BLANkET1 Brief Report 12

Moorina, North–East Tasmania, 28 April 2011

Air Section, EPA Division, May 2011

Context of the BLANkET reports

BLANkET (Base-Line Air Network of EPA Tasmania) reports are compiledusing BLANkET and other Tasmanian air quality data, as well as data fromother sources. The topics and events chosen for these reports are selected forone or more of the following reasons: Scientific interest – for example if the eventdemonstrates a principle or principles of general value in understanding smokemovement and dispersal in the Tasmanian context; Well-documented events –such as if the event is captured by two or more stations and hence providesgeneral information on smoke movement; General public interest – this includeslarge–scale or other smoke events that have generated comment at the time orare of intrinsic public interest for other reasons.

1Base–Line Air Network of EPA Tasmania

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1 Summary

An occurrence of thick smoke at Moorina, north–eastern Tasmania, on the af-ternoon of the 28th of April 2011, was reported to the EPA Division. The sourcewas reported to be a forest coupe burn on Frome Road, Moorina. The reportedlimiting visibility range of 100 to 150 metres implies a very high peak in localsmoke concentration. MODIS and AVHRR satellite images for this time shownorth–eastern Tasmania as partly cloud covered, but with the appearance ofsmoke over Moorina. A ‘hot–spot’ (implied fire) location on the AVHRR imageis consistent with the position of the known Frome Road burn.

The nearest air quality station to Moorina is at Derby, about 6 km south–west of Moorina. PM2.5 data for the 28th of April from the Derby air stationshow a short–duration elevation near 16:00 AEST, followed by a general, near–uniform, elevation of about 10 µg m−3 during the night of the 28th/29th ofApril. The MODIS high–resolution afternoon satellite image indicates that theDerby air station was at the edge of the smoke plume on the afternoon of the28th of April.

The day–averaged PM2.5 levels at Derby for the 28th and 29th of April were8 and 6 µg m−3 respectively. These are slightly elevated compared to adjacentdays, but are well under the 24–hour PM2.5 national advisory reporting standardof 25 µg m−3.

It seems probable that part of the elevation in PM2.5 at Derby on these dateswas due to a contribution from smoke from the Frome Rd burn, but it appearsthat most of the smoke from this burn did not reach the air station at Derby.

2 Data

2.1 Public report

A report from Moorina, north–east Tasmania received by the EPA on the 28thof April 2011 noted thick smoke from a burn in Frome Road Moorina, wasreaching the residence of the reporter. The smoke arrived near midday, andbegan to clear around 3 hours later on a wind change. The reported visibilityduring the interval of thickest smoke was 100 m.

Information provided by Forestry Tasmania recorded a burn of fuel in-dex 1880 units (approximately 18800 tonnes fuel) at the approximate position147.90 E, 41.13 S, with ignition commencing at 12:30 AEST on the 28th ofApril 2011. This was noted as a native forest coupe. The nominal fuel loadingof 400 t per hectare implies a burn area near 47 ha.

2.2 Air quality data

The nearest air monitoring station to Moorina is at Derby, some 6.5 km tothe south–west. Moorina and Derby are at similar elevations above sea–level,however, the local terrain rises around 200 m from Moorina in the direction of

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Derby. This may limit the movement of air, and hence smoke, between theselocations.

Air quality data (PM2.52) and meteorological data for Derby for the 28th

and 29th of April are shown in Figure 1.

Figure 1: Derby BLANkET station data for the 28th and 29th of April 2011.Top panel: 10-minute PM2.5 data. Lower panel: Meteorological data.

Generally low–wind speed or calm conditions prevailed over this interval(lower panel). PM2.5 shows a short duration interval of elevated levels, up tonear 40 µg m−3, from just before 16:00 to just before 18:00 AEST (i.e. just

2PM2.5 means particulate matter (i.e. particles) up to 2.5 µm (micrometres) in effectivediameter.

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before 4 pm to just before 6 pm). The wind was mostly a light north–north–easterly right at smoke onset. From around 22:00 AEST onwards to 08:00 AESTon the 29th PM2.5 stayed relatively constant between 10 and 15 µg m−3.

The smoke onset at Derby near 16:00 AEST was about an hour after thesmoke began to clear at Moorina.

