COMPARISON OF BREWER AND DOBSON TOTAL OZONE

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COMPARISON OF BREWER AND DOBSON TOTAL OZONECOMPARISON OF BREWER AND DOBSON TOTAL OZONE

Brewer and Dobson spectrophotometers are widely used for Total Ozone monitoring.

In Arosa (Switzerland, 46.8N/9.68E 1820 m a.s.l.) there are two semi-automatic Dobson systems and three automatic Brewer systems, which measure total ozone continuously and quasi - simultaneously. The co-location of these two instruments allows a direct comparison for QUALITY CONTROL and for STUDYING THE DIFFERENCES BETWEEN THE TWO TYPES OF SPECTROPHOTOMETERS.

Data (1995-2004) from the two types of sun spectrophotometers (Dobson 62, 101 and Brewer 40, 72, 156) are compared two by two and analysed using the following statistical model:

O3Br - O3

D = C + + XT +

O3 Br; O3 D: Brewer and Dobson Total Ozone C: allows a zero mean difference μ: linear term proportional to the air mass [Thomason et al., 1983]XT: influence of temperatureerror

Two distinct approaches for differences in temperature sensitivities of ozone wavelengths are considered:

Approach 1

Teff: ozone effective temperature calculated from ozone and temperature sounding data from Payerne (200 km from Arosa).

Approach 2

Tp: simplified approach only using temperature at two altitudes (at 31 and 51 hPa given by the soundings of Payerne Aerological Station).

Wave length [nm]

Ozon

e cr

oss s

ectio

n

300 310 320 330 340

5*10

^-22

5*10

^-21

5*10

^-20

5*10

^-19

O3 X-section temperature dependance

202 K221 K241 K273 K293 K

A-pairC-pair

D-pair

DOBSON

MS5

MS6

MS7

BREWER

Reduction of seasonal differences by model (Fig. 3):

Annual Average:

The contribution to the difference from the effective temperature is significantly larger than that from μ (Fig. 4, center plot):

Seasonal Variability:

• The relative contribution to the difference from μ is larger in winter than in summer• Dependence on the magnitude of TOZ:

– TOZ<290 DU: agreement between the 2 instruments is better, with improved agreement in summer than winter. –TOZ>290 DU: the influence of atmospheric parameters is reduced in winter.

Approach 2

Less precise.Further analysis: Attempt to generalize transfer function using other instruments at Arosa.

METHOD

Fig 2. Model of the molecular ozone cross-section from the Global Ozone Monitoring Experiment (satellite UV-visible spectrophotometer) database.

Quasi-simultaneous direct sun measurements from the two types of spectrophotometers yield small, but characteristic seasonal variation in the differences of Total Ozone (Fig. 1).

Causes of differences might include:

• Different wavelengths with different T sensitivities of ozone absorption (Fig. 2)

• Different retrieval algorithms

• Different fields of view

• Differing influence of scattered light

Fig 4. Residual distribution of Total Ozone when applying different explanatory variables.

Contact: barbara.scarnato@env.ethz.ch

Fig 3. Time series of the residuals for B40 and D101(AD) after the model has been applied to the total ozone difference.

MEASUREMENTS

RESULTSApproach 1

Barbara Scarnato1), Johannes Staehelin1), René Stübi 2), Herbert Schill 2)

1) IAC/ETH, Zürich; Switzerland 2) MeteoSwiss, Aerological Station; Payerne Switzerland

Dobson

Brewer

INTRODUCTION

Difference of readings [DU]

Den

sity

Fig 1. Time series for B40 and D101 (AD pair) and their difference. The coincidence criterion is 10 min.