Abstract
The nucleation of atmospheric vapours is an important source of new aerosol particles that can subsequently grow to form cloud condensation nuclei in the atmosphere1. Most field studies of atmospheric aerosols over continents are influenced by atmospheric vapours of anthropogenic origin (for example, ref. 2) and, in consequence, aerosol processes in pristine, terrestrial environments remain poorly understood. The Amazon rainforest is one of the few continental regions where aerosol particles and their precursors can be studied under near-natural conditions3,4,5, but the origin of small aerosol particles that grow into cloud condensation nuclei in the Amazon boundary layer remains unclear6,7,8. Here we present aircraft- and ground-based measurements under clean conditions during the wet season in the central Amazon basin. We find that high concentrations of small aerosol particles (with diameters of less than 50 nanometres) in the lower free troposphere are transported from the free troposphere into the boundary layer during precipitation events by strong convective downdrafts and weaker downward motions in the trailing stratiform region. This rapid vertical transport can help to maintain the population of particles in the pristine Amazon boundary layer, and may therefore influence cloud properties and climate under natural conditions.
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Acknowledgements
Institutional support was provided by the Central Office of the Brazilian Large Scale Biosphere Atmosphere Experiment in Amazonia (LBA), the Brazilian National Institute of Amazonian Research (INPA), the Brazilian National Institute for Space Research (INPE), Amazonas State University (UEA) and Amazonas State (SDS/CEUC/RDS-Uatumã). We acknowledge the Atmospheric Radiation Measurement (ARM) Climate Research Facility, a user facility of the United States Department of Energy (US DOE), Office of Science, sponsored by the Office of Biological and Environmental Research. Funding was obtained from the Atmospheric System Research (ASR) programme (Office of Biological and Environmental Research of US DOE, under contract DE-AC02-98CH10886), the Amazonas State Research Foundation (FAPEAM-GoAmazon), the São Paulo Research Foundation (FAPESP, project numbers 2013/50510-5, 2013/05014-0 and CHUVA 2009/15235-8), the Brazil Scientific Mobility Program (CsF/CAPES-CNPq), the Brazilian Ministry of Science, Technology, and Innovation (MCTI/FINEP contract 01.11.01248.00), the German Max Planck Society (MPG) and the German Federal Ministry of Education and Research (BMBF contract 01LB1001A). This work contains results of research conducted under the Technical/Scientific Cooperation Agreement between the National Institute for Amazonian Research, Amazonas State University, and the Max Planck Society. The work was conducted under scientific licenses 001030/2012-4, 001262/2012-2 and 00254/2013-9 of the Brazilian National Council for Scientific and Technological Development (CNPq). The opinions expressed herein are the entire responsibility of the authors and not of the participating institutions.
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J.W., S.T.M., M.O.A., P.A., L.A.T.M. and R.A.F.S. designed the research. J.W., C.K., H.M.J.B., J.B., S.C., X.C., F.D., J.L., F.M., D.M.-Z., C.P., M.L.P., J.S., S.R.S., J.M.T. and D.Wa. carried out the measurements. J.W. led the analyses, and J.W. and S.T.M. led the writing, with major input from L.A.T.M., M.O.A., S.G., R.K., T.P. and H.M.J.B., and further input from all other authors.
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Reviewer Information Nature thanks H. Coe, B. Wehner and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
Extended Data Figure 1 Locations of the T0a and T3 sites during GoAmazon 2014/5.
Map data: Google Earth.
Extended Data Figure 2 Density of back trajectories of air masses arriving at the T0a site during the wet season from 1 March to 31 May 2014.
Back trajectories were originated at 100 m above the ground every 30 min using the HYSPLIT model with 0.5° resolution winds from the US National Oceanic and Atmospheric Administration Global Data Assimilation System. Map data: Google.
Extended Data Figure 3 Vertical profiles measured onboard G-1.
a–i, Same as Fig. 1 for measurements made onboard G-1 at different altitudes under background conditions from 15:50–16:32 on 14 March 2014 (a–c), from 17:42–19:00 on 17 March 2014 (d–f) and from 14:54–17:00 on 19 March 2014 (g–i). For the flights on 14 March and 17 March the measurements are averaged over horizontal legs that were outside clouds or precipitation regions to avoid artefacts that potentially arise from the shattering of cloud droplets, ice particles or rain drops on the G-1 sampling inlet. The quantities are averages of 77, 92, 126 and 108 1-s measurements over horizontal legs that correspond to spatial scales of 8, 10, 15 and 11 km for the four altitudes shown for 14 March, and averages of 126, 288, 58 and 360 1-s measurements over horizontal legs that correspond to spatial scales of 11, 27, 7 and 42 km for the four altitudes shown for 17 March. For the flight on 19 March 2014 there was widespread stratiform rain and ice precipitation over the G-1 sampling area. The quantities, including aerosol size spectra and particle number concentrations, are averaged over the portion of the horizontal legs with the lowest total water contents measured by a multi-element water content system (Model WCM-2000, SEA) to minimize the impact of the fragmentation of cloud droplets, rain drops and ice particles on the G-1 sampling inlet47 on particle measurements. The quantities are averages of 54, 54, 72, 90 and 108 1-s measurements over horizontal legs that correspond to spatial scales of 5, 5, 7, 10 and 13 km for the five altitudes ranging from 625 m to 7,100 m. The average total water contents measured by the WCM-2000 for the portions of the horizontal legs were 0.007, 0.01, 0, 0.007 and 0.008 g m−3 at the five altitudes. The precipitation water content was derived from the drop spectrum measured by a high-volume precipitation spectrometer (HVPS, version 3, SPEC) and the average values were 0.028, 0.042, 0.0043, 0 and 0.2838 g m−3 for the five altitudes ranging from 625 m to 7,100 m. The nucleation-mode particles with diameters around 10 nm at 1,600 m (Fig. 2g) were probably artefacts due to the remnant impact of rain drops shattering on the G-1 inlet.
