ow has the positive externality affected the economy? How has the negative externality affected the economy?

I have to chosen two following 2 Internet articles from below—one article should discuss a positive externality, and one should discuss a negative externality. Now I need help listing the characteristics of the positive and negative externalities


Positive Externality(characteristics)                  Negative Externality(characteristics)



1) Intra-project externality and layout variables in residential condominium appraisals
by Chan, Su Han,  Chu, Shin-Hering Michelle,  Lentz, George H,  Wang, Ko
Examines the effects of both negative and positive externalities on the sales of condominiums. Several factors such as parks and pools can be both negative and positive, depending on the predilections of the buyer. Greenspace is always a positive externality. The effects of these factors are not used as they should be, and as a result many condos are over or under appraised in value.
http://www.findarticles.com/p/articles/mi_qa3750/is_199801/ai_n8767442/pg_5


2) Externalities and labor market linkages in a dynamic two-sector model of tropical agriculture by GERALD E. SHIVELY
Examines the negative externality of erosion from an upland farm affecting a lowland farm. A model is developed to determine the extent of the externality. The model indicates the attempts to fairly distribute the effects of the externality increase efficiency.
http://www.journals.cup.org/download.php?file=%2FEDE%2FEDE11_01%2FS1355770X05002652a.pdf&code=5dd814830ac0908a1e3a1a9fe14560df

Also could someone please explain to me very well
1) How has the positive externality affected the economy?
2) How has the negative externality affected the economy?