Airmass characterisation along the 20-degree South parallel during the VOCALS-REx campaign

Grant Allen1 H. Coe1, C. Bretherton2, R. Wood2 , T. Clarke3, S. Freitag3, S. Abel4, P. Kreckl5, P. I. Williams1, T. Toniazzo6, Y-N.Lee7, L. Kleinmann7 and the VOCALS Team

1University Of Manchester, UK, 2University of Washington, 3University of Hawaii, 4UK Met Office, 5University of Leeds, 6University of Reading, 7Brookhaven National Laboratory.

The VAMOS Ocean-Climate-Atmosphere-Land Regional Experiment (VOCALS-REx) was conducted from October to December 2008 from Arica, Chile, to record airborne, surface and cruise-ship data to investigate the mechanistic interaction between land, sea and atmosphere in the SEP and to improve the skill of global models in representing the region. This paper synthesises airborne measurements from three aircraft involved in VOCALS-Rex to summarise the atmospheric composition of the marine boundary layer and the lower free troposphere along the 20-degree South parallel in terms of aerosol particle size and composition, carbon monoxide and ozone during distinct meteorological regimes over the course of the campaign. The climatology presented here aims to provide a valuable dataset to inform model simulation and evaluation of dynamical processes in the SEP region for conditions analogous to those during VOCALS.

From 15th October to November 2nd 2008 (period 1), surface conditions remained relatively typical of the South East Pacific (SEP), with a surface anticyclone and barrier flow parallel to the South American coastline. Surface winds were predominantly south-south easterly near the coast, and veering further offshore. A localised trough to the south of the region passed through to the East, reducing cloud cover over the SEP. A disturbed sub-tropical jet aloft resulted in variable wind patterns and sheer throughout the free and upper troposphere. Between 2nd Nov and the end of the campaign on 16th Nov (period 2), a more quiescent jet-stream resulted in less variable free tropospheric wind patterns and sheer. The prevailing surface anticyclone remained and flow conditions were less variable than period 1, with greater cloud cover in general as demonstrated in Figure 1.

•Carbon monoxide (CO) is an long-lived (~ 1 month in the free troposphere) tracer of combustion sources, including anthropogenic and biomass burning.

•Ozone is more short-lived (~few days) and is both created and destroyed in catalytic cycles in the presence of many volatile precursors. It’s correlation with CO can yield clues to airmass photochemical age.

•Figure 2 shows the longitudinal gradient and the sampling variability in both CO and ozone. There is a clear decreasing gradient in CO for both periods offshore from around 72 ppbv in the coastal area, to ~64 ppbv in the remote SEP. Furthermore, the greatest variability is observed near the coast in both cases.

Carbon monoxide, ozone and aerosol size and composition data measured by three aircraft during the VOCALS field campaign, have been synthesized to produce a set of composition statistics relevant to the environment( at the 20 S line of latitude) and meteorological conditions of the South East Pacific typical of Austral Spring. A longitudinal gradient in CO concentration was seen to be correlated with mass loadings of ammonium and sulphate aerosol indicating a common land-based source, rapidly weakening in abundance away from the coastal area. In the remote SEP (west of 76 W), little variability in gas and aerosol was observed, with significant variability in the near coastal zone, consistent with diurnal changes in coastal atmospheric dynamics, coupled with the variability in emissions from land-based sources.

These conclusions were confirmed using back trajectories, which show that coastal air has mostly circulated in the near coast boundary layer and is some cases has circulated from the Brazil to the North of the Andes, whilst MBL air in the remote SEP has in all cases resided over the remote Pacific for the past 10-days at least, with some air having been transported long-range in an upper level jet before experiencing rapid descent. The accuracy of winds used to model back trajectories was investigated by a comparison with aircraft measured winds, yielding excellent agreement and lending confidence in ECMWF wind fields in the SEP. The statistics derived here will aid modellers by providing a set of background composition conditions to initialize climate and cloud-resolving models.

Address: CAS, University Of Manchester, Simon Building, Oxford Rd, Manchester, UK. M13 9PL Email: Grant Allen: grant.allen@manchester.ac.uk Web: http://www.seaes.manchester.ac.ukAcknowledgements: NERC and the UK Met Office for funding the UK contingent to the VOCALS campaign. The NERC Facility for Airborne and Atmospheric Measurement (FAAM).

Figure 4: Aerosol mass loadings measured by the AMS for Period 1 (left column) and Period 2 (right column). Colours correspond to aerosol type (red=sulphate, yellow=ammonium, green=organics)

Figure 6. Comparison of V (left, southerly) and U (right, westerly) wind components as measured during straight and level runs by the BAe-146, with ECMWF 0.125 degree gridded operational analyses, taken at nearest sigma levels (in terms of

pressure. Symbols are colour-coded for time, with red, orange and yellow representing the earlier flights of VOCALS, and green and blue symbols representing

later flights.

•Figure 4 (left) shows aerosol mass loadings for ammonium (NH4), sulphate (SO4) and organics, measured by Aerodyne Aerosol Mass Spectrometers on all three aircraft (nitrate aerosol concentrations were below the limit of detection with retrieved values averaging to zero)

•A longitudinal gradient in SO4 and NH4 was once again observed, which correlated well with CO (R2=0.97 for SO4, R2=0.94 for NH4). Also, the greatest variability (interquartile range) is seen near the coast in both cases. In all locations, there is insufficient NH4 to neutralise SO4, indicating highly acidic aerosol.

