HR: 1340h
AN: B33A-0233    [Abstracts]
TI: Solar dimming and urban aerosol distribution in New York Metropolitan area
AU: * Hannon, P
EM: mgh2001@columbia.edu
AF: Columbia University, 2960 Broadway, New York, NY 10027 United States
AU: Liepert, B
EM: liepert@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Chillrud, S N
EM: chilli@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AB: One impact of human activities on the urban and suburban environment is the dimming of sunlight due to urban air pollution and intensified haze. The spreading of urban aerosol and the optical efficiency depends on the size distribution of the particles and the vertical distribution. Reduced transparency of the atmosphere leads to an increase in scattered light compared to direct sunlight and an overall reduced total solar flux at the surface due to absorption in the atmosphere and backscattering of light to space. The modified solar flux cools the surface and suppresses evaporation and turbulent mass exchange in urban and suburban areas. Increase in diffuse sunlight can also have a positive effect on plant productivity due to increased actinic flux. Hence consequences for the biogeochemical cycles can be expected in urban and suburban areas. The quantification and variability of these effects were investigated in a pilot project in summer 2003 and 2004 where measurements of Aerosol Optical Thickness (AOT) at several wavelengths and particle number concentration for multiple size ranges were made in pilot fashion with the initial goal of better understanding horizontal and vertical distribution of aerosols near a major metropolitan center. Large spatial variability of atmospheric transparency in the New York Metropolitan area was observed in transects through New York City and Long Island to New Jersey in a field campaign in July 2003. Vertical profiles of AOT and particle number concentration were collected on board hot-air balloon flights in July 2004 that were launched from rural/suburban New Jersey. One evening flight was made in clear conditions and 4 evening flights where made under varying hazy conditions with the sunphotometer looking west. One sunrise flight was made in hazy conditions with the sunphotometer looking east through the city. Here we highlight a few results from two evening flights; additional data and plans of future work will be discussed in the poster. On July 6, during clear conditions, AOT at 500 nm is very low (depth integrated mean of 0.07) and fairly constant as a function of altitude (-0.003 change per 100 meters). Particle number concentration in the 0.3 - 0.5 $\mu$m size range are also relatively low but have a steeper vertical gradient than AOT and abruptly drops to near zero above the boundary layer (1200 meters on that flight). The mean AOT at 500 nm (0.5) is a factor of 7 higher on the hazy evening of July 20th and its vertical gradient is also ca. a factor of 7 higher (-0.02 per 100 meters). Particle number concentrations in the the 0.3 - 0.5 $\mu$m size range on this hazy evening were a factor of 3 to 4 higher than the clear evening and fairly constant with altitude, only appreciably dropping off above 1600 meters.
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0345 Pollution--urban and regional (0305)
DE: 0360 Transmission and scattering of radiation
DE: 0399 General or miscellaneous
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting