February 13, 2006

Can the workaholic patterns of “the city that never sleeps” affect New York City’s weather? A recent NASA study reports that they do. Researchers have detected for the first time in an American city a workweek pattern of tiny particles in the air called aerosols, believed to be generated by the comings and goings of people working in the city. The study also finds these urban aerosols, thickest on Wednesdays and lightest on weekends, can influence air temperatures and cloud cover in densely populated cities.

The study, conducted using New York City as its test case, finds that aerosol cycles are affected by city structures, geography, and human activities like vehicle use and construction. The researchers gathered data over four summers from 2000 to 2004 to complete the study. Specifically, they used aerosol data from the Moderate Resolution Imaging Spectroradiometer aboard NASA’s Terra satellite, and daily and hourly measurements from NASA’s AErosol RObotic NETwork (AERONET), a ground-based network of sensors that detect aerosols in the air.

According to researchers, the major rise in human activity over the past century has interrupted the natural relationship between land surfaces and vegetation that contributes to changes in air temperatures and climate. This is especially true when it comes to construction of buildings and roads. This results in small airborne particles called aerosols composed of dust from construction sites, fossil fuel combustion from car and truck exhaust, and power generation. The more dense a city’s residential and commuter populations, the more roads and buildings are built and used. This in turn leads to a greater concentrations of aerosols in the air.

How might these workweek fluctuations in the thickness of aerosols in the air affect weather in the Big Apple? Will it rain more or less as a result? Do these tiny particles make the sky cloudier or clearer?

The study’s observations have added to our understanding of daily, weekly, monthly, seasonal and annual cycles of aerosols, and how these particles might affect clouds and rainfall in urban areas.

“Though longer observations are needed, the weekly pattern of aerosols can indeed be used to better understand weather. Aerosols affect the formation of clouds. In turn, they also affect how much rainfall there is, and land surface temperatures,” said the study’s lead author Menglin Jin, a visiting scientist at NASA’s Goddard Space Flight Center, Greenbelt, Md., and a researcher at the University of Maryland, College Park, Md. “The weekly cycle of aerosols in New York City has been linked to observed weekly cycles in air temperatures and clouds. In particular, rain clouds over the city tend to be thicker and the air temperatures a bit lower on weekends than during the workweek. All of these changes in the air are critical in weather forecasting.”

Aerosols are known to serve as nuclei for water and ice droplets to form around. Previous research indicates that greater concentrations of aerosols disperse water among more nuclei, leading to higher concentrations of cloud drops that are smaller in size. These smaller drops may fail to grow big enough to fall as rain, therefore, reducing rainfall in urban areas. Opposite to this, however, heated urban surfaces such as roads, parking lots and buildings cause stronger convection, or the rising of hot air, which can cause more rainfall. The overall effect on rainfall over New York City is actually a result of these two competing processes. Thus aerosols do indeed play a role in big city weather.

The study distinguished between both distant and local sources of urban aerosols and found a clear weekly cycle in New York City during the summers of 2000 to 2004 for locally-produced aerosols. During New York summers, aerosols were thickest midweek and lightest on weekends. The finding suggests the weekly pattern is caused by the human workweek, since there are no known seven-day natural cycles.

By comparison, clouds were thicker over the weekends. According to the study’s authors, this finding appears to also be connected to aerosols, which may provide nuclei for water droplets to form around, creating more droplets and thicker clouds.

By comparison, clouds had higher concentrations of large droplets over the weekends. According to the study’s authors, midweek aerosols appear to provide higher concentrations of nuclei for water droplets to form around, creating higher concentrations of cloud droplets.

The paper, co-authored by Michael King, NASA’s Earth Observing System Senior Project Scientist and Marshall Shepherd, former NASA Deputy Project Scientist for Global Precipitation Measurement, appeared in the American Geophysical Union’s Journal of Geophysical Research-Atmospheres in 2005.

To read the full text of this study on the Web, visit:
http://climate.gsfc.nasa.gov/publications/fulltext/Jin_et_al._(2005).pdf

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Contact:

Gretchen Cook-Anderson
Goddard Space Flight Center, Greenbelt, Md.

This text derived from http://www.nasa.gov/vision/earth/environment/ny_air.html

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