•Of particular interest is the relationship between organic aerosol and SO4 – in Period 1 we see a gradient in organics which is reasonably well correlated withSO4, yet this is not true of Period 2. The reason for this apparent change in aerosol source is investigated in the following section.

Meteorological Summary

Carbon Monoxide and Ozone

•Figure 3 shows averaged aerosol size spectra covering the fine and accumulation aerosol size modes.

• In period 1 we see a little variability in spectral modality or aerosol number in the coastal and transition region, with evidence of a reduction in aerosol in the remote SEP only.

•In period 2, there is no evidence of a significant gradient in fine and accumulation mode aerosol with longitude.

•This lack of a gradient in accumulation mode aerosol is, however, coincident with a gradient in aerosol total mass and composition (as observed by the AMS instruments – see next section). This suggests that the variability in aerosol mass that is observed is confined to coarse mode (larger size) aerosol particles, with a greater abundance of larger size aerosol near the coast and a reduction offshore. This is consistent with the expected predominance of a source of large mineral (and other) particles on land and blown out to sea.

Introduction:

Summary

Figure 1 GOES10 Infrared (Channel 4) derived brightness temperatures on 26th Oct 2008 (leftmost panels) and 11th November 2008 (rightmost panels) , typical of period 1 and period 2 respectively. Typical morning and evening loud fields are shown to demonstrate the typical filling in of marine SCu cloud overnight. Rainbow coloured clouds represent mid and high level cirrus over the Andes.

• Carbon monoxide and aerosol size and composition data used here are taken from three aircraft which operated out of Arica Airport, Chile, and flew surveys of the marine boundary layer (MBL) and free troposphere between the coast (~70.5 W) and the remote SEP (85 W). A key component of many missions was to fly along the 20 South latitude line to record a set of consistent and representative statistics of SEP composition appropriate to the meteorological conditions in which measurements were taken.

•Data used here are taken from the British BAe-146 FAAM aircraft and the US NCAR C-130 and G-1. Data from a total of 42 flights are used here, representing a total of >250 hours of science flying along the 20 South latitude line. Care was taken to ensure that data from individual instruments was of good quality through careful calibration and inter-platform intercomparison runs during two science flights to examine consistency between instruments. No significant biases were found, with all measurements seen to fall within the natural sampling variability observed through the course of intercomparison runs.

•For the purposes of the statistics presented here, all data are gridded into bins representing two degrees of longitude and divided into the two distinct meteorological periods outlined above. Data are further selected according to the following constraints: 1/ Simultaneously measured cloud liquid water content < 0.05 g/m3 (to screen cloud contaminated data) 2/ ΔT/ΔZ <0.04 K/hPa (using closest available thermodynamic profile, to ensure data are in the MBL and below the inversion height), 3/ -20.5<latitude<-19.5 (to ensure they represent the 20 S latitude).

Figure 2 Longitudinal gradient in marine boundary layer ozone (top) and carbon monoxide (bottom), averaged into 2-degree bins and within 0.5 degrees of 20 South. Central bar represents median concentration, boxes are quartiles and whiskers upper and lower deciles.

Aerosol composition

•Ozone displays little change in concentration with longitude, with median concentrations in the range 25 ppbv to 30 pppv at all longitudes in both periods. However, the near coast region is an exception with an apparent decrease relative to other longitudes. This decrease in ozone, coincident with an increase in CO suggests a relatively fresh (<2 day) combustion source for the CO sampled.

Aerosol size spectra

Figure 3 (above): Averaged ambient aerosol size spectra, normalised for bin width and colour-coded for longitude. Data between 20 nm and 300 nm were recorded by the BAe-146 SMPS instrument , with data between 120 nm and 1.5 microns measured by the internal

BAe-146 internal PCASP

Back Trajectory Analysis

Data selection

•Figure 5 (left) shows 3D ECMWF 10-day back trajectories initiated at 0.5 degree intervals along the 20 S line of latitude between 70.5 W and 90 W for both periods.

•In period 1, air to the East of 77 W mostly resided in the MBL for the previous 10 days, having been circulated around Brazil from the North of the Andes. Such air is might be expected to carry biomass burning and other biogenic material. In contrast, MBL air to the West of 77 W experienced rapid descent from an upper level jet and might be expected to be extremely clean.

•In period 2, all but the air in the near coast (East of 72 W) experienced long range transport over the remote SEP, residing mostly in the lower free troposphere, with air in the near coast having been advected along the coastline from the South. This suggests little land-sourced aerosol in all but the near coastal zone.

Figure 5:10-day ECMWF 3D back trajectories initiated at 0.5 degree intervals along the 20 S latitude line and typical of period 1 (upper panels) and Period 2 (lower

panels). The left hand panels show the path of trajectories mapped horizontally onto the Earth’s surface and colour-coded for their origin along the 20 S latitude line. The right hand panels illustrate the vertical path of trajectories with time and are similarly colour-coded for their origin. Symbols plotted on trajectories in the left hand panels

mark one-day intervals.

•Figure 6 shows a comparison between BAe-146 measured winds and coincident ECMWF analysed winds. The good agreement and Gaussian scatter around the 1:1 line lend confidence in the winds used to model back-trajectories used here.

•Furthermore, there is little evidence of systematic bias with time, suggesting that ECMWF winds are well represented for meteorological conditions during the VOCALS campaign.