[1] Accurate emission inventories and their temporal trends must be incorporated into pollutant inventories to allow for reliable modeling of the country's past, present, and future air quality. Measured carbon monoxide (CO) and nitrogen oxide (NO x ) concentrations from two urban areas show that the CO/NO x vehicular emission ratio has decreased at an average rate of 7-9% per year from 1987 to 1999. This amounts to a factor of nearly 3 over the 12 years. The current U.S. Environmental Protection Agency tabulations of estimated pollutant emission trends indicate a rate of decrease smaller by a factor of 2-3. The trend in maximum ambient CO levels in U.S. cities suggests a 5.2 ± 0.8% per year average annual decrease in CO vehicular emissions, which implies a 2-3% annual increase in NO x emissions from vehicles. Thus over the decade of the 1990s, annual U.S. CO emissions from vehicles have decreased from $65 to $38 Tg, representing approximate decreases of 6 and 3% in the annual global fuel-use CO emissions and in total global anthropogenic CO emissions, respectively. It is expected that the volatile organic compound (VOC)/NO x vehicular exhaust emission ratio has decreased similarly, implying that the character of atmospheric photochemistry in U.S. urban areas has changed significantly over the decade.

Abstract. Isoprene (ISOP) and its oxidation products, methacrolein (MACR) and methyl vinyl ketone (MVK),

[1] Peroxycarboxylic nitric anhydrides (PANs) were measured at Cornelia Fort Airpark during the 1999 Nashville Intensive of the Southern Oxidants Study. The observed concentrations were similar to those measured in the 1994 and 1995 Nashville Intensives. The average daytime ratio of peroxypropionic nitric anhydride (PPN) to peroxyacetic nitric anhydride (PAN) was 13.7%. Extensive measurements were also made of peroxyisobutyric nitric anhydride (PiBN), which confirm the anthropogenic origin of this compound, and its ratio to PAN was 2.4% on average. The daytime ratio of peroxymethacrylic nitric anhydride (MPAN) to PAN was as high as 25% during periods of high biogenic hydrocarbon (BHC) impact. A simple linear combination model was used to estimate anthropogenic hydrocarbon (AHC) and BHC contributions to O 3 production. On average, 80% of the O 3 observed over 80 ppbv was attributed to AHC chemistry and 20% to BHC chemistry. An exception to this was a plume derived from a combination of power plant NO x and isoprene emissions from west of the city, in which the BHC contribution was at least 50 to 75%.