The surface radiative forcing of nitric acid for northern mid-latitudes

Ground-based, high-resolution measurements of downward atmospheric thermal emission spectra are reported for a northern mid-latitude location for summer and winter conditions. These measurements clearly show the presence of the 11.3-mum thermal emission band of nitric acid situated between 850-920 cm(-1). By using the FASCOD3 line-by-line radiation code to simulate the background thermal emission, the measured seasonally averaged surface radiative forcing due to nitric acid is determined to be 0.055 W m(-2) +/- 15%. The zenith column amounts of nitric acid are found to vary between 7.9 x 10(15) and 1.1 x 10(16) molecules cm(-2) +/- 15%. An estimation is made of the contribution of nitric acid to the direct radiative forcing of the Earth's surface since pre-industrial times for northern mid-latitudes. This work suggests that nitric acid may play a role that is comparable to that of other greenhouse gases, such as CFC-11, in the forcing of the Earth's climate system. Under polluted conditions, nitric acid may contribute about half of the radiative forcing that is currently associated with tropospheric ozone. (C) 2000 Elsevier Science Ltd. All rights reserved.