A. "atmospherically" does not describe the way nitrous oxides bond with free ions - it describes where the nitrous oxides are when they bond with free ions. In fact, doing anything "atmospherically" is pretty much nonsensical. Similarly, "up high" doesn't do enough to describe the location of nitrous oxides - we specifically want to say that the nitrous oxides are high *in the atmosphere*. So we can eliminate this answer choice for issues of meaning.
B. We have "however" between two independent clauses: "At ground level, nitrous oxides are bad enough" and "it is in the upper atmosphere in which it may bond with free ions to create dangerous smog particles." We CANNOT use "however" like "but" to separate independent clauses in the middle of a sentence. So we have a sentence structure error. Similarly, "it may bond" refers to "nitrous oxides", but "it" is singular, while "oxides" is plural. So we also have an agreement error. Eliminate.
C. Here, we use "but" to separate two independent clauses, which is totally acceptable. We also resolve all of our meaning issues - the nitrous oxides are in the upper atmosphere when they bond with free ions. Keep this one.
D. This answer is pretty similar to B - we have both "however" and "but", which is redundant. We also have "it is known" referring to "nitrous oxides", which is an agreement error. Eliminate.
E. This answer choice creates a meaning issue - we want a comparison between how bad nitrous oxides are at ground level and how bad they are in the upper atmosphere. However, this answer choice uses "as", which acts like "because". So this answer choice basically says, "Nitrous oxides are bad enough on the ground *because* they bond with free ions in the upper atmosphere." This creates a totally new meaning (that doesn't make much sense), so we can eliminate E.
