School of Geography, Earth and Atmospheric Sciences - Research Publications
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ItemComparison of formaldehyde tropospheric columns in Australia and New Zealand using MAX-DOAS, FTIR and TROPOMI( 2020-07-03)Abstract. South-eastern Australia has been identified by modelling studies as a hotspot of biogenic volatile organic compound (VOC) emissions, however long term observational VOC studies are lacking in this region. Here, two and a half years of MAX-DOAS formaldehyde (HCHO) measurements in Australasia are presented, from Broadmeadows in northern Melbourne, Australia and from Lauder, a rural site in the South Island of New Zealand. Across the measurement period from December 2016 to November 2019, the mean formaldehyde column measured by the MAX-DOAS at Lauder was 2.50 ± 0.61 × 1014 molec cm−2 and at Broadmeadows was 5.40 ± 1.59 × 1015 molec cm−2. In both locations the seasonal cycle showed a pronounced peak in Austral summer (DJF) consistent with temperature-dependent formaldehyde production from biogenic precursor gases. The amplitude of the seasonal cycle at Lauder was 0.7 × 1015 molec cm−2 while it was 2.0 × 1015 molec cm−2 at Broadmeadows. The Lauder MAX-DOAS HCHO measurements are compared with 27 months of co-located fourier-transform infrared (FTIR) observations. The seasonal variation of Lauder MAX-DOAS HCHO, smoothed by the FTIR averaging kernels, correlated strongly with the FTIR measurements, with linear regression slope of 0.91 and R2 of 0.81 for monthly averaged formaldehyde partial columns. In addition to ground-based observations, a clear way to address the VOC measurement gap in areas such as Australasia is with satellite measurements. Here we demonstrate that the Tropospheric Monitoring Instrument (TROPOMI) can be used to distinguish formaldehyde hotspots in forested and agricultural regions of south-eastern Australia. The MAX-DOAS measurements are also compared to TROPOMI HCHO vertical columns at Lauder and Melbourne; very strong monthly average agreement is found for Melbourne (regression slope of 0.61, R2 of 0.95) and a strong agreement is found at Lauder (regression slope of 0.73, R2 of 0.61) for MAX-DOAS vs. TROPOMI between May 2018 and November 2019. This study, the first long term satellite comparison study using MAX-DOAS in the southern hemisphere, highlights the improvement offered by TROPOMI's high resolution over previous satellite products and provides the groundwork for future studies using ground based and satellite DOAS for studying VOCs in Australasia.
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ItemDaytime HONO, NO<sub>2</sub> and aerosol distributions from MAX-DOAS observations in Melbourne( 2018-05-18)Abstract. Nitrogen oxides produced by high temperature combustion are prevalent in urban environments and toxic, contributing to a significant health burden. The chemistry of nitrogen oxides such as NO2 and HONO in pollution are important for hydroxyl radical production and overall oxidative capacity in urban environments, however current mechanisms cannot explain high daytime levels of HONO observed in many urban and rural locations around the world. Here we present HONO, NO2 and aerosol extinction vertical distributions retrieved from MAX-DOAS measurements in suburban Melbourne, which are the first MAX-DOAS results from Australia. Using the optimal estimation algorithm HEIPRO we show that vertical profiles for NO2 and HONO can be calculated with low dependence on the retrieval forward model and a priori parameters, despite a lack of independent co-located aerosol or trace gas measurements. Between December 2016 and April 2017 average peak NO2 values of 8 ± 2 ppb indicated moderate traffic pollution levels, and high daytime peak values of HONO were frequently detected, averaging 220 ± 30 ppt in the middle of the day. HONO levels measured in Melbourne were typically lower than those recorded in the morning in other places around the world, indicating minimal overnight accumulation, but peaked in the middle of the day to be commensurate with midday concentrations in locations with much higher NO2 pollution. Regular midday peaks in the diurnal cycle of HONO surface concentrations have only previously been reported in rural locations. The HONO measured represents an OH radical source in the middle of the day in Melbourne up to ten times stronger than from ozone photolysis. The dependence of the high HONO levels on time since rainfall, combined with the observed diurnal and vertical profiles, provide evidence for a strong photo-activated and ground-based daytime HONO source.
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ItemCoral reef-derived dimethyl sulfide and the climatic impact of the loss of coral reefs( 2020-10-27)Abstract. Dimethyl sulfide (DMS) is a naturally occurring aerosol precursor gas which plays an important role in the global sulfur budget, aerosol formation and climate. While DMS is produced predominantly by phytoplankton, recent observational literature has suggested that corals and their symbionts produce a comparable amount of DMS, which is unaccounted for in models. It has further been hypothesised that the coral reef source of DMS may modulate regional climate. This hypothesis presents a particular concern given the current threat to coral reefs under anthropogenic climate change. In this paper, a global climate model with online chemistry and aerosol is used to explore the influence of coral reef-derived DMS on atmospheric composition and climate. A simple representation of coral reef-derived DMS is developed and added to a common DMS surface water climatology, resulting in an additional DMS flux of 0.3 Tg year−1 S, or 1.7 % of the global flux. By comparing the differences between both nudged and free running ensemble simulations with and without coral reef-derived DMS, the influence of coral reef-derived DMS on regional climate is quantified. In the Maritime Continent-Australian region, where the highest density of coral reefs exist, a small decrease in nucleation and Aitken mode aerosol number concentration and mass is found when coral reef DMS emissions are removed from the system. However, these small responses are found to have no robust effect on regional climate via direct and indirect aerosol effects. This work emphasises the complexities of the aerosol-climate system and the limitations of current modelling capabilities are highlighted, in particular surrounding convective responses to changes in aerosol. In conclusion we find no robust evidence that coral reef-derived DMS influences global and regional climate.
