Changes in surface ozone levels due to convective

GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L10807, doi:10.1029/2006GL025994, 2006

Changes in surface ozone levels due to convective downdrafts over the Bay of Bengal L. K. Sahu1 and S. Lal1 Received 8 February 2006; revised 29 March 2006; accepted 18 April 2006; published 23 May 2006.

[1] Simultaneous measurements of surface level ozone (O3) and meteorological parameters were made over the Bay of Bengal (BOB) during a cruise between 14 September and 12 October 2002. Variations in surface O3, equivalent potential temperature (qE) and associated meteorological parameters clearly show evidence of convective downdrafts during 18– 19 September and 25– 26 September. Sharp increase in O3 mixing ratio is coupled with decrease in qE value in the detrained air. The vertical wind velocity and satellite images also indicate significant convective activities during these periods. The maximum changes in O3 and qE were 26 ppbv and 12 K, respectively. These events comprise of both short and long time scale downdrafts. Citation: Sahu, L. K., and S. Lal (2006), Changes in surface ozone levels due to convective downdrafts over the Bay of Bengal, Geophys. Res. Lett., 33, L10807, doi:10.1029/ 2006GL025994.

1. Introduction [2] Tropospheric O3 plays a vital role in the chemistry of the atmosphere, with effects on both the environment and Earth’s climate. Photolysis of O3 initiates the oxidation process in the troposphere through the formation of hydroxyl radicals (OH). Ozone, because of its strong absorption band centered at 9.6 mm, also plays an important role in global warming, particularly in the upper troposphere [Forster and Shine, 1997; Gauss et al., 2003]. The photochemical reactions are dominated by the cycles of O3 and water vapor in the tropical region, which receives intense solar radiation. [3] Atmospheric convection in the tropics is particularly important, for rapidly transporting material from near the surface to the upper troposphere [Thompson et al., 1996]. Observations have shown efficient redistribution of trace gases by deep convection [Dickerson et al., 1987; Choi et al., 2005]. The convective transport of O3 precursors, specifically NOx to the upper troposphere, can result in a 30% increase in column tropospheric O3 [Pickering et al., 1992]. In contrast, downdrafts of O3 rich air from the upper troposphere toward the surface can decrease the column tropospheric O3 by about 20% [Lelieveld and Crutzen, 1994]. A recent modeling study [Lawrence et al., 2003] reports that the net effect of convective transport of all trace gases results in a 12% increase in the tropospheric O3 burden. In situ observations of convective downdrafts are limited due to sporadic nature of convection. Satellite observations of trace gases in the troposphere are more difficult to make due to cloud interference during convec1

Physical Research Laboratory, Ahmedabad, India.

Copyright 2006 by the American Geophysical Union. 0094-8276/06/2006GL025994$05.00

tive downdraft events. Reports on the downdraft episodes of free tropospheric O3-rich air into the planetary boundary layer (PBL) by in situ surface measurements are limited to the Amazon Basin, Brazil [Betts et al., 2002] and the Po Basin, Italy [Weber and Prevot, 2002]. We are unaware of any reports on the downdrafts of free tropospheric O3-rich air into the marine boundary layer (MBL). [4] As a part of the Bay of Bengal Process Studies (BOBPS) a cruise campaign was conducted on board ORV Sagar Kanya between 14 September and 12 October 2002 over the Bay of Bengal (BOB). Continuous in situ measurements of O3 and meteorological parameters were carried out along the cruise track (Figure 1). Surface level O3 measurements were made using a Dasibi analyzer, which is based on the absorption of UV radiation at 253.7 nm by O3. The details of this analyzer and its calibration are given by Lal and Lawrence [2001]. During the cruise, southwesterly (SW) winds were predominantly transporting cleaner marine air from the southern Indian Ocean over the BOB. This work is the first study of surface O3, which discusses downdraft events of O3-rich free tropospheric air into the MBL observed over the BOB, when the wind flow was from SW direction.

