Pollution influences on atmospheric composition and chemistry at high northern latitudes: Boreal and California forest fire emissions

Abstract

We analyze detailed atmospheric gas/aerosol composition data acquired during the 2008 NASA ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) airborne campaign performed at high northern latitudes in spring (ARCTAS-A) and summer (ARCTAS-B) and in California in summer (ARCTAS-CARB). Biomass burning influences were widespread throughout the ARCTAS campaign. MODIS data from 2000 to 2009 indicated that 2008 had the second largest fire counts over Siberia and a more normal Canadian boreal forest fire season. Near surface arctic air in spring contained strong anthropogenic signatures indicated by high sulfate. In both spring and summer most of the pollution plumes transported to the Arctic region were from Europe and Asia and were present in the mid to upper troposphere and contained a mix of forest fire and urban influences. The gas/aerosol composition of the high latitude troposphere was strongly perturbed at all altitudes in both spring and summer. The reactive nitrogen budget was balanced with PAN as the dominant component. Mean ozone concentrations in the high latitude troposphere were only minimally perturbed (<5 ppb), although many individual pollution plumes sampled in the mid to upper troposphere, and mixed with urban influences, contained elevated ozone (ΔO₃/ΔCO = 0.11 ± 0.09 v/v). Emission and optical characteristics of boreal and California wild fires were quantified and found to be broadly comparable. Greenhouse gas emission estimates derived from ARCTAS-CARB data for the South Coast Air Basin of California show good agreement with state inventories for CO₂ and N₂O but indicate substantially larger emissions of CH₄. Simulations by multiple models of transport and chemistry were found to be broadly consistent with observations with a tendency towards under prediction at high latitudes.

Introduction

The Arctic is one of the most environmentally sensitive regions of the earth system. The warming in this part of the atmosphere is the largest with visible evidence seen in receding sea ice cover (IPCC, 2007). Surface observations have shown that the Arctic is affected by transported Eurasian pollution in winter–spring leading to phenomena such as “Arctic haze” (Rahn, 1981, Barrie, 1986, Shaw, 1995, Quinn et al., 2007, Quinn et al., 2008). In the summer, gas and aerosol emissions from boreal forest fires in Siberia, Canada, and Alaska represent a major chemical and radiative perturbation to the Arctic atmosphere (Stocks et al., 1998, Soja et al., 2007, Shindell et al., 2007, Stone et al., 2008). Black carbon (soot, BC) in Arctic snow can change snow albedo, altering radiative balance in the region (Clarke and Noone, 1985, Hansen and Nazarenko, 2004, Preston and Schmidt, 2006, Flanner et al., 2007). Emissions from boreal fires can be transported long distances influencing air quality in various regions of the Northern Hemisphere and be injected in the stratosphere under special convective conditions (Fromm and Servranckx, 2003, Colarco et al., 2004, Morris et al., 2006, Leung et al., 2007, Real et al., 2007). Recent investigations have suggested that pollution influences in the Arctic are a year round multi-altitude phenomenon that are poorly simulated by models (Klonecki et al., 2003, Stohl, 2006, Law and Stohl, 2007, Shindell et al., 2008).

Within the framework of the 2007–2009 International Polar Year activities (http://www.ipy.org), a number of field campaigns were carried out as part of the POLARCAT (POLar study using Aircraft, Remote sensing, surface measurements and modeling of Climate, chemistry, Aerosols and Transport) program (http://www.polarcat.no/) to better understand the impact of transported and locally generated pollution on the composition, chemistry and climate of the Arctic atmosphere. In collaboration with POLARCAT partners, NASA carried out the ARCTAS (http://cloud1.arc.nasa.gov/arctas/) field campaign in the spring and summer of 2008 utilizing instrumented aircraft for detailed observations of the chemical and radiative properties of gases and aerosols in the Arctic and over California. Although high latitudes were the main focus in ARCTAS, collaboration with California Air Resources Board (CARB) offered a unique opportunity to compare mid-latitude and boreal forest fire emissions as well as provide unique information to test the quality of emission inventories and models in use in California. An overview of the design and implementation of the 2008 ARCTAS campaign and prevailing meteorological conditions has been provided by Jacob et al. (2010) and Fuelberg et al. (2010), respectively.

In this study we use statistical analysis, tracer observations, and models to analyze and interpret ARCTAS data collected largely by the NASA DC-8 (12 km ceiling). We compare the characteristics of boreal and California fire emissions and evaluate the impact of these and anthropogenic sources on atmospheric composition especially as it relates to ozone formation in the troposphere.