MATLAB: PDEPE boundary condition outputting odd results.

boundary conditionsMATLABpdepe

For the boundary condition I've set the left (pl) to a constant value of 280 with the idea being that this value will always be 280, however when i output the results the value is instead 4e10 and appears to be changing. What am I doing incorrectly so that my boundary condition is a) not constant and b) way above the value I've stated.
I've gone over the pdepe page a few times trying to figure it out but unfortauntely haven't made much progress.
Thank you for any help provided.
m = 0;
tspan = linspace(0,365*20*86400,21);
xmesh = linspace(0,20,201);
sol = pdepe(m,@pdefun,@icfun,@bcfun,xmesh,tspan);
u = sol(:,:,1);
figure, plot(u(2,:),xmesh)
ylabel('Depth (m)')
xlabel('Oxygen Concentration (g/m3)');
set(gca, 'XAxisLocation', 'top')
set(gca, 'YDir','reverse')
function[c,f,s]=pdefun(x,t,u,DuDx)
        c = 0.18;
        f = 1e-8*DuDx;
        s = -1e-9*u;
end    
function u=icfun(x)
        u = 0;
end
function[pl,ql,pr,qr]=bcfun(xl,ul,xr,ur,t)
        pl = 280;
        ql = 1;
        pr = ur;
        qr = 0;
end

Best Answer

function main
  m = 0;
  tspan = linspace(0,365*20*86400,21); 
  xmesh = linspace(0,20,201);
  sol = pdepe(m,@pdefun,@icfun,@bcfun,xmesh,tspan);
  u = sol(:,:,1);
  figure, plot(u(2,:),xmesh)
  ylabel('Depth (m)')
  xlabel('Oxygen Concentration (g/m3)');
  set(gca, 'XAxisLocation', 'top')
  set(gca, 'YDir','reverse')
end
function[c,f,s]=pdefun(x,t,u,DuDx)
  c = 0.18;
  f = 1e-8*DuDx;
  s = -1e-9*u;
end    
function u=icfun(x)
  u = 0;
end
function[pl,ql,pr,qr]=bcfun(xl,ul,xr,ur,t)
  u0 = 280;
  pl = ul - u0;  
  ql = 0;
  pr = ur;
  qr = 0;
end
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