2. Results and Discussion 2.1. Convective Downdrafts During 18– 19 September 2002 [5] Figure 2 shows variations in surface O3, equivalent potential temperature (qE), temperature and horizontal wind speed during 18– 19 September 2002 over the central BOB. The approximate locations (8.06N, 88.05E and 9.16N, 88.21E) along the cruise track for these two days of observations are marked in Figure 1. Temporal variation in O3 shows two sharp consecutive peaks, from a background value of 17 ppbv to 22 and 24 ppbv, respectively at around 18:00 IST (Indian Standard Time, 5.5 hours ahead of GMT) on 18 September. These sharp increases in O3 mixing ratios coincide with simultaneous decreases in both qE and temperature. These sudden changes in O3 mixing ratios are tightly coupled with qE and temperature but are in the opposite direction. Similar features but with larger amplitudes are observed at around 05:00 IST on 19 September. In this case, O3 sharply increased from a background value of 16 ppbv to 29 ppbv, whereas qE decreased from 352.5 K to 342 K and temperature from 27.7C to 25.2C. Analogous to the event of 18 September, variation in O3 is inversely coupled with qE and temperature. Further, the event on 19 September is also marked by a dip in surface horizontal wind speed. The observations also show many small sporadic sharp changes in O3, qE and temperature (see Figure 2), which are not being discussed here. [6] The observations during the period (18 – 19 September) were made over the central BOB far away from the coast

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Figure 3. NCEP reanalysis-1 derived average vertical wind (Pascal s 1) field during 18– 19 September 2002 over the Bay of Bengal and surrounding regions.

Figure 1. Cruise track (dotted line) of ORV Sagar Kanya during 14 September to 12 October 2002 over the Bay of Bengal. The four filled circles indicate the locations of convective downdrafts. (Figure 1). The wind field and back trajectory analyses do not reveal any evidence of transport from over the land on these days. The average value of O3 was 17 ± 1.5 ppbv with no noticeable diurnal variation in the absence of downdrafts. Further, no relations between O3 and qE or temperature were observed whenever continental air was encountered along other segments of the cruise track. Thus, advection of air masses from the surrounding continents cannot be the cause of these sharp changes in O3 and meteorological tracers. These changes most likely result from downward transport. [7] Many researchers have used qE to track downdrafts of free tropospheric air into the boundary layer [Betts, 1973; Emanuel, 1994; Betts et al., 2002]. Surface sensible and

latent heat fluxes are considered the main sources of qE in the atmosphere [Betts et al., 1992], while the primary sink is radiative cooling of the troposphere. Consequently, the tropical lower troposphere is characterized by a decrease in qE and an increase in O3 with height [Folkins et al., 1999; Zachariasse et al., 2000; Betts et al., 2002]. Independent meteorological parameters such as temperature and pressure may not be conserved in downdrafts, while qE is near conservative. This suggests that the observed peaks with different amplitudes in O3, qE, and temperature could be due to rapid intrusions of air into the MBL from different altitudes. Since, there were no simultaneous vertical measurements of O3 and meteorological parameters, it is difficult to deduce the origin of detrained air into the MBL. [8] Downdrafts are often linked with convection, as just behind and below the sloping updraft of a squall line there is a convective scale downdraft [Houze and Betts, 1981]. Figure 3 shows average vertical wind velocity (Pascal s 1) at 850 mb level during 18 – 19 September, based on data from the National Centers for Environmental Prediction (NCEP; http://www.cdc.noaa.gov/cdc/data.ncep.reanalysis.html).

Figure 2. Surface level variations in O3, qE, temperature and wind speed observed during 18– 19 September 2002 over the Bay of Bengal. 2 of 4

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Figure 4. Surface level variations in O3, qE and relative humidity during (a) 25 September and (b) 26 September 2002 over the Bay of Bengal. The high magnitude of upward wind velocity (< 0.1 Pascal s 1) centered at around 12N, 90E indicates the presence of a convective system over the BOB. The regional satellite cloud image (DMSP F-142.7 km visible imagery) for 19 September also shows convective activity over parts of the BOB (see http://www.nnvl.noaa.gov/cgi-bin/index.cgi?page = events&category = 2002%20Regional%20Imagery) 2.2. Convective Downdrafts During 25– 26 September 2002 [9] Two examples of relatively large-scale convective downdraft events showing a sudden rise in O3, fall in qE and relative humidity observed on 25 and 26 September are presented in Figure 4. The locations (17.98N, 87.97E and 19.52N 87.99E) of the study are marked in Figure 1. Ozone and qE show clear anti-correlation during both the downdraft events. Ozone suddenly doubled from its initial value of 21 ppbv, with a simultaneous decrease in qE from 355 K to 351 K on the morning of 25 September at around 09:00 IST. The corresponding relative humidity dropped by 12% (from 87% to 75%). In general, convective-scale downdrafts are associated with heavy precipitation [Houze and Betts, 1981]. The heavy rainfall in the afternoon hours on 25 September also suggests a convective downdraft. The O3 analyzer was switched off as a precautionary measure during the rainy period. In addition, the satellite image for

