Atmospheric ice nuclei concentrations and characteristics constraining the role of biological ice nuclei

Transcription

1 Atmospheric ice nuclei concentrations and characteristics constraining the role of biological ice nuclei Paul J. DeMott, Mathews S. Richardson, Daniel.J. Cziczo,, Anthony J. Prenni and Sonia M. Kreidenweis

2 Overview of Today s s Talk How we measure ice nuclei and infer sources and characteristics in the atmosphere The characteristics of ice nuclei based on measurements over the last decade or so Concentrations Sizes Compositions Some present limitations on assessing the likelihood that bio-aerosols play a role as ice nuclei (sizes measured, appearance, where/when sampled)

3 Measuring ice nuclei: Continuous flow diffusion chamber (CFDC) or PCVI extracts to single particle mass spectrometer OPC aerosol μm Activated IN

4 What ice nucleation mechanisms do we measure? Aerosols nucleate ice by varied mechanisms, most depending on T, some on RH as well No instrument yet capable of measuring all mechanisms What can a CFDC measure? Is immersion freezing a singular process on the time scale of measurement? Contact freezing nuclei should be defined by the total numbers of immersion freezing nuclei active at somewhat lower temperature? Yes No, but Yes Yes/maybe

5 Sampling in various locations in various seasons from the surface and from aircraft aircraft aerosol sample inlet Sampling at surface sites CVI inlet (aerosol from evaporated cloud particles) Compact instrument certified for sampling on any aircraft platform

6 What are the concentrations of ice nuclei in the atmosphere?

7 IN concentration dependence on temperature IN IN concentration concentration (per (per liter) liter) IN concentration (per liter) Temperature ( o ( C) o C) ( o C) INSPECT-I INSPECT-I binned binned INSPECT-I binned INSPECT-II binned INSPECT-II binned M-PACE binned INSPECT-II binned FIRE-ACE M-PACE binned WISP94 (Winter- Spring) FIRE-ACE Meyers et binned al. (1992); Meyers RHw et = al. 100% (1992); RHw = 100% Fletcher Meyers et al. (1992); RHw Fletcher = 100% Phillips et al. (2007) 'background' Fletcher CRYSTAL-FACE: SAL dust Phillips et al. (2007) PacDEx: Phillips 'background' Asian et al. dust (2007) 'background'

8 IN concentration dependence on supersaturation with respect to ice IN concentration (per liter) Ice Supersaturation (%) INSPECT-I (Fall) binned INSPECT-II (Spring) binned M-PACE binned WISP94 (Winter- Spring) FIRE-ACE (May 1998) Meyers et al. (1992) Phillips et al. (2007) 'Background' CRYSTAL-FACE: SAL dust PacDEx: Asian dust

9 Cumulative distribution of [IN] and sampling conditions in many projects MPACE: Arctic Fall FIRE-ACE: Arctic Spring INSPECT-2: Western U.S. Spring CRYSTAL-FACE: Florida July Increasing dust particle impacts winter to summer

10 Do ice nuclei relate to ice in clouds?

11 Winter Icing in Storms Project (1994) wave cloud [ice (>50 μm)] versus [IN] IN or Ice Crystal Conc. (L -1 ) CFDC IN Data DCC IN Data Cloud Ice Conc Temperature ( o C)

12 IN data versus ice in clouds for Arctic stratus (spring versus fall seasons) [2Dc>125μm],[IN] (per liter) 2Dc > 125 μm ice versus IN MPACE ice clouds MPACE IN FIREACE ice clouds FIREACE IN Temperature ( o C)

13 Where do IN come from? What are their physical and chemical characteristics?

14 Ice nuclei physical size from TEM analyses in several programs

15 Relation of IN to larger aerosol using a variety of data sets Aerosol conc. > 0.5 μm (cm -3 ) 100 y = x r 2 = INSPECT-2 (>1 hr) CRYSTAL-SAL dust AIRS-11/14/ AIRS-11/19/03 PacDEx Asian dust Power law fit IN concentration (per liter)

16 Mineral dust and metallic particles dominate IN compositions (Cziczo( et al. 2003; Richardson et al. 2007) Results from PALMS (Particle Ablation Laser Mass Spectrometry) analysis of ambient aerosol particles PALMS analyses of IN (residues of ice crystals) activated in the CFDC and aerodynamically extracted from flow.

17 IN composition (TEM/EDS) in several studies (arctic, subtropics, midlatitudes represented) C: Carbonaceous as inferred by absence of elemental signature DM: dust and metallic (some oxides and some not)

18 Many laboratory studies support the role of mineral dusts as IN, but our data suggests larger dusts particles needed to account for ice i formation at very warmest temperatures Homogeneous freezing of sulfates Water Saturation 200 nm Canary Island 300 nm 400 nm RHw = 80%

19 Role of carbonaceous particles as IN (TEM vs PALMS apparent discrepancy) = sulfates+organics 2 = carbonaceous 3 = minerals 4 = metal (oxides) 5 = other Cumulative fraction Si,metal oxide-particles C-particles Length (μm)

20 What are these carbonaceous IN? irregular amorphous structure sooty Lab graphitized soot

21 Biomass burning smoke particles tested in laboratory only rarely a source of measurable IN Most smokes froze at low temperature at the conditions identified for homogenoeus freezing of soluble haze particles Utah sage/rabittbrush mix indicates apparent modest numbers of freezing nuclei (only one of more than 40 tested)

22 PACDEX vertical profile over Central Pacific Ocean (May 23, 2007) Likely dust plumes, but note rise of black carbon (SP-2) particle concentrations in hand with larger particles. [CO] (ppbv) [IN], [SP2], [CCN-0.1%] (cm -3 ) Filter [CO] [IN]-CFDC [CIP_SP2] [UHSAS>500nm] Pressure Alt. (ft) T = -32ºC; RH = %

23 Some present limitations We are designing instruments to detect ice formation by larger aerosols, but by removing the distinction of ice and aerosol by size, how do we extract ice crystals to measure IN composition?

24 New detectors should allow elimination of activated IN detection by size alone

25 Some present limitations We are designing instruments to detect ice formation by larger aerosols, but by removing the distinction of ice and aerosol by size, how do we extract ice crystals to measure IN composition? We have not sampled much directly in regions likely to have strong biological inputs (lower altitude areas over oceans and source plant surfaces) How best to identify biological IN?

26 Summary IN concentrations in the atmosphere range from below cm -3 to as high as a few cm -3. Nevertheless, typical values are to 0.01 cm-3 3 and rarely exceed a 0.1cm -3. Lower IN concentrations are found particularly in cold seasons/regions and at times through elevated layers of the atmosphere. Desert dust is usually responsible for highest [IN]. IN compositional measurements confirm sources from mineral dusts, metallic particles (some may be anthropogenic) and substantial but variable and unidentified numbers of carbonaceous particles. Biomass smokes are mostly poor sources of IN.

27 Summary (continued) IN, while having mode sizes between μm, extend into the supermicron range where we have the weakest instrumental capabilities for identifying their compositions. Niether the apparent physical morphology, nor typical sizes of IN support a strong biological source, but measurements have not emphasized the expected size range of these and sampling has not focused very much on seasons and locations where a strong bio-input input is most expected. Acknowledgments: U.S. NSF grants ATM , ATM , ATM07????????;; NCAR Research Aviation Facility; DOE-ARM; NASA NAG ; U.S.Forest Service, National Park Service, Mike Poellot,, Randy Borys,, Doug Westphal,, Jeff Stith,, David Rogers, Ottmar Moehler.