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The effect of relative humidity (RH) on secondary organic aerosol (SOA) formation from the photooxidation of isoprene under initially high nitric oxide (NO) conditions (i.e., isoprene/NO ~3) was investigated in a dual outdoor smog chamber. Based upon particle volume concentration measurements and the detailed chemical characterization of isoprene SOA using gas chromatography/mass spectrometry (GC/MS) and ultra performance liquid chromatography/electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-Q-TOFMS), enhanced SOA formation was observed under lower RH conditions (15–40 %) compared to higher RH conditions (40–90 %). 2-methylglyceric acid (2-MG) and its corresponding oligoesters, which have been previously shown to form from further oxidation of methacryloylperoxynitrate (MPAN), were enhanced in the particle-phase under lower RH conditions. In addition, an abundant unknown SOA tracer likely derived from the further oxidation of MPAN was detected and enhanced under lower RH conditions. In contrast, the 2-methyltetrols, which are known to mainly form from the reactive uptake of isoprene epoxydiols (IEPOX) under low-NO conditions in the presence of acidified aerosol, did not substantially vary under different RH conditions; however, isoprene-derived organosulfates were found to be enhanced under high-RH conditions, indicating the likely importance of the aqueous aerosol phase in their formation. Based upon the detailed chemical characterization results, particle-phase organic esterification is tentatively proposed to explain the observed enhancements of isoprene SOA mass under lower RH conditions. Alternative mechanisms explaining the enhancement of 2-MG and its corresponding oligoesters cannot be completely ruled out. This is one of only a few chamber studies that have examined the effect of RH on isoprene SOA formation. In comparison to our recent results obtained from aromatic SOA formation, the effect of RH on isoprene SOA formation is reversed. The results of this study highlight the importance of elucidating the key reactive intermediates that lead to SOA formation, especially since RH likely affects their ability in forming SOA. Furthermore, ignoring the effects of RH may significantly affect the accuracy of both regional and global SOA models.
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We performed airborne measurements of aerosol particle concentration, composition, size distribution and optical properties over Western Africa in the corridor 2–17° N and 3–5° E. Data were collected on board the French ATR-42 research aircraft in June–July 2006 as part of the African Monsoon Multidisciplinary Analysis (AMMA) intensive field phases in June–July 2006 using the AVIRAD airborne aerosol sampling system. <br><br> The aerosol vertical distribution was documented on an almost daily basis. In particular, the vertical distribution of mineral dust emitted locally by Mesoscale Convective Systems (MSC) was distinguished from that of mineral dust that was transported from the Saharan by the African Easterly Jet (AEJ). Mineral dust emitted in the Sahel by convection-driven erosion was mostly confined in the boundary layer. One episode of injection of Sahelian mineral dust in the AEJ was observed. The elemental composition was found to be an element of difference between episodes, whereas the volume size distribution was more uniform. <br><br> For non-mixed dust, the single scattering albedo ranged between 0.88–0.93 at 370 nm and between 0.91–0.99 at 950 nm, lower values being observed for Sahelian dust. In correspondence, the imaginary part of the complex refractive index varied between 0.002–0.004 at 370 nm and between 0.001–0.002 at 950 nm.
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Ambient particles collected on teflon filters at the Peak of Whistler Mountain, British Columbia (2182 m a.s.l.) during spring and summer 2009 were measured by Fourier transform infrared (FTIR) spectroscopy for organic functional groups (OFG). The project mean and standard deviation of organic aerosol mass concentrations (OM) for all samples was 3.2±3.3 (μg m<sup>−3</sup>). Measurements of aerosol mass fragments, size, and number concentrations were used to separate fossil-fuel combustion and burning and non-burning forest sources of the measured organic aerosol. The OM was composed of the same anthropogenic and non-burning forest components observed at Whistler mid-valley in the spring of 2008; during the 2009 campaign, biomass burning aerosol was additionally observed from fire episodes occurring between June and September. On average, organic hydroxyl, alkane, carboxylic acid, ketone, and primary amine groups represented 31 %±11 %, 34 %±9 %, 23 %±6 %, 6 %±7 %, and 6 %±3 % of OM, respectively. Ketones in aerosols were associated with burning and non-burning forest origins, and represented up to 27 % of the OM. The organic aerosol fraction resided almost entirely in the submicron fraction without significant diurnal variations. OM/OC mass ratios ranged mostly between 2.0 and 2.2 and O/C atomic ratios between 0.57 and 0.76, indicating that the organic aerosol reaching the site was highly aged and possibly formed through secondary formation processes.