25 September shows development of a cyclone over northeast region of the BOB. On 26 September, another example of such an anti-correlation was observed at around 10:30 IST at a different site. Ozone increased from 25 ppbv to 40 ppbv whereas qE decreased from 357 K to 347– 348 K (Figure 4b). The relative humidity changed from 88% to 80% during this event. [10] Similar relations between O3 and qE have been reported over the Amazon [Betts et al., 2002], which were attributed to convective downdrafts of free tropospheric air into the planetary boundary layer (PBL). In agreement with typical qE values in the tropical MBL [Folkins et al., 1999], the present observations show values of 354 – 357 K outside the downdraft regime for these two days of measurements (Figure 4). [11] A simultaneous rapid increase in the O3 mixing ratio and a decrease in qE below characteristic MBL values indicate downdrafts of free tropospheric air. Compared to the observations during 18– 19 September, the change in qE by 4 K and 25 ppbv change in O3 are disproportional during the downdraft on 25 September (Table 1). This could be due to sampling of already detrained air. Under these circumstances, qE would not be fully conserved and would increase due to exchange of thermal and meteorological properties with surrounding marine air. Other possibility could be changes in the vertical gradients of O3 and qE from

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Table 1. Maximum Changes in qE and O3 During Different Convective Downdraft Events Over the Bay of Bengal Downdraft Event 1 2 3 4

Date (Year 2002) 18 19 25 26

September September September September

Time, Latitude, Longitude, IST N E 18:35 04:40 08:55 11:40

8.03 9.02 17.85 18.80

88.05 88.02 87.90 88.11

DqE, K 5.6 11.7 4.1 8.8

DO3, ppbv 6.7 12.7 26.2 16.5

event to event. After these transitions almost constant levels of 40 ppbv in O3 and 348 K in qE were observed for the next 12 hours on 26 September. During these downdrafts not only qE and relative humidity but other meteorological parameters, e.g., wind speed and wind direction, also showed anomalies (not shown). Vertical profiles of O3 and qE over locations near the downdrafts are not available; these could have provided estimates of original altitudes of these air parcels.

3. Summary and Conclusions [12] Continuous in situ measurements of surface level O3 and meteorological parameters were made between 14 September and 12 October 2002 over the BOB. This paper presents the distribution of O3 and its relation to meteorological parameters in convective downdrafts in the MBL. The observed sharp rises in O3 mixing ratios are tightly coupled with decreases in qE and temperature during 18– 19 September in the detrained air masses. Decrease in qE values up to 10 K accompanied by 13 ppbv increase in O3 indicate recent intrusion of free tropospheric air into the MBL. Analyses of vertical wind velocity and satellite cloud images indicate the presence of a convective system near the observations sites, supporting the interpretation of O3-qE results. [13] The second set of relatively large-scale downdraft events was observed during 25– 26 September. A sharp increase in O3 of 26 ppbv is marked by simultaneous decreases in qE and relative humidity from their background values on 25 September. The heavy rainfall observed just after this event provides an additional evidence for convective downdraft process. Another transition, showing a 15 ppbv increase in O3 and a fall in qE of 10 K in detrained air persisted for the next 12 hours after the event on 26 September. The maximum changes observed in qE and O3 with respect to their marine background levels during different downdraft events are given in Table 1. The disproportional changes in qE and O3 can result from difference in time delay between detrainment and their measurement. While, another cause could be changes in the vertical gradients of qE and O3 from event to event. [14] The fractional area covered by convective clouds associated with the summer monsoon circulation during July – September is greatest over south Asia [Gettelman et al., 2002]. The widespread downdrafts of free tropospheric air would result in elevated background O3 mixing ratios in the MBL. The observed background O3 value of 17 ppbv over the BOB is significantly higher than the southern Indian Ocean background value of 9 ppbv [Chand et al., 2003] and 8 ppbv observed over the Pacific Ocean [Singh et al., 1996]. Downdrafts of O3 rich free tropospheric air into the MBL could have substantial impact on the local photochemistry and its budget in the troposphere, as the destruction of O3 in the MBL by photolysis to O1(D) followed by its

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reaction with H2O is a significant sink. However, convective processes are highly variable in nature and their impact on the redistribution of chemical species may vary significantly. [15] Acknowledgments. We thank S. Venkataramani, K. S. Modh, T. K. Sunil and S. Desai for help in the experiment, P. C. Pandey and M. Sudhakar for providing us berth in the cruise and the officers and the scientific team of ORV Sagar Kanya for logistical help. The NCEP reanalysis data were downloaded from NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado, USA. We also thank to the anonymous reviewers for their very useful comments and suggestions.

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S. Lal and L. K. Sahu, Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India. ([email protected])

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