Semiannual NO₂ plumes during the monsoon transition periods over the central Indian Ocean

Abstract

[1] In this study we identify recurring plumes of tropospheric NO 2 originating from Africa and Indonesia during the monsoon transition periods over the central Indian Ocean (CIO, 5°N-30°S, 55°E-95°E), based on GOME satellite observations and global model (MATCH-MPIC) simulations. Despite the relatively short lifetime of NO x , these strong plumes can develop due to the pronounced anti-cyclonic circulation over the CIO, and the weak maritime convection, which limits vertical mixing. Model results indicate that the plumes are mainly transported in the middle troposphere (MT). Thus, the CIO in the Southern Hemisphere (SH) is not always as pristine as found in INDOEX during the winter monsoon.

Analysing spatio-temporal patterns of the global NO<sub>2</sub>-distribution retrieved from GOME satellite observations using a generalized additive model

“…5a). For most of the central areas of the Indian Ocean (latitudes between 20 • and 30 • S), we observe mostly two local maxima in the annual signal, again in good agreement with Kunhikrishnan et al (2004). In this example, an apparent wind pattern ("Monsoon") has been absorbed into the annual cycle.…”

Sensitivity of NOx over the Indian Ocean to emissions from the surrounding continents and nonlinearities in atmospheric chemistry responses

“…During the winter monsoon, over the northern part of the CIO (down to the ITCZ at about 5-10°S), the scaled sensitivity to Indian emissions is <10%, while south of the ITCZ the response is negligible. The most prominent feature in this region is the strong sensitivity to emissions from Africa (20 -30%) and SE Asia (>30%) in the MT and UT, especially during the monsoon transition periods, as discussed in Kunhikrishnan et al [2004b]. Chinese emissions also influence the UT near the tropopause due to outflow from summer monsoon deep convection.…”

“…Detailed descriptions and evaluations of the model are given by Rasch et al [1997], Lawrence et al [1999, 2003a, 2003b], and von Kuhlmann et al [2003a, 2003b]. The model output has recently been compared with observations over the Indian Ocean and southern Asian region [ Lawrence et al , 2003a; Kunhikrishnan et al , 2004a, 2004b]. The surface NO x emissions used in MATCH‐MPIC for the regions surrounding the Indian Ocean are given in Table 1; NO x emissions are greatest for Africa (partly due to the large area) and China, and are lowest for the Middle East.…”

Satellite‐observed pollution from Southern Hemisphere biomass burning

“…In addition to CO and hydrocarbons, biomass burning also produces other ozone precursors. A plume of NO 2 , attributed primarily to African biomass burning, is observed by the ERS‐2 satellite Global Ozone Monitoring Experiment (GOME) crossing the Indian Ocean during September [ Kunhikrishnan et al , 2004]. This is the only time of the year when NO 2 levels reach sufficiently high values that they are observable by satellite in the remote SH ocean.…”