Differential Optical Absorption Spectroscopy Observations Research Articles

Modeling the reactive halogen plume from Ambrym and its impact on the troposphere with the CCATT-BRAMS mesoscale model

Abstract. Ambrym Volcano (Vanuatu, southwest Pacific) is one of the largest sources of continuous volcanic emissions worldwide. As well as releasing SO2 that is oxidized to sulfate, volcanic plumes in the troposphere are shown to undergo reactive halogen chemistry whose atmospheric impacts have been little explored to date. Here, we investigate with the regional-scale model CCATT-BRAMS (Coupled Chemistry Aerosol-Tracer Transport model, Brazilian developments on the Regional Atmospheric Modeling System, version 4.3) the chemical processing in the Ambrym plume and the impact of this volcano on the atmospheric chemistry on both local and regional scales. We focus on an episode of extreme passive degassing that occurred in early 2005 and for which airborne DOAS (differential optical absorption spectroscopy) measurements of SO2 and BrO columns in the near-downwind plume between 15 and 40 km from the vents have been reported. The model was developed to include reactive halogen chemistry and a volcanic emission source specific to this extreme degassing event. In order to test our understanding of the volcanic plume chemistry, we performed very high-resolution (500 m × 500 m) simulations using the model nesting grid capability and compared each DOAS measurement to its temporally and spatially interpolated model counterpart “point-by-point”. Simulated SO2 columns show very good quantitative agreement with the DOAS observations, suggesting that the plume direction as well as its dilution in the near-downwind plume are well captured. The model also reproduces the salient features of volcanic chemistry as reported in previous work, such as HOx and ozone depletion in the core of the plume. When a high-temperature chemistry initialization is included, the model is able to capture the observed BrO ∕ SO2 trend with distance from the vent. The main discrepancy between observations and model is the bias between the magnitudes of observed and simulated BrO columns that ranges from 60 % (relative to the observations) for the transect at 15 km to 14 % for the one at 40 km from the vents. We identify total in-plume depletion of ozone as a limiting factor for the partitioning of reactive bromine into BrO in the near-source (concentrated) plume under these conditions of extreme emissions and low background ozone concentrations (15 ppbv). Impacts of Ambrym in the southwest Pacific region were also analyzed. As the plume disperses regionally, reactive halogen chemistry continues on sulfate aerosols produced by SO2 oxidation and promotes BrCl formation. Ozone depletion is weaker than on the local scale but still between 10 and 40 % in an extensive region a few thousands of kilometers from Ambrym. The model also predicts the transport of bromine to the upper troposphere and stratosphere associated with convection events. In the upper troposphere, HBr is re-formed from Br and HO2. Comparison of SO2 regional-scale model fields with OMI (Ozone Monitoring Instrument) satellite SO2 fields confirms that the Ambrym SO2 emissions estimate based on the DOAS observations used here is realistic. The model confirms the potential of volcanic emissions to influence the oxidizing power of the atmosphere: methane lifetime (calculated with respect to OH and Cl) is increased overall in the model due to the volcanic emissions. When considering reactive halogen chemistry, which depletes HOx and ozone, the lengthening of methane lifetime with respect to OH is increased by a factor of 2.6 compared to a simulation including only volcanic SO2 emissions. Cl radicals produced in the plume counteract 41 % of the methane lifetime lengthening due to OH depletion. Including the reactive halogen chemistry in our simulation also increases the lifetime of SO2 in the plume with respect to oxidation by OH by 36 % compared to a simulation including only volcanic SO2 emissions. This study confirms the strong influence of Ambrym emissions during the extreme degassing event of early 2005 on the composition of the atmosphere on both local and regional scales. It also stresses the importance of considering reactive halogen chemistry when assessing the impact of volcanic emissions on climate.

Gas emission strength and evolution of the molar ratio of BrO/SO2 in the plume of Nyiragongo in comparison to Etna

AbstractAirborne and ground‐based differential optical absorption spectroscopy observations have been carried out at the volcano Nyiragongo (Democratic Republic of Congo) to measure SO2 and bromine monoxide (BrO) in the plume in March 2004 and June 2007, respectively. Additionally filter pack and multicomponent gas analyzer system (Multi‐GAS) measurements were carried out in June 2007. Our measurements provide valuable information on the chemical composition of the volcanic plume emitted from the lava lake of Nyiragongo. The main interest of this study has been to investigate for the first time the bromine emission flux of Nyiragongo (a rift volcano) and the BrO formation in its volcanic plume. Measurement data and results from a numerical model of the evolution of BrO in Nyiragongo volcanic plume are compared with earlier studies of the volcanic plume of Etna (Italy). Even though the bromine flux from Nyiragongo (2.6 t/d) is slightly greater than that from Etna (1.9 t/d), the BrO/SO2 ratio (maximum 7 × 10−5) is smaller than in the plume of Etna (maximum 2.1 × 10−4). A one‐dimensional photochemical model to investigate halogen chemistry in the volcanic plumes of Etna and Nyiragongo was initialized using data from Multi‐GAS and filter pack measurements. Model runs showed that the differences in the composition of volcanic volatiles led to a smaller fraction of total bromine being present as BrO in the Nyiragongo plume and to a smaller BrO/SO2 ratio.