2.3 Satellite Images

The MODIS Aqua image for north–east Tasmania for 13:40 AEST (1:40 pm) onthe 28th of April is shown in Figure 2. The locality of Moorina is marked by thehouse icon. Also shown are the location of EPA air quality stations (red balloonsymbols). A burn conducted by Forestry Tasmania at Frome Road, Moorina,on the 28th of April is also marked. The white cloud extending south–west fromthe burn locality has a blue edge. This is likely to be a smoke signature.

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Figure 2: MODIS Aqua image for north–east Tasmania for 13:40 AEST (1:40pm) on the 28th of April. Moorina is marked with a house icon. Also shownare the locations of air quality monitoring stations (red balloon symbols) andthe location of a Foresty Tasmania burn at Frome Road, Moorina, conductedon this day.

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A false-colour NOAA AVHRR3 satellite image for 14:08 AEST 28th April2011 for Tasmania is shown in Figure 3. This false–colour image has beenproduced using band 1 (visible) for blue light, band 2 (near infrared) for green,and band 3 (also near–infrared) for red. An inferred fire location in north–east Tasmania is marked with a red cross. The deduced position of 147.89 E,41.15 S, corresponds essentially exactly with the known location of the ForestryTasmania Frome Road burn (noted above). The false–colour imagery shows thefeature that extends south–west from the burn location is unusual in spectralcomposition compared to nearby cloud. Taken together with the visible lightAqua image it appears to indicate this is likely to be a smoke plume.

2.4 Inferred source

Based on the first–hand report from the member of the public from Moorinaand the satellite imagery showing evidence of a plume over Moorina, the FromeRoad burn was the most likely source of the reported smoke. Figure 4 shows aGoogle Earth oblique view of the line of sight from Moorina to the Frome Rdburn. The burn was approximately 2.5 km from the Moorina locality.

3 Discussion

3.1 Smoke at Moorina

The public report indicated that during the peak of the smoke impact visibilitywas limited to 100 m. Estimating particulate concentrations from visibility mea-surements or estimates is intrinsically difficult. The visibility range is definedas the distance at which objects, normally visible from a particular location,just become undetectable. A visibility estimate of smoke concentration will beinfluenced by the local relative humidity, time of day, look–direction relative tothe sun, cloudiness, presence of other aerosols (e.g. dust), and the extent anduniformity of the smoke, as well as the need for landmarks at known distancesto provide a limiting visual–range estimate. Additionally, and importantly, itneeds to be borne in mind that an ambient particle mass concentration (asproduced by the BLANkET dustrak particle counters) and a visibility range(determined by eye or from a photographic image) are measurements of intrin-sically different quantities. A particle mass concentration is, as implied by thename, a measure of particle mass per volume. A visibility range is a measureof the total number of particles in a viewing column – i.e. an ‘optical depth’.To explicitly consider this further, consider a uniform smoke plume that coversa wide area. The mass concentration of this uniform plume is the same every-where, but the number of particles in a viewing column depends on the lengthof the column under consideration. The longer the column (that is, the furtherone looks) the more particles one looks through. A particle counter, such as

3The US National Ocean Atmosphere Administration Advanced Very High–ResolutionRadiometer

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Figure 3: NOAA AVHRR false–colour image for Tasmania for 14:09 AEST (2:08pm) on the 28th of April. The ‘X’ in north–east Tasmania corresponds to aninferred fire location based on the infrared spectral signature. This coincideswith the location of the Frome Road burn.

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Figure 4: Google Earth oblique view showing the line–of–sight from Moorina tothe Frome Road burn. The burn was approximately 2.5 km from the Moorinalocality.

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a dustrak as used in the BLANkET stations, would measure the same massconcentration everywhere in the column, no matter what length of column isconsidered, as long as the smoke remained uniform throughout.

These differences are important, and ultimately limit the validity of derivinga mass concentration from visibility (range) estimates.

However, for the purposes of completeness, an approximate estimate of parti-cle concentration can be determined. This has been derived from data obtainedat Fairbanks, Alaska, of smoke concentrations and visibility.4

Figure 5 shows a log–log plot of PM2.5 data and visibility (in km) derivedfrom data collected at Fairbanks5. The dotted line represents a linear fit to thedata. The validity of this line, and, in particular, the validity of extrapolatingthis line to higher and lower ranges, is limited, hence the following should beinterpreted cautiously.