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ItemCloud, precipitation and radiation responses to large perturbations in global dimethyl sulfide( 2017-12-22)Abstract. Natural aerosol emission represents one of the largest uncertainties in our understanding of the climate system. Sulfur emitted by marine organisms, as dimethyl sulfide (DMS), constitutes one fifth of the global sulfur budget and yet the distribution, fluxes and fate of DMS remain poorly constrained. In this study we quantify the role of DMS in the chemistry-climate system and determine the climate's response to large DMS perturbations. By removing all marine DMS in the Australian Community Climate and Earth System Simulator (ACCESS) – United Kingdom Chemistry and Aerosol (UKCA), we find a top of atmosphere radiative effect of 1.7 W m−2. The largest responses to removing marine DMS are in stratiform cloud decks in the Southern Hemisphere's eastern ocean basins. These regions show significant differences in low-cloud (−9 %), radiation (+7 W m−2 in short wave incoming surface radiation) and large-scale rainfall (+15 %) when all DMS is removed. We demonstrate a precipitation suppression effect of DMS-derived aerosol in stratiform cloud deck regions, coupled with an increase in low cloud fraction. The increase in low cloud fraction is an example of the aerosol lifetime effect. Other areas of low cloud formation, such as the Southern Ocean and stratiform cloud decks in the Northern Hemisphere, have a relatively weak response to DMS perturbations. Our study highlights the need for further modelling and field studies of natural aerosols and their impact on cloud and precipitation, in particular in Southern Hemisphere stratiform cloud regions.
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ItemNo Preview AvailableStratospheric ozone intrusion events and their impacts on tropospheric ozone(Copernicus GmbH, 2017-01-05)Abstract. Stratosphere-to-troposphere transport (STT) provides an important natural source of ozone to the upper troposphere, but the characteristics of STT events in the southern hemisphere extratropics and their contribution to the regional tropospheric ozone budget remain poorly constrained. Here, we develop a quantitative method to identify STT events from ozonesonde profiles. Using this method we estimate the seasonality and quantify the ozone transported across the tropopause over Davis (69° S), Macquarie Island (54° S), and Melbourne (38° S). STT seasonality is determined by two distinct methods: a Fourier bandpass filter of the vertical ozone profile, and an analysis of the Brunt-Viäsälä frequency. Using a bandpass filter on 7–9 years of ozone profiles from each site provides clear detection of STT events, with maximum occurrences during summer and minimum during winter above all three sites. The majority of tropospheric ozone enhancements from STT events occur within 2.5 km, 3 km of the tropopause at Davis, and Macquarie Island. Events are more spread out at Melbourne, occurring frequently up to 7.5 km from the tropopause. The mean fraction of total tropospheric ozone attributed to STT during STT events is 2–4 % at each site; however, during individual events over 10 % of tropospheric ozone may be directly transported from the stratosphere. The cause of STTs is determined to be largely due to synoptic low pressure frontal systems, determined using coincident ERA-Interim reanalysis meteorological data. Ozone enhancements can also be caused by biomass burning plumes transported from Africa and South America, these are apparent during austral winter and spring, and are determined using satellite measurements of CO. To provide regional context for the ozonesonde observations, we use the GEOS-Chem chemical transport model, which is too coarsely resolved to distinguish STT events but is able to accurately simulate the seasonal cycle of tropospheric ozone columns over the three southern hemisphere sites. Combining the ozonesonde-derived STT event characteristics with the simulated tropospheric ozone columns from GEOS-Chem, we conservatively estimate that the annual tropospheric ozone flux over the Southern Ocean due to STT events is ~ 3.2 ×1016 molecules cm−2 yr−1. This value is significantly lower than expected from previous global estimates due to the conservative nature of several components of our calculation, in particular the contribution of STT to total tropospheric ozone during an event (STT impact). Using an assumed STT impact of 35 % based on prior modelling studies rather than our observational estimate of 2–4 % increases the estimated Southern Ocean flux by an order of magnitude. Despite lingering uncertainties in scaling ozonesonde measurements to regional values, ozonesonde datasets provide a useful tool for STT detection, and the analysis methods described in this paper could be applied to many existing long-term records.
