Estimation of night time NO3 and OH radical levels in the UK urban atmosphere, using the steady state approximation method

The steady state method has been applied to the UK NETCEN data archive at four urban sites (Marylebone Road London, Southampton, Birmingham and Middlesbrough) of UK over the period 1997-2001. At each site hourly measurements of NO, NO2, O3, CO and 22 VOCs were used to determine nighttime OH and NO3 radical levels using the steady-state approximation. The average night-time OH concentration ranges from 104-105 molecule cm-3, with the lowest value returned by the most polluted site (Marylebone Road), a similar result is observed for NO3 whose concentration ranges from 105-108 molecule cm-3. Both radicals appear to peak in spring, where night-time O3 levels pass through a maximum and have typical timescales to establish steady state of a few seconds for NO3 to minutes for N2O5, emphasising the applicability of the method. Further analysis suggests that a non-negligible level of NO3 exists during the day, confirming previous measurements. Although the level of NO3 is generally sub pptv, it makes a significant contribution to nitrate formation during the day.