The reported 100 m visibility implies a very high local smoke concentration,over several hundred µg m−3, which is well above what was measured at Derbyon this day. Informal eye estimates of visibility can tend to overestimate theactual range from quoting a distance where obscuration is significantly apparent,rather than quoting the limiting distance when vision is completely obscured.Consequently, the member of the public who supplied the original report wascontacted for clarification. The person was able to report that the tree–linealong the lower property boundary was not visible at peak smoke impact. Thistree–line is approximately 150 m from the viewing location. Hence the reported100 m visibility measure appears substantially correct, and therefore the localsmoke concentrations were likely to be extremely high during the interval ofpeak smoke impact.

The highest instantaneous PM2.5 level seen at any station in the BLANkETnetwork so far (up to May 2011) was just under 1000 µg m−3 at Emu River(5 km south of Burnie) on the night of the 20th of April 2010. In that instanceit appears likely that smoke from a burn, or burns, was topographically con-fined in a river valley by a katabatic wind6. In October 2010 Sheffield stationrecorded PM2.5 levels around 250 µg m−3 due to smoke from a planned burnaround 5 km distant7. Given these instances, the indicated very high smokeconcentrations at Moorina, 2.5 km from the 28th April burn location, do notappear unprecedented.

3.2 Smoke at Derby

Elevated PM2.5 levels at Derby were seen during the afternoon of the 28th ofApril, but the peak levels were well below the likely smoke concentrations at

4Fairbanks is located in interior Alaska, where summer temperatures can exceed 30 C.Annual rainfall is low in the Alaskan interior, with much of the precipitation coming as wintersnow rather than as rain. In late summer thunderstorms can start fires in the extensive borealforests (principally spruce and birch) that surround Fairbanks. In consequence Fairbanksoften experiences a number of days of high particle levels in late summer.

5www.co.fairbanks.ak.us/airquality/Docs/ParticulateLevels.pdf6BLANkET Technical report 10, available at http://www.environment.tas.gov.au/index.aspx?base=75297BLANkET Technical Report 11, available from http://www.environment.tas.gov.au/index.aspx?base=7529

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Figure 5: A log–log plot of data from the Fairbanks–North Star Borough,Alaska, of estimated visibility versus PM2.5 concentration. The dotted linerepresents a linear fit to the data. The validity of the fitted line, and, in partic-ular, the validity of extrapolating this line to higher and lower ranges, is limited,hence conclusions drawn from these data should be interpreted cautiously.

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Moorina. It is important to note that short–duration elevations of PM2.5 near16:00 AEST are relatively common occurrences at Derby in cooler weather. Thiseffect was studied in detail in 2009, and is described in BLANkET Technicalreport 28. In brief, often in daytime, in the cooler months of the year at Derby,a light breeze blows down the Ringarooma River valley. This moves air from thetown itself generally towards the BLANkET air station, located at the south–east end of the town. During the night–hours a katabatic flow is establisheddown the steep gulley of the Cascades Rivulet. This effectively reverses thewind direction through the town and moves air (and chimney smoke, which ispredominantly present in late–afternoon and evening) away from the station.During the late afternoon, on synoptically calm days, the wind reversal mostlyoccurs near 16:00 AEST during the cooler months. During this interval of windchange some smoke from the town can be taken directly towards the air station,giving rise to a short–term but marked elevation in PM2.5. The wind changecan be marked by several oscillations in wind direction before the evening flowbecomes established. Examples of the ‘afternoon elevation’ in PM2.5 at Derbycan be seen in Figure 6, which shows data from the 5th and 6th of May 2011.Short–duration elevated PM2.5 intervals near 16:00 are indicated on both daysby asterisks.

Detailed views of PM2.5 data for Derby station for the 25th to the 30th ofApril are given in Figure 7. The thick arrows at top and bottom of the plotmark 16:00 AEST. As noted above, near 16:00 AEST in the cooler months ashort–term enhancement of the PM2.5 levels are often seen at Derby. The burnat Moorina was conducted on the 28th of April. There is an elevated intervalof PM2.5 near 16:00 AEST on this day, as was discussed earlier. There are alsoclear elevations of PM2.5 near 16:00 AEST on the 25th and 27th of April, aswell as a small but real increase near this time on the 29th.

