take air samples from aircraft or tall towers to measure gas
concentrations remote from sources. Bottom-up methods take measurements
directly at facilities. Top-down methods provide a more complete and
unbiased assessment of emissions sources, and can detect emissions over
broad areas. However, they lack specificity and face difficulty in
assigning emissions to particular sources. Bottom-up methods provide
direct, precise measurement of gas emissions rates. However, the high
cost of sampling and the need for site access permission leads to small
sample sizes and possible sampling bias.
Photographs, videos and other images provided by the Precourt Institute for Energy at Stanford University are to be used - with appropriate credit - for editorial purposes only. Flopping, editing, altering or otherwise embellishing these images in any way that changes the images' editorial content is prohibited. Photographs and images provided for web use must be resized to low resolution. Permission is for one-time use only. Supplied photo/video/image file must be deleted after use.