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Boreal regions comprise about 17 % of the global land area, and they both affect and are influenced by climate change. To better understand boreal forest fire emissions and plume evolution, 947 whole air samples were collected aboard the NASA DC-8 research aircraft in summer 2008 as part of the ARCTAS-B field mission, and analyzed for 79 non-methane volatile organic compounds (NMVOCs) using gas chromatography. Together with simultaneous measurements of CO<sub>2</sub>, CO, CH<sub>4</sub>, CH<sub>2</sub>O, NO<sub>2</sub>, NO, HCN and CH<sub>3</sub>CN, these measurements represent the most comprehensive assessment of trace gas emissions from boreal forest fires to date. Based on 105 air samples collected in fresh Canadian smoke plumes, 57 of the 80 measured NMVOCs (including CH<sub>2</sub>O) were emitted from the fires, including 45 species that were quantified from boreal forest fires for the first time. After CO<sub>2</sub>, CO and CH<sub>4</sub>, the largest emission factors (EFs) for individual species were formaldehyde (2.1 ± 0.2 g kg<sup>−1</sup>), followed by methanol, NO<sub>2</sub>, HCN, ethene, α-pinene, β-pinene, ethane, benzene, propene, acetone and CH<sub>3</sub>CN. Globally, we estimate that boreal forest fires release 2.4 ± 0.6 Tg C yr<sup>−1</sup> in the form of NMVOCs, with approximately 41 % of the carbon released as C<sub>1</sub>-C<sub>2</sub> NMVOCs and 21 % as pinenes. These are the first reported field measurements of monoterpene emissions from boreal forest fires, and we speculate that the pinenes, which are relatively heavy molecules, were detected in the fire plumes as the result of distillation of stored terpenes as the vegetation is heated. Their inclusion in smoke chemistry models is expected to improve model predictions of secondary organic aerosol (SOA) formation. The fire-averaged EF of dichloromethane or CH<sub>2</sub>Cl<sub>2</sub>, (6.9 ± 8.6) × 10<sup>−4</sup> g kg<sup>−1</sup>, was not significantly different from zero and supports recent findings that its global biomass burning source appears to have been overestimated. Similarly, we found no evidence for emissions of chloroform (CHCl<sub>3</sub>) or methyl chloroform (CH<sub>3</sub>CCl<sub>3</sub>) from boreal forest fires. The speciated hydrocarbon measurements presented here show the importance of carbon released by short-chain NMVOCs, the strong contribution of pinene emissions from boreal forest fires, and the wide range of compound classes in the most abundantly emitted NMVOCs, all of which can be used to improve biomass burning inventories in local/global models and reduce uncertainties in model estimates of trace gas emissions and their impact on the atmosphere.
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Organic aerosols (OA) can be separated with factor analysis of aerosol mass spectrometer (AMS) data into hydrocarbon-like OA (HOA) and oxygenated OA (OOA). We develop a new method to parameterize H:C of OOA in terms of <i>f</i><sub>43</sub> (ratio of <i>m/z</i> 43, mostly C<sub>2</sub>H<sub>3</sub>O<sup>+</sup>, to total signal in the component mass spectrum). Such parameterization allows for the transformation of large database of ambient OOA components from the <i>f</i><sub>44</sub> (mostly CO<sub>2</sub><sup>+</sup>, likely from acid groups) vs. <i>f</i><sub>43</sub> space ("triangle plot") (Ng et al., 2010) into the Van Krevelen diagram (H:C vs. O:C) (Van Krevelen, 1950). Heald et al. (2010) examined the evolution of total OA in the Van Krevelen diagram. In this work total OA is deconvolved into components that correspond to primary (HOA and others) and secondary (OOA) organic aerosols. By deconvolving total OA into different components, we remove physical mixing effects between secondary and primary aerosols which allows for examination of the evolution of OOA components alone in the Van Krevelen space. This provides a unique means of following ambient secondary OA evolution that is analogous to and can be compared with trends observed in chamber studies of secondary organic aerosol formation. The triangle plot in Ng et al. (2010) indicates that <i>f</i><sub>44</sub> of OOA components increases with photochemical age, suggesting the importance of acid formation in OOA evolution. Once they are transformed with the new parameterization, the triangle plot of the OOA components from all sites occupy an area in Van Krevelen space which follows a ΔH:C/ΔO:C slope of ~ −0.5. This slope suggests that ambient OOA aging results in net changes in chemical composition that are equivalent to the addition of both acid <i>and</i> alcohol/peroxide functional groups without fragmentation (i.e. C-C bond breakage), and/or the addition of acid groups with fragmentation. These results provide a framework for linking the bulk aerosol chemical composition evolution to molecular-level studies.