The elevation of PM2.5 at Derby on the afternoon of the 28th of April is oflonger duration than usual. This is illustrated in Figure 8. The top panel ofFigure 8 shows detail of the Derby PM2.5 data for the afternoon of the 28thof April. The lower panel shows a composite of Derby PM2.5 data for theafternoons of the10th to the 22nd of May 2010. This was an interval of cold,calm weather. On most days a short–duration elevated PM2.5 instance occurredin the afternoon. The lower panel shows that most of these occurrences tookplace typically on a 30–minute timescale, with a few instances nearer 1–hourin length. In contrast the 28th April 2011 elevation was present for around3 hours. This alone does not preclude the elevation as being due solely to alocal (chimney smoke) source, but given the unusual signature of this extendedafternoon PM2.5 elevation, it seems probable that there was a contribution froma source external to Derby. The satellite imagery showed the edge of the plumefrom the Frome Road burn to be nearly overhead at Derby at 13:40 AEST.Hence the Frome Road burn appears to be the most obvious candidate externalsource.

It is worth noting that the PM2.5 levels at Derby overnight on the 28th–29th8Available at http://www.environment.tas.gov.au/index.aspx?base=7529.

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Figure 6: Derby BLANkET station data for the 5th and 6th of May 2011.Top panel: 10-minute PM2.5 data. Lower panel: Meteorological data. Short–duration elevated PM2.5 intervals near 16:00 are indicated on both days byasterisks.

of April were slightly elevated compared to the preceding nights, as shown inFigure 9, even though overnight weather conditions were similar. The 24–houraveraged PM2.5 values are given below the top panel. The relatively smoothvariation overnight on the 28th–29th is also suggestive of a source at somedistance from the station.

Based on the above analysis it appears the most consistent interpretation ofthe elevated PM2.5 levels is that there was likely to have been a contributionfrom the Frome Road burn, along with local (Derby) sources (from domesticwoodheating). It is not possible to apportion the relative contributions.

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Figure 7: Derby BLANkET station data for selected days in April 2011. Topto bottom panels: PM2.5 data from the 25th, 26th, 27th, 28th, 29th, and 30thof April. The thick arrows at top and bottom mark 16:00 AEST.

4 Conclusions

The report received from Moorina indicated significantly higher smoke levelswere present there than seen at the BLANkET station at Derby, some 2.5 kmaway. This is important as it points to large variations in smoke concentra-tion even over small spatial scales. The reasons for the difference in smokeconcentrations at the two locations probably relate to a combination of of therelatively complex local topography and to the prevailing meteorological con-ditions at the time. The analysis strongly indicates that significantly highersmoke smoke levels can occur nearer to the burn locations than at sites evenonly a few kilometres farther away.

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5 Acknowledgements

BLANkET stations are sited at properties owned or operated by Ben LomondWater, Break O’Day Council, Cradle Mountain Water, Dorset Council, ForestryTasmania Huon District, Hobart City Council, Huon Valley Council, SouthernWater, and West Tamar Council.

MODIS images are from the NASA/Goddard Space Flight Center ‘MODISRapid Response’ web site9. Use of the NOAA AVHRR satellite images is alsoacknowledged.

Report compiled by J. Innis.

9http://rapidfire.sci.gsfc.nasa.gov/

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Figure 8: Top panel: Derby BLANkET station 10–minute PM2.5 data for theafternoon of 28 April 2011. Lower panel: Composite plot of 10–minute PM2.5

data from the afternoons of 10th to 22nd May 2010 for Derby. The duration ofthe 28th April 2011 afternoon enhancement was significantly greater than wasseen for elevations for the 12 days of May 2010 data shown in the lower panel.The May 2010 elevations are typical of what is normally seen at Derby.

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Figure 9: Top: Derby BLANkET station data for 20th to 29th of April 2011.Lower panel: Meteorological data. The 24–hour averaged PM2.5 values are givenbelow the top panel. The averages for the 28th and 29th were slightly higherthan for the preceding days.

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