Nocturnal loss of NOx during the 2010 CalNex‐LA study in the Los Angeles Basin

AbstractThe chemical removal of NOx at night in urban areas remains poorly constrained due to uncertainties in the contribution of various loss pathways and the impact of the suppressed nocturnal vertical mixing. Here we present long‐path differential optical absorption spectroscopy observations of nocturnal vertical concentration profiles of O3, NO2, and NO3 in the lower atmosphere (33–556 m above ground level) measured during the CalNex‐LA 2010 study. Positive nocturnal vertical gradients of O3 and NO3 and negative gradients of NO2 were observed during the night. Relatively short lifetime of nocturnal NO3 (less than 1000 s) and high nighttime steady state N2O5 mixing ratios (up to 2 ppb) indicated active nocturnal chemistry during CalNex. Comparison of modeled and observed altitude‐resolved NO3 loss frequencies shows that hydrolysis of N2O5 on aerosols was the dominant loss pathway of NO3 and NOx. Based on this argument, the nocturnal loss rates of NOx, L(NOx), at different altitudes and averaged over the lowest 550 m of the atmosphere were calculated. The nocturnally averaged L(NOx) ranged between 0.8 and 1.3 ppb h−1 for the lower atmosphere with the L(NOx) for the first 8 days at about 1 ppb h−1. This number is close to the one previously determined in Houston in 2009 of ~0.9 ppb h−1. Comparisons between daytime NOx loss due to the OH + NO2 reaction and nighttime L(NOx) show that during CalNex, nocturnal chemistry contributed an average of 60% to the removal of NOx in a 24 h period in the lower atmosphere.

Daytime OIO in the Gulf of Maine

The potential importance of iodine for marine boundary layer (MBL) chemistry has found increasing recognition in recent years. However, observations of the key iodine species are sparse and the chemical reactions of the iodine oxides are not well understood. Here we present Differential Optical Absorption Spectroscopy observations of IO, OIO in the MBL of the Gulf of Maine, U.S., during Summer 2004. We report the first daytime observation of OIO, indicating that this compound is rather photostable. Mixing ratios of IO were similar to, and those of OIO higher than, values reported for European coastal sites. Calculations with the one‐dimensional model MISTRA show that the observed simultaneous presence of elevated OIO and NOx cannot be explained by currently known iodine chemistry. Our results lead to the conclusion that thus far unknown chemical reactions of iodine oxides, probably involving iodine nitrates, might occur in the MBL.

Cloudy sky optical paths as derived from differential optical absorption spectroscopy observations

Recently, the influence of tropospheric clouds on zenith sky observations of atmospheric gases (O3, NO2, O4, H2O, and others) has been reported [Van Roozendael et al., 1994; Erle et al., 1995]. For measurements under heavy cloud cover, Erle et al. [1995] determined from simultaneously measured absorptions of different tropospheric species an average optical path for photons reaching the instrument of >100 km. This path length was much larger than expected from previous reports [Feigelson, 1981]. Thus an important question was whether the magnitude of the observation was a typical situation rather than a rare exception. Here we add a large set of new “cloud event” observations evaluated from data which were recorded in the Arctic, at midlatitudes, and the tropics. It is shown that the observed absorption enhancements of O3, H2O, and O4 perfectly coincide with indirect cloud indicators, i.e., changes in the average intensity and the color index as well as direct observations of cloud passages detected by satellite. In particular, we demonstrate that the observed optical paths have the same magnitude as the path lengths reported by Erle et al [1995], and thus optical paths under cloudy skies may indeed become much larger than previously reported. Besides these observations made during specific meteorological conditions we also studied optical paths continuously for several weeks in Arctic winter/spring conditions. We derived a frequency distribution of path lengths for cloudy skies with an average mean optical path of 29 km.

A DOAS study on the origin of nitrous acid at urban and non-urban sites

Measurements have been performed to study the origin of the tropospheric nighttime concentrations of HNO 2 The experiments were carried out at three locations with different levels of pollution by using a commercial differential optical absorption spectroscopy (DOAS) and a point sampling analyser. The DOAS observations allowed study of the concentrations of HNO 2, NO 2, SO 2 and O 3. Light intensities given by the DOAS system were used to estimate rough values for the total surface of aerosols S along the beam path. NO was measured by a chemiluminescent monitor. A HNO 2 formation is clearly indicated by the nighttime increase of the HNO 2/NO x ratio. Correlation studies suggest the participation of the aerosol surface in the heterogeneous formation in the less polluted areas. In Milan the measured HNO 2 concentrations are considered to be influenced by the nighttime boundary layer and the heterogeneous formation on permanent surfaces like buildings.