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The spatial distribution of the source-receptor relationship (SRR) of sulfur over Northeast Asia was examined using a chemical transport model (RAQM) off-line coupled with a meteorological model (MM5). The simulation was conducted for the entire year of 2002. The results were evaluated using monitoring data for six remote stations of the Acid Deposition Monitoring Network in East Asia (EANET). The modeled SO<sub>2</sub> and O<sub>3</sub> concentrations agreed well with the observations quantitatively. The modeled aerosol and wet deposition fluxes of SO<sub>4</sub><sup>2−</sup> were underestimated by 30 % and 50 %, respectively. The domain was divided into 5 source-receptor regions: (I) North China; (II) Central China; (III) South China; (IV) South Korea; and (V) Japan. The sulfur deposition in each receptor region amounted to about 50–75 % of the emissions from the same region. The largest contribution to the deposition in each region was originated from the same region, accounting for 53–84 %. The second largest contribution was due to Region II, supplying 14–43 %. The spatial distributions of the SRRs revealed that subregional values varied by about two times more than regional averages due to nonuniformity across the deposition fields. Examining the spatial distributions of the deposition fields was important for identifying subregional areas where the deposition was highest within a receptor region. The horizontal distribution changed substantially according to season.
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Cloud condensation nuclei act as cores for water vapour condensation, and their composition and chemical properties may enhance or depress the ability for droplet growth. In this study we use molecular dynamics simulations to show that model humic-like substances (HULIS) in systems containing 10 000 water molecules mimic experimental data well referring to reduction of surface tension. The model HULIS compounds investigated in this study are cis-pinonic acid (CPA), pinic acid (PAD) and pinonaldehyde (PAL). The structural properties examined show the ability for the model HULIS compounds to aggregate inside the nanoaerosol clusters.
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Ensemble climate model simulations used for the Intergovernmental Panel on Climate Change (IPCC) assessments have become important tools for exploring the response of the Earth System to changes in anthropogenic and natural forcings. The systematic evaluation of these models through global satellite observations is a critical step in assessing the uncertainty of climate change projections. This paper presents the technical steps required for using nadir sun-synchronous infrared satellite observations for multi-model evaluation and the uncertainties associated with each step. This is motivated by need to use satellite observations to evaluate climate models. We quantified the implications of the effect of satellite orbit and spatial coverage, the effect of variations in vertical sensitivity as quantified by the observation operator and the impact of averaging the operators for use with monthly-mean model output. We calculated these biases in ozone, carbon monoxide, atmospheric temperature and water vapour by using the output from two global chemistry climate models (ECHAM5-MOZ and GISS-PUCCINI) and the observations from the Tropospheric Emission Spectrometer (TES) instrument on board the NASA-Aura satellite from January 2005 to December 2008. <br><br> The results show that sampling and monthly averaging of the observation operators produce zonal-mean biases of less than ±3 % for ozone and carbon monoxide throughout the entire troposphere in both models. Water vapour sampling zonal-mean biases were also within the insignificant range of ±3 % (that is ±0.14 g kg<sup>−1</sup>) in both models. Sampling led to a temperature zonal-mean bias of ±0.3 K over the tropical and mid-latitudes in both models, and up to −1.4 K over the boundary layer in the higher latitudes. Using the monthly average of temperature and water vapour operators lead to large biases over the boundary layer in the southern-hemispheric higher latitudes and in the upper troposphere, respectively. Up to 8 % bias was calculated in the upper troposphere water vapour due to monthly-mean operators, which may impact the detection of water vapour feedback in response to global warming. Our results reveal the importance of using the averaging kernel and the a priori profiles to account for the limited vertical resolution and clouds of a nadir observation during model application. Neglecting the observation operators resulted in large biases, which are more than 60 % for ozone, ±30 % for carbon monoxide, and range between −1.5 K and 5 K for atmospheric temperature, and between −60 % and 100 % for water vapour.
