ORCID Profile
0000-0002-5311-2570
Current Organisation
Australian Bureau of Meteorology
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Publisher: Copernicus GmbH
Date: 20-05-2014
Abstract: Abstract. During recent years, elevated ozone (O3) values have been observed repeatedly in the Upper Green River basin (UGRB), Wyoming, during wintertime. This paper presents an analysis of high ozone days in late winter 2011 (1 h average up to 166 ppbv – parts per billion by volume). Intensive operational periods (IOPs) of ambient monitoring were performed, which included comprehensive surface and boundary layer measurements. On IOP days, maximum O3 values are restricted to a very shallow surface layer. Low wind speeds in combination with low mixing layer heights (~ 50 m above ground level around noontime) are essential for accumulation of pollutants within the UGRB. Air masses contain substantial amounts of reactive nitrogen (NOx) and non-methane hydrocarbons (NMHC) emitted from fossil fuel exploration activities in the Pinedale Anticline. On IOP days particularly in the morning hours, reactive nitrogen (up to 69%), aromatics and alkanes (~ 10–15% mostly ethane and propane) are major contributors to the hydroxyl (OH) reactivity. Measurements at the Boulder monitoring site during these time periods under SW wind flow conditions show the lowest NMHC / NOx ratios (~ 50), reflecting a relatively low reactive NMHC mixture, and a change from a NOx-limited regime towards a NMHC-limited regime as indicated by photochemical indicators, e.g., O3 /NOy, O3 /NOz, and O3 / HNO3 and the EOR (extent of reaction). OH production on IOP days is mainly due to nitrous acid (HONO). On a 24 h basis and as determined for a measurement height of 1.80 m above the surface HONO photolysis on IOP days can contribute ~ 83% to OH production on average, followed by alkene ozonolysis (~ 9%). Photolysis by ozone and HCHO photolysis contribute about 4% each to hydroxyl formation. High HONO levels (maximum hourly median on IOP days: 1096 pptv – parts per trillion by volume) are favored by a combination of shallow boundary layer conditions and enhanced photolysis rates due to the high albedo of the snow surface. HONO is most likely formed through (i) abundant nitric acid (HNO3) produced in atmospheric oxidation of NOx, deposited onto the snow surface and undergoing photo-enhanced heterogeneous conversion to HONO (estimated HONO production: 10.2 ± 40% ppbv h−1) and (ii) combustion-related emission of HONO (estimated HONO production: ~ 0.1 ± 30% ppbv h−1). HONO production is confined to the lowermost 10 m of the boundary layer. HONO, serves as the most important precursor for OH, strongly enhanced due to the high albedo of the snow cover (HONO photolysis rate 10.7 ± 30% ppbv h−1). OH radicals will oxidize NMHCs, mostly aromatics (toluene, xylenes) and alkanes (ethane, propane), eventually leading to an increase in ozone.
Publisher: Elsevier BV
Date: 03-2019
Publisher: Copernicus GmbH
Date: 15-02-2022
Abstract: Abstract. Marine atmospheric boundary layer clouds cover vast areas of the Southern Ocean (SO), where they are commonly organized into mesoscale cellular convection (MCC). Using 3 years of Himawari-8 geostationary satellite observations, open and closed MCC structures are identified using a hybrid convolutional neural network. The results of the climatology show that open MCC clouds are roughly uniformly distributed over the SO storm track across midlatitudes, while closed MCC clouds are most predominant in the southeast Indian Ocean, with a second maximum along the storm track. The ocean polar front, derived from ECMWF-ERA5 sea surface temperature gradients, is found to be aligned with the southern boundaries for both MCC types. Along the storm track, both closed and open MCCs are commonly located in post-frontal, cold air masses. The hourly classification of closed MCC reveals a pronounced daily cycle, with a peak occurring late night/early morning. Seasonally, the diurnal cycle of closed MCC is most intense during the summer months (December–February DJF). Conversely, almost no diurnal cycle is evident for open MCC.
Publisher: Informa UK Limited
Date: 18-07-2013
DOI: 10.1080/10962247.2013.822438
Abstract: Nitrous acid (HONO) and formaldehyde (HCHO) are important precursors for radicals and are believed to favor ozone formation significantly. Traffic emission data for both compounds are scarce and mostly outdated. A better knowledge of today's HCHO and HONO emissions related to traffic is needed to refine air quality models. Here the authors report results from continuous ambient air measurements taken at a highway junction in Houston, Texas, from July 15 to October 15, 2009. The observational data were compared with emission estimates from currently available mobile emission models (MOBILE6 MOVES [MOtor Vehicle Emission Simulator]). Observations indicated a molar carbon monoxide (CO) versus nitrogen oxides (NO(x)) ratio of 6.01 +/- 0.15 (r2 = 0.91), which is in agreement with other field studies. Both MOBILE6 and MOVES overestimate this emission ratio by 92% and 24%, respectively. For HCHO/CO, an overall slope of 3.14 +/- 0.14 g HCHO/kg CO was observed. Whereas MOBILE6 largely underestimates this ratio by 77%, MOVES calculates somewhat higher HCHO/CO ratios (1.87) than MOBILE6, but is still significantly lower than the observed ratio. MOVES shows high HCHO/CO ratios during the early morning hours due to heavy-duty diesel off-network emissions. The differences of the modeled CO/NO(x) and HCHO/CO ratios are largely due to higher NO(x) and HCHO emissions in MOVES (30% and 57%, respectively, increased from MOBILE6 for 2009), as CO emissions were about the same in both models. The observed HONO/NO(x) emission ratio is around 0.017 +/- 0.0009 kg HONO/kg NO(x) which is twice as high as in MOVES. The observed NO2/NO(x) emission ratio is around 0.16 +/- 0.01 kg NO2/kg NO(x), which is a bit more than 50% higher than in MOVES. MOVES overestimates the CO/CO2 emission ratio by a factor of 3 compared with the observations, which is 0.0033 +/- 0.0002 kg CO/kg CO2. This as well as CO/NO(x) overestimation is coming from light-duty gasoline vehicles.
