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New Yorkers' Work Week Clouds Air With Tiny Particles, Study Finds
According to a recent NASA study, the millions of people who work in New York City not only alter the population density each work day, they add to the density of tiny particles in the air called aerosols. Researchers have discovered for the first time in an American city a workweek pattern of aerosols clouding the air, believed to be created 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 affect air temperatures and clouds in big cities.

Pie chart showing the sources of aerosol particles. Image to right This picture shows sources of tiny particles in the air called aerosols. Aerosol particles larger than about 1 micrometer in size are produced by windblown dust and sea salt from sea spray and bursting bubbles. Aerosols smaller than 1 micrometer are mostly formed by condensation processes such as conversion of sulfur dioxide (SO2) gas (released from volcanic eruptions) to sulfate particles and by formation of soot and smoke during burning processes. After formation, the aerosols are mixed and transported by atmospheric motions and are primarily removed by cloud and precipitation processes. Click on image to enlarge. Credit: NASA's Earth Observatory website

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 information 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 detects aerosols in the air.

According to researchers, the major rise in human activity over the past one-hundred years has disturbed the natural relationship between land surfaces, and trees and plants that helps to influence air temperatures and climate. This is especially true when it comes to construction of buildings and roads. New construction causes small airborne particles called aerosols composed of dust from construction sites, fossil fuel burning from car and truck exhaust, and power generation. The more residents and daily commuters a city has, the more roads and buildings are built and used. This all leads to a greater concentration of aerosols in the air.

The STS92 Space Shuttle astronauts photographed upstate New York at sunset on October 21, 2000. Image to left: The STS-92 Space Shuttle astronauts photographed upstate New York at sunset on October 21, 2000. The view looks toward the southwest from southern Canada, and captures a regional smog layer extending across central New York, western Lake Erie and Ohio, and further west. Winds bring ozone and some chemicals that participate in its formation to rural areas downwind of emission sources. Ozone itself is invisible. Credit: NASA JSC

How might these workweek changes 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 also added to our understanding of daily, weekly, monthly, seasonal and annual cycles of aerosols, and how these particles might impact 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.

This image shows a magnified view of aerosol particles collected in the industrial city of Port Talbot, England. Image to right: This image shows a magnified view of aerosol particles collected in the industrial city of Port Talbot, England. Many of the particles measure roughly 2.5 microns across, small enough to easily enter and damage human lungs. This Micrograph adapted from Sixth Annual UK Review Meeting on Outdoor and Indoor Air Pollution Research 15th–16th April 2002 Web Report W12, Leicester, UK. Credit: MRC Institute for Environment and Health

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.

"According to the United Nations Population Fund prediction, 60 percent of all people will soon be living in urban areas," said Jin. "Urban areas affect a lot of people even though cities cover only 0.2 percent of the global land surface. So, it's very important for us to know if and how the workweek in big urban areas influences aerosols in the air that in turn may affect the climate in those areas."

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.

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Gretchen Cook-Anderson
Goddard Space Flight Center