Variability in tropical tropospheric ozone: Analysis with Global Ozone Monitoring Experiment observations and a global model

Abstract

Tropical tropospheric ozone columns (TTOCs) have been determined with a convective‐cloud‐differential (CCD) method, using ozone column and cloud measurements from the Global Ozone Monitoring Experiment (GOME) instrument. GOME cloud top pressures, derived with the Fast Retrieval Scheme for Clouds from the Oxygen A‐band (FRESCO) method, indicate that most convective cloud top levels are between 300 and 500 hPa and do not extend to the tropical tropopause. The new GOME‐CCD method takes this tropical transition layer below the tropopause into account and uses above‐cloud and clear‐sky ozone column measurements to derive a monthly mean TTOC below 200 hPa. Validation of the GOME‐TTOCs with seven Southern Hemisphere Additional Ozonesondes (SHADOZ) sites shows good agreement, with an RMS difference of about 5 Dobson units. In the northern tropics the GOME‐TTOC compares most of the time well with in situ measurements at Paramaribo (6°N, 55°W) and Abidjan (5°N, 4°W). Analysis of the GOME‐TTOCs for 2000 and 2001, with the aid of the chemistry‐transport model TM3, illustrates that the variability in the TTOC depends on a complex interaction of several processes, including biomass burning, lightning, and large‐scale transport. The much larger extent of the South Atlantic TTOC maximum in September–October 2001, compared to September–October 2000, can be attributed to differences in large‐scale transport. An exceptional situation in the northern tropics occurred during the biomass burning season December 2001 to January 2002, when there were almost no fires over northern Africa. This resulted in strongly reduced TTOCs over the Atlantic between the equator and 10°N.

Tropical tropospheric ozone column retrieval for GOME-2

“…The CCD method can only be applied in the tropics where the assumption of a zonally invariant stratospheric column is valid. An improved CCD method has been developed by Valks et al (2003) that is based on total ozone and cloud measurements from GOME. In contrast to TOMS, GOME is able to determine cloud fraction, cloud pressure and cloud albedo (optical thickness) by using measurements in the visible and near-infrared wavelength region.…”

“…In Valks et al (2003) comparisons of the CCD method for the GOME/ERS-2 instrument with stratospheric ozone columns based on ozonesonde data from the SHADOZ network have been made. The monthly mean stratospheric ozone columns derived with the GOME CCD method have been compared with ozonesonde measurements for eight tropical sites.…”

Tropical tropospheric ozone columns from nadir retrievals of GOME-1/ERS-2, SCIAMACHY/Envisat, and GOME-2/MetOp-A (1996–2012)

“…In Sect. 3, we describe the CCD technique and the modifications we made to the method applied in the past by Ziemke et al (1998) and Valks et al (2003Valks et al ( , 2014. In Sect.…”

“…In a subsequent step, the monthly-mean above-cloud ozone columns (ACCOs) are subtracted from the cloud-free TCOs, assuming a zonally invariant stratospheric column, resulting in monthly averaged tropical tropospheric columns of ozone (TTCO). The same method was improved and applied to GOME (Valks et al, 2003) and GOME-2 data by Valks et al (2014). In contrast to TOMS, GOME was able to determine cloud fractions (CFs), cloud albedos, and cloud-top pressures by using spectral measurements in the near-infrared wavelength (oxygen A-band) region combined with broadband spectral data from polarisation measurement devices that have better spatial resolution than the spectra used in trace gas retrievals.…”

Influence of stratospheric airmasses on tropospheric vertical O<sub>3</sub> columns based on GOME (Global Ozone Monitoring Experiment) measurements and backtrajectory calculation over the Pacific

“…The tropospheric excess ozone is globally derived from GOME data by applying the Tropospheric Excess Method (TEM) (Fishman et al, 1990). Another method to derive tropospheric O 3 columns from the measurements of a nadir-looking instrument is the Convective-CloudDifferential (CCD) method which is discussed by (Valks et al, 2003) and (Ziemke et al, 1998). …”