Estimates of the rate of production of excited oxygen atoms due to the
photolysis of ozone (

It is widely recognised that the chemistry of the clean troposphere is driven
by a few key oxidising species, with a major contributor being the hydroxyl
radical (OH)

The primary source of OH is through the photolysis of ozone to produce
O(

The rate of ozone photolysis in Reaction (R1),

There are a range of radiometric techniques used for the determination
of actinic flux, and the strengths of various detectors have been
assessed by a field comparison experiment

The ideal viewing geometry for the determination of

Most quantitative UV observations measure global irradiance (

If it is assumed that there is no upwelling radiation (surface
albedo

Estimating the ratio of the direct beam to global irradiance
(

Three types of systems have been used in the past to determine

The Cape Grim Baseline Air Pollution Station (“Cape Grim”),
(40

As part of the Cape Grim measurement programme, spectral UV-B irradiance (both
global and diffuse) has been measured routinely. The purpose of this work is
to use the spectral UV-B measurements to estimate

All UV-B irradiance measurements reported here have been made in the
radiation enclosure at the Cape Grim Baseline Air Pollution Station. This is
located some 300

It is worth noting that the cosine error of the diffuser is determined from the solar zenith angle dependence of the ratio of the SRAD-derived direct beam irradiance to the sun photometer. A correction for this variation can then be applied during the calibration.

The resulting database of measurements includes alternating estimates
of global and diffuse irradiance at each wavelength and time. The
determination of the components of the irradiance at a single time is
based on interpolation of the diffuse/global measurements before and
after the global/diffuse measurement in question

The input diffuser was constructed from PTFE but was not temperature-controlled. The phase change reported for this material at around
292

As the Cape Grim UV data set includes both the diffuse and global
irradiance, Eq. (5) can be used, as the direct beam irradiance can be
derived from the difference between the global and diffuse component (see
Eq. 4). This leaves the determination of the ratio

For the ozone absorption cross section (

The UV-B measurements span the region 298–335

The uncertainty in these derived

Comparison of clear-sky calculation values to measurements.
Calculations have been performed with the column ozone amount reported by the
TOMS satellite.

In the analysis of data the model TUV (Tropospheric Ultraviolet and Visible Radiation Model) version 5.0 has been used

The measurements can be compared with the clear-sky calculations performed
using TUV 5.0, where the experimentally derived values have been estimated
using both a clear-sky and cloudy estimate of

The data collected for the period 2000–2005 are shown in Fig. 2. The data set comprises over 108 000 measurements. The gaps in the data set represent times when the equipment failed.

Photolysis rate

The annual cycle is the dominant feature in this plot. To quantify this, monthly mean values have been calculated by sorting all data from a month into 24 hourly bins, and from these bins producing an average daily cycle for each month. It is assumed that if no measurement is made in one of the 24 hourly bins during the month then the average is zero. The average of the 24 hourly averages is then calculated for each month in the 6 years. This method has been used to limit the impact of possible biases from collecting spectra at varying time intervals.

Despite the variability seen in the individual measurements (see Fig. 2), the
monthly averages are relatively stable (Fig. 3, top panel). The lower panel
of Fig. 3 shows that for midsummer and midwinter the interannual
variability in the monthly averages is 3–5 %, with the increases
in between presumably driven by the the observed annual cycle. That is,
during spring and autumn it matters more when in the month the measurements
have been made. Changes in ozone column amount could be a contributing
factor. Using the coincident satellite ozone data shows a maximum in ozone
variability in midwinter, suggesting that ozone is not the main driving
force. The resultant average monthly

Monthly mean photolysis rate

Measurements of

Earlier measurements of interannual variability of UV-B have been reported
for Ushuaia in Argentina

Results for fitting

To investigate any trend in the data, both monthly trends for each month and
trends as a function of season have been calculated. The most significant
linear trend is in summer (December–February) (

Annual cycle of

The dependence of

Solar zenith angle dependence of

A significant fraction of the variability can be due to the differences in
the ozone column during the year. To characterise the dependence, functions
of the following form were fitted to the measured

Cape Grim measurements of

Using this derived ozone RAF, the data set was normalised to both 300 DU and
1 a.u. as shown in Fig. 5. Given the large difference between the median and
average values for the bins, a second fit was performed to the median of the
binned values of Fig. 5, and the fits are also included in Table 2. For
reference, the fits with two exponential terms, using all data and the
medians, are included in Fig. 5. It should be noted that the increase in

The removal of the variation due to changes in stratospheric ozone, as
described by the satellite ozone measurements, reduces the interquartile
variability by up to 20 % as shown in Fig. 6. The effect on high-sun
(small solar zenith angle) measurements is smaller, as this is only collected
in midsummer, and so the ozone variability is small. At larger solar zenith
angles (

Clouds can both reduce and enhance solar radiation at the ground level.
Figure 5 shows that the 99th-percentile value is close to the clear-sky
calculated value at solar zenith angles less than 50

To assess the overall impact of clouds, the ratio of the median value to the
calculated clear-sky value was determined (Fig. 7). This shows that for solar
zenith angles less than 70

The interquartile (75–25 %) difference as a percentage of the median value as a function of solar zenith angle. UVB is the global irradiance, and the other two terms are the derived photolysis rates, with the red curve measurements having been corrected to a constant column ozone amount.

