...I am to understand that CO2 and NOx are inversely related; if you tune for one, the other suffers...
That is not quite right. I suppose if you were to "richen" the mixture, thus increasing CO2 due to higher consumption it may also lower NOx some as well, but it would still be much higher than a gas engine with a 3 way catalyst.
The balance of emissions that are targeted are Hydrocarbons (HC) and Carbon MONOxide (CO, no 2) vs NOx. In a gas engine, when you richen the mixture you get more HC and CO, but less NOx. When you lean it out, you reduce HC and CO but increase NOx. That is the inverse relationship where it comes to harmful pollutants. CO2 is not a harmful pollutant nor is it currently regulated here in the U.S. CO2 is regulated in Europe as a way of encouraging the adoption of more fuel efficient vehicles and thus spewing less carbon into the atmosphere.
With that having been said, diesels are EXCEPTIONALLY low producers of HC and CO emissions. As in their levels are so low of those pollutants that they could get away with ZERO emissions equipment and still be far, far under the limit with regards to those two gases. NOx is another story, of course.
Going off the top of my head, when your ALH was new, the federal regulations allowed somewhere around 1.0 g/mi of NOx. The ALH might have beaten that output by a tenth or two. But where the regs also allow 1.0 g/mi of CO, the TDI was a tiny fraction of that.
Actually had an interesting discussion and made a somewhat silly point to bhtooefr last week. I suggested that a modern diesel emits less CO than any gas vehicle. So, he came up with one of the most efficient internal combustion gas setups he could think of: 2017 Prius. And then looked up the numbers for a new Chevy Cruze diesel.
The result? The Prius emits 0.15 g/mi. The Cruze diesel emits 0.10 g/mi. Not much difference, right? There are many other vehicles that emit far more than that, too. The Prius is definitely exceptionally clean.
But just to prove a point about how data is presented can make all the difference in perception, I then came back with all caps: "BREAKING NEWS! 2017 TOYOTA PRIUS EMITS 50% MORE POISONOUS CARBON MONOXIDE THAN DIESEL CHEVY CRUZE!"
Nothing I said was incorrect. Carbon monoxide is quite literally a poison that will kill you in a much more instant sense than NOx. It is absolutely a poison. And the Prius does indeed emit 50% more, which sounds like a lot. But, like tier 2 bin 5 NOx rules that VW was cheating, multiplying a number close to zero is still a very small number... Although, if the numbers are accurate, the cheating TDIs were actually emitting more NOx than your ALH was certified to... So there's that.
But, when you say "Emits 10x the limit of NOx," it's also good to take into consideration that the limit was REDUCED by 10x in the move to tier 2. A BRM code TDI was certified to emit 0.7 g/mi NOx. The "Clean diesels" were only supposed to emit 0.07 g/mi. Literally moved the decimal point on that one.
But, in every other regard, the new clean diesel is definitely cleaner than your ALH. While NOx emissions were probably about the same between the two (if not higher for the cheating new diesels), the new diesels emit ZERO particulates thanks to the particulate filter, less sulfur oxides thanks do a deSOx cat along with the shift to ultra low sulfur diesel which you didn't have when your ALH was new.
Standards have shifted and as far as NOx is concerned, yes, your TDI emits more than most gas engines. But you can also argue, especially if the person knocking you for your vehicle choice drives something not incredibly fuel efficient or new, that yours produces less CO and HC than theirs. And, if you were to hypothetically close yourself in a garage with an engine running, would you choose the old diesel or any gas engine? I'd choose the diesel every time because they naturally produce a tiny fraction of CO of most gas engines and even beat some of the cleanest gas engines out there. And CO is the one gas that is much more of an immediate threat to you staying alive than any other emission.
Not that I recommend locking yourself in a garage with any running vehicle... but the hypothetical still rings true.
An interesting read about NOx is here:
https://engineering.berkeley.edu/2009/03/ozone-weekend-effect
This is an article by UC Berkeley about a study that was done about NOx and its effects on air quality. For some time there was debate on whether lowering NOx standards were a good idea as there was evidence that suggested lowering NOx made the ratio of NOx to VOCs in the atmosphere just right to maximize production of smog (where NO can be recycled to more harmful NO2 and O3). Gas engines produce more VOCs, although California rules and gas pumps have been setup to basically eliminate most VOCs from internal combustion engine sources along with aerosol cans, so they're convinced VOCs have been reduced to naturally-produced background levels, so they figure instead of allowing NOx go more or less unabated, regulating it down to nearly nothing was the better option. So that's what they did.
Not that it's immediately harmful to breath, as most of diesel's exhaust emissions are NO, not NO2 (regulations lump all nitrogen oxides together...). NO2 you don't want to breath. NO is OK (in fact, your body produces NO to help keep your veins from clogging... seriously. Look up endothelial cells). Although I've also read a study that suggested what comes out of a diesel's tailpipe these days creates more NO2 than they used to because of catalytic converters. Majority of what comes out of the engine is NO, after it's passed through the cat it might be something like 2/3 NO, 1/3 NO2. And if atmospheric conditions are right (very low wind, abundant sun and high temperature and VOCs in the atmosphere--conditions often present in much of California) NO goes through chemical reactions to make NO2 and O3 as mentioned above.
Now, for all the *****ing and moaning some of us do about how difficult the regulations have made life for the diesel world, it turns out the environmentally-minded scientists were indeed right. There have been astounding reductions in photochemical smog in just a few short years in places where diesel traffic is normally heavy. Places like the Port of Oakland and Long Beach. One article I was reading a few months ago made mention of a 70-something % reduction in Oakland in just the first 3 years of the rollout of DPF and effective NOx reduction diesel trucks. That's astounding.
But, this is also why we have state's rights. Hell, there are many counties within California that don't require a biannual smog (but they do require a smog to be performed when selling a vehicle). Some areas don't have a smog problem. And in those areas one could argue it's perfectly fine to remove emissions equipment should they become problematic. But in the densely populated regions that also happen to frequently experience the weather patterns that make smog worse, I cannot condone doing so and of course neither does the government.
Long story short, the regulations do in fact make a very noticeable improvement in air quality for those places that are notoriously problematic. Your tiny diesel is such a small fraction of that, though. And in some ways not buying a new car is better for the environment than frequently buying a new one. Also, you can point out to your friends that NOx is just one small sliver of the whole emissions pie. Your diesel is trading off higher NOx for much lower poisonous CO and HC along with superb fuel economy. That has to count for something. Of course, that is the same reasoning that got Europe into trouble. They then encouraged mass adoption of diesel through tax incentives and air quality got worse because when they began incentivizing diesel, they were not yet clean. No particulate filters and NOx control that is simply not super effective compared to what we have now. But you need to have cumulative effect of mass adoption of the technology for any real pollution like that to happen. In the U.S., the take up rate for all diesels is very low compared to gas engines. So, it's really small potatoes...
I just hope to see improvements in the serviceability, cost and reliability of the emissions components of new diesels. I could go on and on and argue that maybe having a diesel and only drive it in the city is not only a bad idea for pollution but also the vehicle itself. It does seem diesels have more issues when only driven in the city... But they do really shine out on the open highway. I will maintain that diesel is the optimum long distance highway vehicle until battery electrics can do 500-600 miles on a charge and only take a few minutes to recharge to do it again--also providing that the infrastructure is in place and there's not too much demand vs. supply. Until then, there is still a major convenience factor in favor of diesel. Diesel should still be an important technology among others to help us reduce oil consumption until that day comes. And what kind of driving you do most will determine which kind of vehicle best fits what you do. And diesels are definitely not great for the stuck in traffic cycle long term...