I've prudently refrained from posting on this particular thread until now, but here it is, since LuckySandals bumped it.
There is no decrease in value that can not be accounted for, and each can be determined to be a cause of one or more persons, but that doesn't mean that everyone should be compensated for all loss of value.
Say Person A buys property out in the wilderness, and his property is worth a lot because there is some value to be had from particular buyers because of the seclusion. 20 years later, he has 10 neighbors, and his property is now devalued. It has lost the value added due to seclusion, and this is a direct result of the 10 neighbors moving in. However, there is no logical case that Person A needs to be compensated for this loss. Person A realized, or should have if he didn't, that in order to maintain the seclusion value, he'd need to buy up the additional neighboring X number of acres, to be preserved for their scenic value and add a buffer from his land and the rest of the world. This is the same case where one neighbor is particularly dirty and his yard devalues the neighbors. Unless there are conditions attached to the land at the time of sale, the neighbor cannot be held liable for reparations due to property loss; he was acting within his property rights, and all the neighbors should have realized that this might happen. The value added from living in a beautiful neighborhood was no product of their labor(at least not entirely so), and therefore no person owned it, nor could any person claim loss of it. The seller of the property should have attached that condition as a part of the sales contract if they wanted to maintain it. If the neighbors want to keep the value from a beautiful neighborhood, perhaps they could offer a monetary incentive to the seller, such that collectively, the value offered was much lower than the value obtained, yet enough incentive to cause the seller to add such a minor thing to his sales contract.
In the case of air pollution it could be argued that perhaps the molecules of methane which cause such a stench are infringing upon Person B's property and driving down the value. In a ridiculous world, Person B could charge the trespass for each molecule. In a more rational world Person B might build a system to filtrate the stench and then make a case in court that Person A's cows were the reason for it in the first place and therefore Person A should assist to pay for the filtration system. In an even more rational world, Persons B-Z, the neighbors affected by the smell, would offer an incentive for Person A to filtrate the smell on his side. The monetary incentive could very easily be much less collectively than the combined loss of property values, whilst still giving Person A ample benefit from the exchange. In both scenarios, the stinky cattleherd has plenty of reason, whether through contractual obligation, legal retribution, or monetary incentive, to take care of the problem of his own volition. You wouldn't need the Homeowner's Association to get involved, and most times, you wouldn't need the federal government to get involved.