Constraining remote oxidation capacity with ATom observations

Abstract

The global oxidation capacity, defined as the tropospheric mean concentration of the hydroxyl radical (OH), controls the lifetime of reactive trace gases in the atmosphere such as methane and carbon monoxide (CO). Models tend to underestimate the methane lifetime and CO concentrations throughout the troposphere, which is consistent with excessive OH. Approximately half of the oxidation of methane and non-methane volatile organic compounds (VOCs) is thought to occur over the oceans where oxidant chemistry has received little validation due to a lack of observational constraints. We use observations from the first two deployments of the NASA ATom aircraft campaign during July-August 2016 and January-February 2017 to evaluate the oxidation capacity over the remote oceans and its representation by the GEOS-Chem chemical transport model. The model successfully simulates the magnitude and vertical profile of remote OH within the measurement uncertainties. Comparisons against the drivers of OH production (water vapor, ozone, and NO _y concentrations, ozone photolysis frequencies) also show minimal bias, with the exception of wintertime NO _y . The severe model overestimate of NO _y during this period may indicate insufficient wet scavenging and/or missing loss on sea-salt aerosols. Large uncertainties in these processes require further study to improve simulated NO _y partitioning and removal in the troposphere, but preliminary tests suggest that their overall impact could marginally reduce the model bias in tropospheric OH. During the ATom-1 deployment, OH reactivity (OHR) below 3 km is significantly enhanced, and this is not captured by the sum of its measured components (cOHR_obs) or by the model (cOHR_mod). This enhancement could suggest missing reactive VOCs but cannot be explained by a comprehensive simulation of both biotic and abiotic ocean sources of VOCs. Additional sources of VOC reactivity in this region are difficult to reconcile with the full suite of ATom measurement constraints. The model generally reproduces the magnitude and seasonality of cOHR_obs but underestimates the contribution of oxygenated VOCs, mainly acetaldehyde, which is severely underestimated throughout the troposphere despite its calculated lifetime of less than a day. Missing model acetaldehyde in previous studies was attributed to measurement uncertainties that have been largely resolved. Observations of peroxyacetic acid (PAA) provide new support for remote levels of acetaldehyde. The underestimate in both model acetaldehyde and PAA is present throughout the year in both hemispheres and peaks during Northern Hemisphere summer. The addition of ocean sources of VOCs in the model increases cOHR_mod by 3% to 9% and improves model-measurement agreement for acetaldehyde, particularly in winter, but cannot resolve the model summertime bias. Doing so would require 100 Tg yr^-1 of a long-lived unknown precursor throughout the year with significant additional emissions in the Northern Hemisphere summer. Improving the model bias for remote acetaldehyde and PAA is unlikely to fully resolve previously reported model global biases in OH and methane lifetime, suggesting that future work should examine the sources and sinks of OH over land.

Figure 1.

Annual mean 2016 (a) surface (log scale) and (b) zonal mean cOHR calculated from individual model species. The GEOS-Chem species included in the calculation of cOHR are listed in Table S3.

Figure 2.

ATom-1 ocean-only flight tracks colored by altitude.

Figure 3.

Median OH concentrations for the Northern Hemisphere (> 0° N) and Southern Hemisphere (< 0° S) from the ATHOS instrument described in Table 2 during ATom-1 (July–August 2016) and ATom-2 (January–February 2017) compared against the GEOS-Chem model in 0.5 km altitude bins. The observations have been filtered to remove biomass burning (acetonitrile > 200 ppt) and stratospheric (O₃/CO > 1.25) influence. The dashed lines show the observed 25th–75th percentiles.

Figure 4.

The same as Fig. 3 for median water vapor concentrations. Water vapor mixing ratio was measured by the DLH instrument as described in Table 2.

Figure 5.

The same as Fig. 3 for median photolysis frequencies for ozone (j (O¹D)). The actinic flux measured by the CAFS instrument is used to calculate j (O¹D) as described in Table 2.

Figure 6.

The same as Fig. 3 for median ozone concentrations. Ozone was measured by the NOAA NO_yO₃ instrument as described in Table 2.

Figure 7.

The same as Fig. 3 for median NO_y (a) and NO (b) concentrations. NO_y and NO were measured by the NOAA NO_yO₃ instrument as described in Table 2.

Figure 8.

Comparison of modeled and observed HNO₃, ozone, NO_y, and NO with sensitivity studies including scaling emissions from the US and Asia, improved chlorine chemistry (X. Wang et al., 2019), and the photolysis of particulate nitrate on coarse-mode sea-salt aerosols (Kasibhatla et al., 2018) as described in Sect. 4.1. HNO₃ was measured by the Caltech CIMS; ozone, NO_y, and NO were measured by the NOAA NO_yO₃ instrument (Table 2).

Figure 9.

The same as Fig. 3 for median OHR. OHR was measured by the ATHOS instrument as described in Table 2. The calculation of cOHR in the model and observations includes the species described in Table 2. In order to allow for a point-by-point comparison of cOHR in the model and observations, missing values are filled in the observational components of cOHR using linear interpolation. All calculated reactivity values are determined using the temperature and pressure of the ATHOS instrument inlet, which differ from ambient values. The sensitivity tests are described in Sect. 5.

Figure 10.

Impact of all ocean emissions (Tables 3 and 4) on annual simulated 2016 surface cOHR as described in the text.

Figure 11.

Median observed and modeled OHR and cOHR (see text) below 3 km in the Northern Hemisphere (> 0° N) and Southern Hemisphere (< 0° S) during ATom-1. The “Other” category is the following species as described in Table 2: ethanol, propane, ethane, acetone, > C₃ aldehydes, methyl ethyl ketone, methyl vinyl ketone, methacrolein, benzene, toluene, > C₄ alkanes, peroxyacetic acid, peroxynitric acid, dimethyl sulfide, nitric acid, NO, and NO₂. The diameter of each pie chart is scaled relative to the maximum cOHR for ATom-1.

Figure 12.

Same as Fig. 10 but for ATom-2. The diameter of each pie chart is scaled relative to the maximum cOHR for ATom-2.

Figure 13.

The same as Fig. 3 for median acetaldehyde profiles. Acetaldehyde was measured by the TOGA instrument as described in Table 2. The sensitivity studies are described in Sect. 5.1.

Figure 14.

The same as Fig. 3 for median peroxyacetic acid (PAA) profiles. PAA was measured by the Caltech CIMS instrument as described in Table 2. The sensitivity studies are described in Sect. 5.1.

Figure 15.

The same as Fig. 3 for median peroxyacetyl nitrate (PAN) profiles. PAN was measured by the PANTHER instrument as described in Table 2. The sensitivity studies are described in Sect. 5.1.