Comments on Pattern of ozone precursor emissions in the OTAG domain

Executive Summary

I disagree with the contention that tall stack point sources have emissions that are invariate with time is not correct (at least with respect to sources in the midwest region).
Typically, most midwestern fossil units (at least on our large system) are operated in a cycling mode and not a base load mode.
This has been the case on our system since the early 80's. I would also point out that in the mid Ohio River Valley and the Kanawha River Valley, there are numerous large petrochemical\
facilities with large emissions of VOC's, so the Ohio River Valley is not just the home of point source NOx.

Submitted by David Long on 2/21/97 RecID: DavidLong

Re: Executive Summary

David's comment (From an obviously reliable "source" (: > ) raises an important point, and underscores a key missing element in the body of AQA analyses, to date. We haven't really done much analysis of emissions data - in comparison with ambient air quality or meteorological data. For example, an OTAG regional map which shows the ratio of average (or peak, or ...) Ozone to average (or whatever) emissions (of NOx or VOC or some combination of the 2) could be informative.

Several aspects of the long-term ambient ozone pattern analysis show features in the Midwest which could be at least partly accounted for by emissions characteristics which were relatively more invariate in time than ground level emissions patterns in urban centers. Specifically: relatively high 8-hour concentrations (compared with relative absence of peak 1-hour concentrations); high 10th percentile concentrations (compared with urban centers where 90% concentrations are much higher); lack of perceptible Friday to Sunday ozone concentration ratio (compared with a very high comparable ratio in urban centers). If these observed patterns can't be accounted for by differences in regional (source-type-oriented) emissions patterns, then we need to look for alternative explanations. One thing I'm curious about is the extent to which utility (and/or automotive) NOx emissions might tend to increase with increasing ambient temperature (air conditioning) in the summer. Anthropogenic and biogenic VOC emissions would clearly increase with temperature, but NOx?

Although time is short, I think it could be very productive to have a joint session (or conference call) between the AQA and Emissions Inventory workgroups.

Submitted by Richard Poirot on 2/22/97 RecID: RichardPoirot

Re: Executive Summary

Do you know whether or not the variation of the emissions from the point sources follows a diurnal and/or seasonal cycle? and if so how large a variation?
Also, the Ohio River Valley may have significant VOC sources, (urban and point), but according to the UAMV basecase VOC inventory for 7/14/95, the Ohio river valley and states northward (IL, IN, OH) have low total VOC emissions compared to their surroundings.

Submitted by Bret Schichtel on 2/21/97 RecID: BretSchichtel

Re: Re: Executive Summary

Sorry for taking so long to respond to these comments. Both Brett and Rich raise some good questions about how generating units are typically operated. While I can only speak definitively for my company, what I understand about adjoining systems suggests that this is most likely true for them too. Virtually every fossil fired unit we operate has a noticeable diurnal variation in load. How much depends on the weather conditions and load requirements of our system and those systems to which we are able to sell power.

A typical diurnal cycle will go from its minimum sometime between 0300 and 0500 EST (we do all of our dispatching on standard time year round)with the daily peak coming anywhere from 0800 to 1600 EST. The peak varies depending on weather conditions and demand. In addition to weather impacting on demand, it also impacts on the ability to load the units. In extremely hot weather, a number of our units cannot be loaded as highly as they can be in cooler weather. The primary reasons are the inability to reject sufficient heat from the cycle to allow the unit to operate within safe limits or due to the reduced density of the air available for combustion. What this means from an emissions standpoint is that during very hot conditions, emissions would peak in the late morning or early afternoon and drop off with time (based on ambient temperature) until the air cooled after dark or load was reduced on the unit in response to lowered demand.

There are also day of week and seasonal variations, with seasonal peaks typically coming in the summer and winter. Typically, weekends have lower total emissions than weekdays, as the peak tends to be shorter in duration (and most likely lower) than on week days.

Submitted by David Long on 3/11/97 RecID: DavidLong1

Volume I: EXECUTIVE SUMMARY--Precursor emissions

Paragraph 1, Sentence 1, insert after Anthropogenic and biogenic sentence 1, delete anthropogenic in front of ozone.... sentence 3, insert after low-level mobile source.

Closing comment: This is true of baseload power plants but an equal number of point sources do follow diurnal and seasonal patterns.

Submitted by Jeffrey West on 2/18/97 RecID: JeffreyWest

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