Verification of expected trends in urban traffic NOx emissions from long-term measurements of ambient NO2 concentrations in urban air.

Data from long-term measurements of ambient NO2 concentrations at roof level in 15 Swedish cities have been used to verify expected trends in urban traffic NOx emissions, resulting mainly from the growth in the number of threeway catalyst (TWC) cars in Sweden since the mid 1980s. The result show that, with few exceptions, all cities exhibit a highly significant downward trend in ambient NO2 concentration since the winter season 1986/1987, as regards both winter season averages and 98th percentiles of daily averages, with an average decrease in both cases of approximately 30% through the winter season 1993/1994. The same trend is also observed when meteorological variations between years are taken into account. Corrections for NO2 in background air yield an even stronger downward trend, or an average 40% decrease for the study period. Simultaneously, rough calculations indicate a 30% decrease in urban traffic NOx emissions during the study period. The conclusions are that, since emission calculations always involve a high degree of uncertainty, use of data from long-term measurements of NO2 concentrations in urban air can be very helpful in establishing real-world trends for urban traffic NOx emissions, as soon as NOx-levels are low enough for the NO+ozone reaction to become 'NOx-limited'.