Extended Data Figure 4 Radar reflectivity and vertical air motion.
a–d, Reflectivity factor measurements (a, c) and vertical air motion estimates (b, d) from the 1,290 MHz wind profiler deployed at the T3 site for a deep convection system on 19 March 2014 (a, b) and 20 March 2014 (c, d). Larger reflectivity factor measurements are associated with heavier precipitation. Vertical air motions were derived from profiler Doppler velocity48. Strong surface-driven convective updrafts reaching altitudes above 12 km were observed at 08:40 on 19 March and 11:30 on 20 March. Adjacent to deep convective updraft cores, deep convective downdrafts from the free troposphere to the surface were often observed, as well as compensating downward motions over the extended trailing stratiform regions. Such downward motions (which have maximum velocities up to 10 m s−1) were consistently observed during precipitation in the wet season, and are anticipated to transport free tropospheric air with high concentrations of small particles into the boundary layer.
Extended Data Figure 5 Forward trajectories of air masses starting at the T0a site.
Forward trajectories starting at altitudes ranging from 625 m to 4,850 m above the T0a site around the precipitation event (that is, 04:00–06:30) on 19 March 2014 and ending at the time of G-1 sampling at the respective altitudes on the same day. The trajectories are coloured by altitude, and the end of each trajectory is indicated by a star symbol. Filled circles represent the G-1 sampling locations coloured by sampling altitude. The average distances between the ends of the trajectories and the G-1 sampling locations are 140, 260, 300 and 140 km for the sampling altitudes of 625, 1,600, 3,200 and 4,850 m, respectively. This suggests that the air masses over T0a around the precipitation event reached similar locations and altitudes to those of G-1 at the time of G-1 sampling, especially at the altitudes of 625 m and 4,850 m. Map data: Google.
Extended Data Figure 6 Impact of vertical transport during precipitation on boundary layer aerosol.
Another example demonstrating the contribution of the vertical transport of free tropospheric small particles to the particle concentration in the atmospheric boundary layer at the T0a site during a precipitation event on 4 May 2014. a, Water vapour mixing ratio, precipitation rate and θe. b, The total particle number concentration (N), the concentration of small particles with a diameter, Dp, of less than 50 nm (N<50) and the concentration of CCN-sized particles with Dp larger than 100 nm (N>100). c, d, Particle size spectra (dN/dlogDp) at ground level.
Extended Data Figure 7 Seasonal trend and diel variation of equivalent potential temperature.
a, Time series (blue line) of θe derived from surface measurements at the T0a site from 1 March to 31 May 2014 and its seasonal trend (red line) fitted using a second-order polynomial. b, Diel variation of the detrended equivalent potential temperature (
) at the T0a site from 1 March to 31 May 2014. The error bars represent 1 s.d. of 11,040 1-min measurements.
Extended Data Figure 8 Average precipitation as a function of Δθe.
a, Number of data points for each Δθe bin. b, Statistics of the average precipitation rate during the two hours immediately before the Δθe measurement. The box and whisker plots are drawn for the 10th, 25th, 50th, 75th and 90th percentiles. The black circles represent the mean values. The negative Δθe values were typically observed following precipitation, especially for values below −5 K.
Extended Data Figure 9 Variation of particle number concentration with Δθe under polluted conditions.
Statistics of total particle number concentration measured by a condensation particle counter (TSI, model 3010) at the T3 site from 1 March to 31 May 2014 for each Δθe bin (that is, same as Fig. 3d except that measurements were taken at the T3 site). A reduced particle number concentration was observed at low Δθe values at the T3 site, in contrast to the almost constant particle number concentration with Δθe under background conditions at the T0a site. This suggests that, under polluted conditions, the vertical transport of free tropospheric aerosol cannot compensate particles removed by precipitation. The impact of vertical transport of free tropospheric particles becomes masked as anthropogenic emissions become the major source of particle number in the boundary layer under polluted conditions.
Extended Data Figure 10 Variations in the particle number concentrations with filtered θe.
Same as Fig. 3 except that the analysis is based on θe processed by a first-order Butterworth filter to remove the variations in θe with timescales longer than 20 h (that is, with a frequency of less than 1.2 d−1). a, Number of data points for each filtered θe bin. b, Statistics of small particle (<50 nm in diameter) concentrations for each filtered θe bin. c, Statistics of the CCN-sized particle (>100 nm in diameter) concentration. d, Statistics of total particle number concentration. The box and whisker plots are drawn for the 10th, 25th, 50th, 75th and 90th percentiles. The black circles represent the mean values.
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Wang, J., Krejci, R., Giangrande, S. et al. Amazon boundary layer aerosol concentration sustained by vertical transport during rainfall. Nature 539, 416–419 (2016). https://doi.org/10.1038/nature19819
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DOI: https://doi.org/10.1038/nature19819
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