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ItemEvaluation of the Australian Community Climate and Earth-System Simulator Chemistry-Climate Model(Copernicus GmbH, 2015-07-13)Abstract. Chemistry climate models are important tools for addressing interactions of composition and climate in the Earth System. In particular, they are used for assessing the combined roles of greenhouse gases and ozone in Southern Hemisphere climate and weather. Here we present an evaluation of the Australian Community Climate and Earth System Simulator-Chemistry Climate Model, focusing on the Southern Hemisphere and the Australian region. This model is used for the Australian contribution to the international Chemistry-Climate Model Initiative, which is soliciting hindcast, future projection and sensitivity simulations. The model simulates global total column ozone (TCO) distributions accurately, with a slight delay in the onset and recovery of springtime Antarctic ozone depletion, and consistently higher ozone values. However, October averaged Antarctic TCO from 1960 to 2010 show a similar amount of depletion compared to observations. A significant innovation is the evaluation of simulated vertical profiles of ozone and temperature with ozonesonde data from Australia, New Zealand and Antarctica from 38 to 90° S. Excess ozone concentrations (up to 26.4 % at Davis during winter) and stratospheric cold biases (up to 10.1 K at the South Pole) outside the period of perturbed springtime ozone depletion are seen during all seasons compared to ozonesondes. A disparity in the vertical location of ozone depletion is seen: centered around 100 hPa in ozonesonde data compared to above 50 hPa in the model. Analysis of vertical chlorine monoxide profiles indicates that colder Antarctic stratospheric temperatures (possibly due to reduced mid-latitude heat flux) are artificially enhancing polar stratospheric cloud formation at high altitudes. The models inability to explicitly simulated supercooled ternary solution may also explain the lack of depletion at lower altitudes. The simulated Southern Annular Mode (SAM) index compares well with ERA-Interim data. Accompanying these modulations of the SAM, 50 hPa zonal wind differences between 2001–2010 and 1979–1998 show increasing zonal wind strength southward of 60° S during December for both the model simulations and ERA-Interim data. These model diagnostics shows that the model reasonably captures the stratospheric ozone driven chemistry-climate interactions important for Australian climate and weather while highlighting areas for future model development.
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ItemThe impact of overshooting deep convection on local transport and mixing in the tropical upper troposphere/lower stratosphere (UTLS)(Copernicus GmbH, 2015-01-14)Abstract. In this study we examine the simulated downward transport and mixing of stratospheric air into the upper tropical troposphere as observed on a research flight during the SCOUT-O3 campaign in connection to a deep convective system. We use the Advanced Research Weather and Research Forecasting (WRF-ARW) model with a horizontal resolution of 333 m to examine this downward transport. The simulation reproduces the deep convective system, its timing and overshooting altitudes reasonably well compared to radar and aircraft observations. Passive tracers initialised at pre-storm times indicate the downward transport of air from the stratosphere to the upper troposphere as well as upward transport from the boundary layer into the cloud anvils and overshooting tops. For example, a passive ozone tracer (i.e. a tracer not undergoing chemical processing) shows an enhancement in the upper troposphere of up to about 30 ppbv locally in the cloud, while the in situ measurements show an increase of 50 ppbv. However, the passive carbon monoxide tracer exhibits an increase, while the observations show a decrease of about 10 ppbv, indicative of an erroneous model representation of the transport processes in the tropical tropopause layer. Furthermore, it could point to insufficient entrainment and detrainment in the model. The simulation shows a general moistening of air in the lower stratosphere but it also exhibits local dehydration features. Here we use the model to explain the processes causing the transport and also expose areas of inconsistencies between the model and observations.
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ItemNo Preview AvailableBoundary layer new particle formation over East Antarctic sea ice - Possible Hg driven nucleation?(Copernicus GmbH, 2015-07-16)Abstract. Aerosol observations above the Southern Ocean and Antarctic sea ice are scarce. Measurements of aerosols and atmospheric composition were made in East Antarctic pack ice on-board the Australian icebreaker Aurora Australis during the spring of 2012. One particle formation event was observed during the 32 days of observations. This event occurred on the only day to exhibit extended periods of global irradiance in excess of 600 W m−2. Within the single air-mass influencing the measurements, number concentrations of particles larger than 3 nm (CN3) reached almost 7700 cm−3 within a few hours of clouds clearing, and grew at rates of 5.6 nm h−1. Formation rates of 3 nm particles were in the range of those measured at other Antarctic locations at 0.2–1.1 ± 0.1 cm−3 s−1. Our investigations into the nucleation chemistry found that there were insufficient precursor concentrations for known halogen or organic chemistry to explain the nucleation event. Modelling studies utilising known sulfuric acid nucleation schemes could not simultaneously reproduce both particle formation or growth rates. Surprising correlations with Total Gaseous Mercury (TGM) were found that, together with other data, suggest a mercury driven photochemical nucleation mechanism may be responsible for aerosol nucleation. Given the very low vapour pressures of the mercury species involved, this nucleation chemistry is likely only possible where pre-existing aerosol concentrations are low and both TGM concentrations and solar radiation levels are relatively high (~ 1.5 ng m−3 and &geq; 600 W m−2, respectively), such as those observed in the Antarctic sea ice boundary layer in this study or in the global free-troposphere, particularly in the Northern Hemisphere.