Atmospheric Lifetime and the Range of PM2.5 Transport Background and Rationale Atmospheric Residence Time and Spatial Scales Residence Time Dependence on Height Range of Transport Resource Links Contact: Rudolf Husar,

Background and Rationale Residence time refers to the time span between the PM emission (or the emission of their precursor gases) and its removal from the atmosphere. Residence time determines the range of impact of a specific sources.

Atmospheric residence time and transport distance are related by the average wind speed, say 5 m/s. Residence time of several days yields ‘long range transport’ and more uniform spatial pattern. On the average, PM2.5 particles are transported 1000 or more km from the source of their precursor gases. Atmospheric Residence Time and Spatial Scales PM2.5 sulfates reside 3-5 days in the atmosphere Ultrafine 0.1 m coagulate while coarse particles above 10 m settle out more rapidly. PM in the m size range has the longest residence time because they neither settle, nor coagulate.

Residence Time Dependence on Height. The PM2.5 residence time increased with height. Within the atmospheric boundary layer (the lowest 1-2 km), the residence time is 3-5 days. If aerosols are lifted to 1-10 km in the troposphere, they are transported for weeks and many thousand miles before removal. The lifting of boundary layer air into the free troposphere occurs by deep convective clouds and by converging airmasses near weather fronts.

Range of Transport The residence time determines the range of transport. For example, given a residence time of 4 days (~100 hrs) and a mean transport speed of 10 mph, the transport distance is about 1000 miles. The range of transport determines the ‘region of influence’ of specific sources.

The three major airmass source regions that influence North America are the northern Pacific, Arctic, and the tropical Atlantic. During the summer, the eastern US is influenced by the tropical airmass, from Gulf of Mexico. The three transport processes that shape regional dispersion are wind shear, veer, and eddy motion. Homogeneous hazy airmasses are created through shear and veer at night followed by vigorous vertical mixing during the day. Transport Mechanisms Pollutants are transported by the atmospheric flow field which consists of the mean flow and the fluctuating turbulent flow

Low wind speeds over a source region allows for pollutants to accumulate. High wind speeds ventilate a source region preventing local emissions from accumulating. Horizontal DilutionVertical Dilution In urban areas, during the night and early morning, the emissions are trapped by poor ventilation. In the afternoon, vertical mixing and horizontal transport tend to dilute the concentrations. Influence of Transport on Source Regions

Plume transport varies diurnally from a ribbon-like layer near the surface at night to well mixed plume during the daytime. Even during the daytime mixing, individual power plant plumes remain coherent and have been tracked for 300+ km from the source. Most of the plume mixing is due to nighttime lateral dispersion at night followed by daytime vertical mixing. Plume Transport Much of the man-made PM2.5 in the East is from SO2 emitted by power plants.

Long Range Transport In many remote areas of the US, high concentration of PM2.5 have been observed. Such events are have been attributhe Long range Long range transport events occur when there is an airmass stagnation over a source region, such as the Ohio River Valley and the PM2.5 accumulates. Following the accumulation, the hazy airmass is transported to the receptor areas. Satellite and surface observations of fine particles in hazy airmasses has provides a clear manifestation of long range pollutant transport over Eastern N. America.

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