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The Tropospheric Emission Spectrometer (TES), a hyperspectral infrared instrument on the Aura satellite, retrieves a vertical profile of tropospheric ozone. However, polar-orbiting instruments like TES provide limited nadir-view coverage. This work illustrates the value of these observations when taken in context with geostationary imagery describing synoptic-scale weather patterns. The goal of this study is to create map-view products of upper troposphere (UT) ozone through the integration of TES ozone measurements with two synoptic dynamic tracers of stratospheric influence: specific humidity derived from the GOES Imager water vapor absorption channel, and potential vorticity (PV) from an operational forecast model. As a mixing zone between tropospheric and stratospheric reservoirs, the upper troposphere (UT) exhibits a complex chemical makeup. Determination of ozone mixing ratios in this layer is especially difficult without direct in situ measurement. However, it is well understood that UT ozone is correlated with dynamical tracers like low specific humidity and high potential vorticity. Blending the advantages of two remotely sensed quantities (GOES water vapor and TES ozone) is at the core of the Multi-sensor Upper Tropospheric Ozone Product (MUTOP). <br><br> Our results suggest that 72 % of TES-observed UT ozone variability can be explained by its correlation with dry air and high PV. MUTOP reproduces TES retrievals across the GOES-West domain with a root mean square error (RMSE) of 18 ppbv (part per billion by volume). There are several advantages to this multi-sensor derived product approach: (1) it is calculated from two operational fields (GOES specific humidity and GFS PV), so maps of layer-average ozone can be created and used in near real-time; (2) the product provides the spatial resolution and coverage of a geostationary image as it depicts the variable distribution of ozone in the UT; and (3) the 6 h temporal resolution of the derived product imagery allows for the visualization of rapid movement of this dynamically-driven ozone, as illustrated in the animation Supplement. This paper presents the scientific basis and methodology behind the creation of this unique ozone product, as well as a statistical comparison of the derived product to an evaluation dataset of coincident TES observations.
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Australian dust emissions are highly episodic, and this may increase the importance of Australian dust as a climate feedback agent. We compare two 160-year coupled atmosphere-ocean simulations of modern-day climate using the CSIRO Mark 3.6 global climate model (GCM). The first run (DUST) includes an interactive treatment of mineral dust and its direct radiative effects. The second run (NODUST) is otherwise identical, but has the Australian dust source set to zero. We focus on the austral spring season, when the correlation between rainfall and the El Niño Southern Oscillation (ENSO) is strongest over Australia. The ENSO-rainfall relationship over eastern Australia is stronger in the DUST run: dry (El Niño) years tend to be drier, and wet (La Niña) years wetter. The amplification of ENSO-related rainfall variability over eastern Australia represents an improvement relative to observations. The effect is driven by ENSO-related anomalies in radiative forcing by Australian dust over the south-west Pacific Ocean; these anomalies increase (decrease) surface evaporation in La Niña (El Niño) years. Some of this moisture is advected towards eastern Australia, where increased (decreased) moisture convergence in La Niña (El Niño) years increases the amplitude of ENSO-related rainfall variability. The modulation of surface evaporation by dust over the south-west Pacific occurs via surface radiative forcing and dust-induced stabilisation of the boundary layer. The results suggest that (1) a realistic treatment of Australian dust may be necessary for accurate simulation of the ENSO-rainfall relationship over Australia, and (2) radiative feedbacks involving dust may be important for understanding natural rainfall variability over Australia.
