230 UTC. It is near the same area forecast by GEOS-5 to contain elevated dust aerosol optical depth at 2300 UTC (see third figure, this post).

In this case, the GEOS-5 model and MODIS imagery
do not agree on the timing and location of an aerosol (possibly dust) plume. Our instruments also did not encounter dust spectra at a rate above background level, so it may be that the GEOS-5 forecast lead us on a goose chase yesterday.

Going forward, we will continue to use global model dust forecasts to guide our measurement strategies, but they must be taken with a large remote-sensing grain of salt.

Tomorrow’s post: Medium range convergence of global model forecasts.

-Andy