The results for solar zenith angles greater than 70

Attempts to capture the cloud variability through independent observations
have not been very successful. Measures such as visual observations and
automatic sky cameras have not been implemented at Cape Grim. While
sun photometers make measurements during this period, they do not make
measurements of cloud optical depth as has been used elsewhere

Top panel shows the ratio of the median measured

A dependence of

The atmospheric composition at Cape Grim is dependent on wind direction, and
clean or “baseline” conditions are defined by standard measures

Another important question is how reliably the climatology measured here is
representative of a larger region. Cape Grim, sitting on the coast, could
have a cloud environment different to locations out to sea and inland.
A study of the global irradiance at a number of locations concluded that Cape
Grim experienced cloud conditions similar to the southern ocean in this area

Modelling studies

The results of this study permit the prediction of

Six years of estimates of

In this Appendix an estimate of the uncertainty in the
measurements using SRAD is presented. Given the unusual nature of the
calibration used for this instrument, this analysis is preceded by a
description of the calibration method, followed by estimates of the
uncertainties in the components of the calibration to derive an uncertainty
in the determination of the irradiance. This is then used to derive an
uncertainty in the spectral actinic flux. Then the uncertainty in

The strategy for calibration of the spectral radiometer (SRAD) has been
described elsewhere

Atmospheric transmittance can be expressed in terms of the signal at ground
level

Here, the subscript sunp refers to measurements made with a sun photometer, and
SRAD to measurements made with a spectral radiometer. For the calibration of
sun photometers, various techniques have been developed to determine

For other wavelengths we can determine the relative calibration using the
ratio-Langley technique. The Langley technique is a well-known implementation
of the Beer–Lambert law where the top-of-the-atmosphere signal is derived
from direct beam solar measurements at a range of solar zenith angles.
Fundamental to this method is the assumption that during the period of the
analysis the atmospheric optical depth does not change. Alternative methods
have been developed, such as the ratio-Langley technique

The ratio-Langley technique provides estimates of the ratio of top-of-the-atmosphere signals, with this information derived from the
spectral radiometer measurements:

In practice this means dividing the direct beam irradiance spectrum (from
SRAD) by the direct beam irradiance at

With the calibration at

The derived direct beam irradiance depends therefore on four independent factors as given on the right-hand side of this equation, including the measurement itself. Each term will therefore now be considered and then combined to produce an overall uncertainty estimate.

The sun photometer measures the solar direct beam irradiance at a range of
visible and UV wavelengths chosen to be relatively free from the influence of
molecular absorption. The wavelength relevant for these measurements is
342 nm. This wavelength has been calibrated in situ through the use of the
general method

From Eq. (A1), the SRAD top-of-the-atmosphere signal at the reference
wavelength is given by

The ratio of the direct beam signal from SRAD to the sun photometer can be determined every time there are valid coincident measurements of the direct sun.

In determining the calibration of the Carter–Scott SPO1A sun photometer

The ratio of the direct beam signals of the two instruments (last term in
Eq. A4) depends on both the absolute sensitivity of SRAD, which varied during
the time period, and any non-ideal solar zenith angle response of the SRAD
diffuser (which did not alter significantly during the measurements reported
here). This solar zenith angle dependance (cosine error) has been assessed by
determining the solar zenith angle dependence of the ratio of SRAD to the
sun photometer and corrected. the uncertainty in the cosine correction,
determined by the scatter around a smooth curve, is of the order of 1 % at
solar zenith angles less than 80

Following the correction for the solar zenith dependence, the ratio of the direct beam irradiance signals are quite stable, except when there have been significant instrument changes. In the period 2003–2005 when the instruments were not changed the estimated central value derived from the median has a standard deviation of 0.5 %.

The accuracy of ratio-Langley-derived ratio has been assessed for
sun photometers

The irradiance at the top of the atmosphere is taken from

The direct beam irradiance signal (

The observed scatter in

The uncertainty in the diffuse and global signals can be estimated from the
relationship given in Eq. (4) (main text) with an assumption in the relative
uncertainty in the diffuse and global measurement. Since both measurements
need to be interpolated, both are subject to the same error sources. If the
percent scatter is the same for both diffuse and global, the implied uncertainty
in both is approximately 8 %. Note that these two quantities are
independently measured. The wavelength dependence of this uncertainty in

For

The uncertainty in

Assuming that the correlation between variables is small (Sect. 5.1.2,

The remaining quantity to be estimated is the uncertainty in

Equation (A6) gives an uncertainty that is dependent upon the solar zenith
angle. At around 56

Equation (A6) has been evaluated for the whole data set presented here. On
average, it is found that the final term is 80 % of the total uncertainty,
with the other two terms being around 10 % each. The derived median
uncertainty for

When considering averages of measurements, the importance of the signal
uncertainty will decrease, and the uncertainty in

To evaluate the uncertainty in the integral given in Eq. (2) (main text), it
is necessary to consider all terms over an extended wavelength range. The
combined uncertainty of the cross section and quantum yield for the
production of

The UV-B measurements span the region 298–335 nm, and this can lead to an
underestimate of the photolysis rate. A study by

The resultant estimated uncertainties are summarised in Table A1. It should be noted that this does not include any estimate of the impact of model assumptions, including the assumption of isotropic diffuse irradiance or the assumption of the surface albedo equal to zero.

For comparison with the model TUV 5.0

Summary of the percentage uncertainties

This work would not have been possible without the ongoing dedication and support of the staff at the Cape Grim Baseline Air Pollution Station and the financial support provided for work at Cape Grim by the Bureau of Meteorology. The inspiration of the other scientists involved in the Cape Grim programme is also gratefully acknowledged. The thoughtful, detailed and constructive comments from the referees have significantly improved this paper. Edited by: A. Hofzumahaus