Publisher: Taiwan Association for Aerosol Research
Date: 2018
Publisher: Copernicus GmbH
Date: 05-07-2013
DOI: 10.5194/ACPD-13-17953-2013
Abstract: Abstract. During recent years, elevated ozone (O3) values have been observed repeatedly in the Upper Green River Basin (UGRB), Wyoming during wintertime. This paper presents an analysis of high ozone days in late winter 2011 (1 h average up to 166 ppbv). Intensive Operational Periods (IOPs) of ambient monitoring were performed which included comprehensive surface and boundary layer measurements. On IOP days, maximum O3 values are restricted to a very shallow surface layer. Low wind speeds in combination with low mixing layer heights (~50 m a.g.l. around noontime) are essential for accumulation of pollutants within the UGRB. Air masses contain substantial amounts of reactive nitrogen (NOx) and non-methane hydrocarbons (NMHC) emitted from fossil fuel exploration activities in the Pinedale Anticline. On IOP days in the morning hours in particular, reactive nitrogen (up to 69%), aromatics and alkanes (~10–15% mostly ethane and propane) are major contributors to the hydroxyl (OH) reactivity. Measurements at the Boulder monitoring site during these time periods under SW wind flow conditions show the lowest NMHC/NOx ratios (~50), reflecting a relatively low NMHC mixture, and a change from a NOx-limited regime towards a NMHC limited regime as indicated by photochemical indicators, e.g. O3/NOy, O3/NOz, and O3/HNO3 and the EOR (Extent of Reaction). OH production on IOP days is mainly due to nitrous acid (HONO). Until noon on IOP days, HONO photolysis contributes between 74–98% of the entire OH-production. Ozone photolysis (contributing 2–24%) is second to HONO photolysis. However, both reach about the same magnitude in the early afternoon (close to 50%). Photolysis of formaldehyde (HCHO) is not important (2–7%). High HONO levels (maximum hourly median on IOP days: 1096 pptv) are favored by a combination of shallow boundary layer conditions and enhanced photolysis rates due to the high albedo of the snow surface. HONO is most likely formed through (i) abundant nitric acid (HNO3) produced in atmospheric oxidation of NOx, deposited onto the snow surface and undergoing photo-enhanced heterogeneous conversion to HONO (estimated HONO production: 2250 pptv h−1) and (ii) combustion related emission of HONO (estimated HONO production: ~585 pptv h−1). HONO, serves as the most important precursor for OH, strongly enhanced due to the high albedo of the snow cover (HONO photolysis rate 2900 pptv h−1). OH radicals will oxidize NMHCs, mostly aromatics (toluene, xylenes) and alkanes (ethane, propane), eventually leading to an increase in ozone.
Publisher: American Geophysical Union (AGU)
Date: 05-05-2014
DOI: 10.1002/2013JD020287
Publisher: Elsevier BV
Date: 04-2018
Publisher: American Geophysical Union (AGU)
Date: 11-06-2016
DOI: 10.1002/2015JD024279
Publisher: Copernicus GmbH
Date: 05-07-2013
Publisher: American Meteorological Society
Date: 07-2021
Abstract: Understanding the key dynamical and microphysical mechanisms driving precipitation in the Snowy Mountains region of southeast Australia, including the role of orography, can help improve precipitation forecasts, which is of great value for efficient water management. An intensive observation c aign was carried out during the 2018 austral winter, providing a comprehensive range of ground-based observations across the Snowy Mountains. We used data from three vertically pointing rain radars, cloud radar, a PARSIVEL disdrometer, and a network of 76 pluviometers. The observations reveal that all of the precipitation events were associated with cold front passages. About half accumulated during the frontal passage associated with deep, fully glaciated cloud tops while the rest occurred in the post-frontal environment and was associated with clouds with supercooled liquid water (SLW) tops. About three quarters of the accumulated precipitation were observed under blocked conditions, likely associated with blocked stratiform orographic enhancement. Specifically, more than a third of the precipitation resulted from moist cloudless air being lifted over stagnant air, upwind from the barrier, creating SLW-top clouds. These SLW-clouds then produced stratiform precipitation mostly over the upwind slopes and mountain tops, with hydrometeors reaching the mountain tops mostly as rimed snow. Two precipitation events were studied in detail, which showed that during unblocked conditions, orographic convection invigoration and unblocked stratiform enhancement were the two main mechanisms driving the precipitation with the latter being more prevalent after the frontal passage. During these events, ice particle growth was likely dominated by vapor deposition and aggregation during the frontal periods, while riming dominated during the post-frontal periods.
No related grants have been discovered for Luis Ackermann.