Preview of 1994 Ozone Precursor Concentrations in the Norteastern U.S.
One of the simplest applications of PAMS data is a comparison of VOC and NOx concentrations. Following EPA's Empirical Kinetic Modeling Approach (EKMA, maximum afternoon ozone levels are dependent on the morning concentrations and mixing ratios of VOC and NOx. The morning VOC to NOx ratio can provide a useful starting point for evaluating the relative effectiveness of cost-efficient ozone precursor control strategies.
A VOC to NOx ratio of 8 to 1 is often cited as an approximate decision point for determining the relative benefits of NOx vs. VOC controls. At low VOC to NOx ratios (< about 4 to 1), an area is considered to be VOC-limited; VOC reductions will be most effective in reducing ozone, and NOx controls may lead to ozone increases. At high VOC to NOx ratios (> about 15 to 1), an area is considered NOx limited, and VOC controls may be ineffective. When VOC to NOx ratios are at intermediate levels (4 to 15), a combination of VOC and NOx reductions may be warranted.
Figure 4.1 Conceptial Ekma Diagram
While the EKMA approach has largely been replaced by more sophisticated grid models, the VOC to NOx ratio may still provide a useful perspective for developing local and regional control strategies and/or assessing their effectiveness over space and time. However, in many ozone non-attainment areas, direct measurements of collocated and/or regionally representative NOx and VOC concentrations are limited. Strategy development is based entirely on emissions estimates and photochemical model results of questionable or at best unknown accuracy.
The State of New Jersey has maintained several sites in a long-term EPA monitoring program for morning (6 to 9 AM) NMOC (non-methane organic carbon).
Figure 4.2 displays the Summer, 1993 NOx and NMOC data for 2 of these sites. The majority of data points at the suburban Plainfield site fall to the upper left (NOx-limited) side of the 8 to 1 line. The urban Newark data suggest a more VOC-limited regime. However, few points at either site are substantially above or below the 8 to 1 line, suggesting that both VOC and NOx controls may be effective
.
Figure 4.2 Morning NMOC:NOx Ratios at Urban and Suburban New Jersey Sites, Summer, `93
The time period of 6 to 9 AM is traditionally considered most appropriate for assessing precursor levels which lead to production of peak ozone concentrations later in the day. Mobile source emissions of VOC and NOx are high, and photochemical activity is minimal. It should be recognized, however, that the day's ozone production is not entirely limited to precursor concentrations present in this 6 to 9 AM time period. Figure 4.3 displays the average diurnal cycles of the TNMHC (total non-methane hydrocarbon) to NOx ratio for 3 PAMS sites averaged over the Summer (JJA) of 1993
Figure 4.3 Average Diurnal TNMHC to NOx Ratios at Selected PAMS Sites During Summer, 1993
.
The sites exhibit a wide range of morning ratios - from well below to well above the 8 to 1 line. They also exhibit very distinct diurnal cycles, which ascend rapidly during the morning hours. This makes the exact time period over which the ratio is calculated of critical importance. For example, a control strategy based on VOC to NOx ratios during the 6 to 9 AM time period, might change substantially depending on whether that time period is taken as Eastern Standard Time or Local Standard Time (values here are displayed in Eastern Standard Time).
Figure 4.4 Morning TNMHC to NOx Ratios for NE PAMS Sites During July, 1994 Episodes
The hourly values of TNMHC and NOx Concentrations between 6 and 9 AM for NESCAUM region PAMS Sites during the two 3-day 1994 Episodes are displayed in Figure 4.4. These data are much too limited to draw substantive conclusions - although it may be observed that most of the data points (during the morning of these days of high afternoon ozone levels) fall within the range where both NOx and VOC controls may be effective.
As more PAMS data become available, a simple comparison of TNMOC to NOx ratios, and their changes over space and time may provide a useful basis for developing or evaluating the effectiveness of local and regional control strategies.
It should also be cautioned that there are a number of limitations to, and potential biases in, the use of VOC to NOx ratios. For example, the Cape Elizabeth ratios plotted in Figure 4.4 are clearly well to the NOx-limited side of the chart. However, the absolute concentrations of precursors at this site are insufficient to support the formation of the high ozone concentrations measured here during these episodes (such as 148 ppb ozone on the afternoon of 7/21/94). In cases (like this) where transport is a significant (dominant) factor, the precursor concentrations at upwind locations along the transport path need to be determined. It will ultimately be critical to distinguish between ozone contributions from local precursor emissions, from transported ozone formed in upwind locations, and from in-situ ozone production from transported upwind precursors.
There are also questions about which species should be included in a calculation of "total VOCs" and "total NOx". There is a remarkable inconsistency in the literature among different calculations of total VOCs at different sites and time periods. In some cases methane is included (usually excluded). In other cases biogenics and/or carbonyls are included, excluded, not well quantified, or estimated. Traditional NOx measurements include NO (generally quantified accurately) plus variable combinations of NO2 and some (but not all) other reactive nitrogen species (NOy).
Even if appropriate quantities of "total VOC" and "total NOx (or NOy)" could be quantified, the simple ratio of the two disregards the composition and reactivity of individual hydrocarbon and nitrogen compounds. During a multi-day episod, concentrations of total non-methane hydrocarbons may build to high concentrations under a stagnating high pressure system. However, without fresh injections of VOCs, the most reactive compounds will become depleted, leaving high VOC levels composed of less reactive species with little potential for ozone production. A variety of "reactivity" schemes have been proposed and employed in different studies, but as yet, there is no consensus on which of these adjustment schemes is most appropriate.
Focusing exclusively on the 6 to 9 AM time frame may severely underestimate contributions from highly reactive VOC compounds like isoprene and formaldehyde - which tend to peak in mid-day (when most other reactive compounds have been depleted). Also, reactive nitrogen compounds like PAN, HONO and nitric acid exhibit diurnal patterns which differ from those of NO and NO2.
