Abstract
Abstract. Agriculture comprises a substantial, and increasing, fraction of land use in many regions of the world. Emissions from agricultural vegetation and other biogenic and anthropogenic sources react in the atmosphere to produce ozone and secondary organic aerosol, which comprises a substantial fraction of particulate matter (PM2.5). Using data from three measurement campaigns, we examine the magnitude and composition of reactive gas-phase organic carbon emissions from agricultural crops and their potential to impact regional air quality relative to anthropogenic emissions from motor vehicles in California's San Joaquin Valley, which is out of compliance with state and federal standards for tropospheric ozone PM2.5. Emission rates for a suite of terpenoid compounds were measured in a greenhouse for 25 representative crops from California in 2008. Ambient measurements of terpenoids and other biogenic compounds in the volatile and intermediate-volatility organic compound ranges were made in the urban area of Bakersfield and over an orange orchard in a rural area of the San Joaquin Valley during two 2010 seasons: summer and spring flowering. We combined measurements from the orchard site with ozone modeling methods to assess the net effect of the orange trees on regional ozone. When accounting for both emissions of reactive precursors and the deposition of ozone to the orchard, the orange trees are a net source of ozone in the springtime during flowering, and relatively neutral for most of the summer until the fall, when it becomes a sink. Flowering was a major emission event and caused a large increase in emissions including a suite of compounds that had not been measured in the atmosphere before. Such biogenic emission events need to be better parameterized in models as they have significant potential to impact regional air quality since emissions increase by several factors to over an order of magnitude. In regions like the San Joaquin Valley, the mass of biogenic emissions from agricultural crops during the summer (without flowering) and the potential ozone and secondary organic aerosol formation from these emissions are on the same order as anthropogenic emissions from motor vehicles and must be considered in air quality models and secondary pollution control strategies.
Highlights
Biogenic compounds are emitted from vegetation via several mechanisms and pathways
This work includes a survey of volatile organic compound (VOC) and intermediate-volatility organic compound (IVOC) emissions from agricultural crops studied via plant enclosure measurements in a greenhouse to develop emission factors and emission parameters (Table 2), and an assessment of seasonal emissions from an orange orchard located in a rural area of the San Joaquin Valley
The relative magnitude of biogenic versus anthropogenic emissions and compound concentrations vary depending on location in the San Joaquin Valley as shown by the comparison of the Bakersfield and Lindcove sites (Fig. 6)
Summary
Biogenic compounds are emitted from vegetation via several mechanisms and pathways. Emissions are typically a function of environmental parameters (e.g., light, temperature) or specialized responses to communicate with, attract, or repel animals, insects, or other plants (Bouvier-Brown et al, 2009; Goldstein and Galbally, 2007). Plant species can emit a variety of these isomers with one or more double bonds and can include cyclic or bicyclic rings, but a certain suite of compounds has been observed more frequently (Bouvier-Brown et al, 2009; Goldstein and Galbally, 2007). This work includes a survey of volatile organic compound (VOC) and intermediate-volatility organic compound (IVOC) emissions from agricultural crops studied via plant enclosure measurements in a greenhouse to develop emission factors and emission parameters (Table 2), and an assessment of seasonal emissions from an orange orchard located in a rural area of the San Joaquin Valley. Further objectives of this work include characterizing emissions associated with spring flowering, examining the relative importance of biogenic emissions from agriculture on ozone and SOA formation in the San Joaquin Valley, and modeling the net effect of orange trees in our case study orchard on ambient ozone concentrations
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