Cape Grim

Abstract. Model-measurement comparisons of HOx in extremely clean air ([NO]<3 ppt) are reported. Measurements were made during the second Southern Ocean Photochemistry Experiment (SOAPEX-2), held in austral summer 1999 at the Cape Grim Baseline Air Pollution Station in northwestern Tasmania, Australia. 

Cape Grim is an ideal location to study free-radical chemistry in extremely clean conditions (Penkett et al., 1997). It frequently experiences air masses characterized by low condensation nuclei (CN) and Radon counts (<462 cm-3 and < 100 mBq m-3, respectively) with the local wind direction in the sector 190°-280°. In these baseline conditions, air has not passed over land for 5 days or more and is therefore relatively free of anthropogenic influence. Four days, which were characterised by the lowest NOx and NMHCs levels experienced during the campaign, have been selected to be representative of baseline conditions in the Southern Ocean. 

Light non-methane hydrocarbons (NMHCs) were measured using an automated GC-FID system with large volume sample collection onto a Peltier cooled carbon sieve trap followed by on-line thermal desorption, and separation on an aluminium oxide PLOT capillary column. The system deployed at Cape Grim has been described in more detail in a previous paper (Lewis et al., 2001). 

ACE1, Cape Grim, Tasmania, Nov.-Dec. 1995 Baseline sector 40.7° S, 144.7° E 2.0xl0-7... 

Two observationally constrained box-models, based on the Master Chemical Mechanism and with different levels of chemical complexity, have been used to study the HOx radical chemistry during the SOAPEX-2 campaign, which took place during the austral summer of 1999 (January-February) at the Cape Grim Baseline Air Pollution Station in northwestern Tasmania, Australia. The box-models were constrained to the measured values of long lived species and photolysis rates and physical parameters (NO, NO2, O3, HCHO, j(01D), j(N02), H2O and temperature). In addition the detailed model was constrained to the measured concentration of CO, CH4 and 17 NMHCs, while the simple model was additionally constrained only to CO and CH4. The models were updated to the latest available kinetic data and completed with a simple description of the heterogeneous uptake and dry deposition processes. 

Ayers, G. P. and Galbally, I. A preliminary investigation of a boundary layer - free troposphere entrainment velocity at Cape Grim, Baseline 92, Department of Arts, Sport, Environment, Tourism and Territories, Canberra A.C.T., Australia, 1994. 

Covert, D. S., Gras, J. L., Wiedensohler, A., and Stratmann, F. Comparison of directly measured CCN with CCN modeled from the number-size distribution in the marine boundary layer during ACE 1 at Cape Grim, Tasmania, J. Geophys. Res.-A., 103, 16597-16608,1998. 

FIGURE 6.31 Measured peroxy radical concentrations ([H02 ] + [ROj]) and ozone photolysis rate /(O D) (a) or /(O D) 05 (b) in clean marine air at Cape Grim, Tasmania (adapted from Penkett et al., 1997). 

Figure 6.31 shows plots of measured peroxy radical concentrations for low-NOx conditions at Cape Grim, Tasmania (Penkett et al., 1997). Overlaid are plots of 7(0 D) and /(O D) 05. The plot of /(O D) 05 provides a better match. The slower decay in the peroxy radical concentration at dusk is due to the slow decay due to self-reactions, with some contribution from the CH302 + O, reaction (k 1 X 10"17 cnr1 molecule 1 s 1 Tyndall et al., 1998) and perhaps a small contribution from deposition (Monks et al., 1996). 

Murphy, D. M., D. S. Thomson, and A. M. Middlebrook, Bromine, Iodine, and Chlorine in Single Aerosol Particles at Cape Grim, Geophys. Res. Lett, 24, 3197-3200 (1997). 

FIGURE 11.54 Diurnal profile of average (H02 + R02) concentrations measured at Cape Grim, Tasmania ( ), and at Mace Head, Ireland ( ), under clean air conditions using a chemical amplification technique. (Adapted from Carpenter el at., 1997.)... 

Although not fully fluorinated, HFC-23 (CHF3) also has a long lifetime ( 250 years WMO, 1995) because its reaction with OH is slow, k298 = 2.8 X 10 1(1 cm1 molecule-1 s-1 (DeMore et al., 1997). A major source is the production of HCFC-22 (CHC1F2), where HFC-23 is a byproduct. As the use of HCFC-22 and its atmospheric levels have increased (Fig. 13.6), the levels of HFC-23 would be expected to increase as well, and indeed, this is the case. Oram et al. (1998) reported the first atmospheric measurements of this compound at Cape Grim, Australia. HFC-23 increased from 2 ppt in 1978 to 11 ppt in 1995, with a growth rate of 5% per year in 1995. 

FIGURE 14.17 Atmospheric methane concentrations over the past 1000 years. Different symbols represent data from ice cores in Antarctica and Greenland and the Antarctic firm layer. Line from 1978 includes air measurements at Cape Grim, Tasmania (adapted from Etheridge et al., 1998). 

It should be explored whether halogen activation reactions may also occur under different circumstances than at polar sunrise (33, 34). Recent observational studies by Ayers and colleagues at Cape Grim, Tasmania, indicate Br release from seasalt (35). 

Cox ML, Sturrock GA, Fraser PJ, Siems ST, Krummel PD, O Doherty S (2003) Regional Sources of Methyl Chloride, Chloroform and Dichloromethane Identified from AGAGE Observations at Cape Grim, Tasmania, 1998-2000. J Atmos Chem 45 79... 

Cox ML, Fraser PJ, Sturrock GA, Siems ST, Porter LW (2004) Terrestrial Sources and Sinks of Halomethanes Near Cape Grim, Tasmania. Atmos Environ 38 3839... 

Case Study II — Photochemical control of ozone in the remote marine boundary layer (MBL) - An elegant piece of experimental evidence for the photochemical destruction of ozone comes from studies in the remote MBL over the southern ocean at Cape Grim, Tasmania (41 In the MBL, the photochemical processes are coupled to physical processes that affect the observed ozone concentrations, namely deposition to the available surfaces and entrainment from the free troposphere. The sum of these processes can be represented in the form of an ozone continuity equation (a simplified version of Equation 2.6), viz... 

Figure 14 Average diurnal cycles for peroxide (open squares) and ozone (filled squares) in baseline air at Cape Grim, Tasmania (41° S) for January 1992 ...

Galbally I. E., Bentley S. T., and Meyer C. P. M. (2000) Midlatitude marine boundary-layer ozone destruction at visible sunrise observed at Cape Grim, Tasmania, 41° S. Geophys. Res. Lett. 27, 3841-3844. 

Middlebrook A. M., Murphy D. M., and Thomson D. S. (1998) Observations of organic material in individual marine particles at Cape Grim during the First Aerosol Characterization Experiment (ACE 1). J. Geophys. Res. 103, 16475-16483. 

Figure 4 Transmission electron microscope (TEM) image of an aggregate of NaCl and sulfate salts from a small marine particle. This particle is at the low end of the observed size range of sea salt particles. The sample was collected near Cape Grim, Tasmania, Southern Ocean during the ACE-1 experiment (source Buseck and Posfai, 1999).

Figure 13 Mass spectrum of a single marine aerosol particle (collected at Cape Grim, Australia). It shows both the rich chemical detail as well as the complex interpretative back-calculation problem connected with trying to sort out which species were present in such a particle it presumably contained sea salt, sulfate, and organic material (Murphy et aL, 1998 reproduced by permission of Nature Publishing Group from Nature, 1998, 392, 62-65).

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